143 KiB
gMock Cookbook
You can find recipes for using gMock here. If you haven’t yet, please read this first to make sure you understand the basics.
Note: gMock lives in the testing
name
space. For readability, it is recommended to write
using ::testing::Foo;
once in your file before using the
name Foo
defined by gMock. We omit such using
statements in this section for brevity, but you should do it in your own
code.
Creating Mock Classes
Mock classes are defined as normal classes, using the
MOCK_METHOD
macro to generate mocked methods. The macro
gets 3 or 4 parameters:
class MyMock {
public:
(ReturnType, MethodName, (Args...));
MOCK_METHOD(ReturnType, MethodName, (Args...), (Specs...));
MOCK_METHOD};
The first 3 parameters are simply the method declaration, split into 3 parts. The 4th parameter accepts a closed list of qualifiers, which affect the generated method:
const
- Makes the mocked method aconst
method. Required if overriding aconst
method.override
- Marks the method withoverride
. Recommended if overriding avirtual
method.noexcept
- Marks the method withnoexcept
. Required if overriding anoexcept
method.Calltype(...)
- Sets the call type for the method (e.g. toSTDMETHODCALLTYPE
), useful in Windows.
Dealing with unprotected commas
Unprotected commas, i.e. commas which are not surrounded by
parentheses, prevent MOCK_METHOD
from parsing its arguments
correctly:
cpp {.bad} class MockFoo { public: MOCK_METHOD(std::pair<bool, int>, GetPair, ()); // Won't compile! MOCK_METHOD(bool, CheckMap, (std::map<int, double>, bool)); // Won't compile! };
Solution 1 - wrap with parentheses:
cpp {.good} class MockFoo { public: MOCK_METHOD((std::pair<bool, int>), GetPair, ()); MOCK_METHOD(bool, CheckMap, ((std::map<int, double>), bool)); };
Note that wrapping a return or argument type with parentheses is, in
general, invalid C++. MOCK_METHOD
removes the
parentheses.
Solution 2 - define an alias:
cpp {.good} class MockFoo { public: using BoolAndInt = std::pair<bool, int>; MOCK_METHOD(BoolAndInt, GetPair, ()); using MapIntDouble = std::map<int, double>; MOCK_METHOD(bool, CheckMap, (MapIntDouble, bool)); };
Mocking Private or Protected Methods
You must always put a mock method definition
(MOCK_METHOD
) in a public:
section of the mock
class, regardless of the method being mocked being public
,
protected
, or private
in the base class. This
allows ON_CALL
and EXPECT_CALL
to reference
the mock function from outside of the mock class. (Yes, C++ allows a
subclass to change the access level of a virtual function in the base
class.) Example:
class Foo {
public:
...
virtual bool Transform(Gadget* g) = 0;
protected:
virtual void Resume();
private:
virtual int GetTimeOut();
};
class MockFoo : public Foo {
public:
...
(bool, Transform, (Gadget* g), (override));
MOCK_METHOD
// The following must be in the public section, even though the
// methods are protected or private in the base class.
(void, Resume, (), (override));
MOCK_METHOD(int, GetTimeOut, (), (override));
MOCK_METHOD};
Mocking Overloaded Methods
You can mock overloaded functions as usual. No special attention is required:
class Foo {
...
// Must be virtual as we'll inherit from Foo.
virtual ~Foo();
// Overloaded on the types and/or numbers of arguments.
virtual int Add(Element x);
virtual int Add(int times, Element x);
// Overloaded on the const-ness of this object.
virtual Bar& GetBar();
virtual const Bar& GetBar() const;
};
class MockFoo : public Foo {
...
(int, Add, (Element x), (override));
MOCK_METHOD(int, Add, (int times, Element x), (override));
MOCK_METHOD
(Bar&, GetBar, (), (override));
MOCK_METHOD(const Bar&, GetBar, (), (const, override));
MOCK_METHOD};
Note: if you don’t mock all versions of the
overloaded method, the compiler will give you a warning about some
methods in the base class being hidden. To fix that, use
using
to bring them in scope:
class MockFoo : public Foo {
...
using Foo::Add;
(int, Add, (Element x), (override));
MOCK_METHOD// We don't want to mock int Add(int times, Element x);
...
};
Mocking Class Templates
You can mock class templates just like any class.
template <typename Elem>
class StackInterface {
...
// Must be virtual as we'll inherit from StackInterface.
virtual ~StackInterface();
virtual int GetSize() const = 0;
virtual void Push(const Elem& x) = 0;
};
template <typename Elem>
class MockStack : public StackInterface<Elem> {
...
(int, GetSize, (), (override));
MOCK_METHOD(void, Push, (const Elem& x), (override));
MOCK_METHOD};
Mocking Non-virtual Methods
gMock can mock non-virtual functions to be used in Hi-perf dependency injection.
In this case, instead of sharing a common base class with the real
class, your mock class will be unrelated to the real class, but
contain methods with the same signatures. The syntax for mocking
non-virtual methods is the same as mocking virtual methods
(just don’t add override
):
// A simple packet stream class. None of its members is virtual.
class ConcretePacketStream {
public:
void AppendPacket(Packet* new_packet);
const Packet* GetPacket(size_t packet_number) const;
size_t NumberOfPackets() const;
...
};
// A mock packet stream class. It inherits from no other, but defines
// GetPacket() and NumberOfPackets().
class MockPacketStream {
public:
(const Packet*, GetPacket, (size_t packet_number), (const));
MOCK_METHOD(size_t, NumberOfPackets, (), (const));
MOCK_METHOD...
};
Note that the mock class doesn’t define AppendPacket()
,
unlike the real class. That’s fine as long as the test doesn’t need to
call it.
Next, you need a way to say that you want to use
ConcretePacketStream
in production code, and use
MockPacketStream
in tests. Since the functions are not
virtual and the two classes are unrelated, you must specify your choice
at compile time (as opposed to run time).
One way to do it is to templatize your code that needs to use a
packet stream. More specifically, you will give your code a template
type argument for the type of the packet stream. In production, you will
instantiate your template with ConcretePacketStream
as the
type argument. In tests, you will instantiate the same template with
MockPacketStream
. For example, you may write:
template <class PacketStream>
void CreateConnection(PacketStream* stream) { ... }
template <class PacketStream>
class PacketReader {
public:
void ReadPackets(PacketStream* stream, size_t packet_num);
};
Then you can use
CreateConnection<ConcretePacketStream>()
and
PacketReader<ConcretePacketStream>
in production
code, and use CreateConnection<MockPacketStream>()
and PacketReader<MockPacketStream>
in tests.
;
MockPacketStream mock_stream(mock_stream, ...)...;
EXPECT_CALL.. set more expectations on mock_stream ...
<MockPacketStream> reader(&mock_stream);
PacketReader... exercise reader ...
Mocking Free Functions
It’s possible to use gMock to mock a free function (i.e. a C-style function or a static method). You just need to rewrite your code to use an interface (abstract class).
Instead of calling a free function (say, OpenFile
)
directly, introduce an interface for it and have a concrete subclass
that calls the free function:
class FileInterface {
public:
...
virtual bool Open(const char* path, const char* mode) = 0;
};
class File : public FileInterface {
public:
...
virtual bool Open(const char* path, const char* mode) {
return OpenFile(path, mode);
}
};
Your code should talk to FileInterface
to open a file.
Now it’s easy to mock out the function.
This may seem like a lot of hassle, but in practice you often have multiple related functions that you can put in the same interface, so the per-function syntactic overhead will be much lower.
If you are concerned about the performance overhead incurred by virtual functions, and profiling confirms your concern, you can combine this with the recipe for mocking non-virtual methods.
Old-Style
MOCK_METHODn
Macros
Before the generic MOCK_METHOD
macro was introduced,
mocks where created using a family of macros collectively called
MOCK_METHODn
. These macros are still supported, though
migration to the new MOCK_METHOD
is recommended.
The macros in the MOCK_METHODn
family differ from
MOCK_METHOD
:
- The general structure is
MOCK_METHODn(MethodName, ReturnType(Args))
, instead ofMOCK_METHOD(ReturnType, MethodName, (Args))
. - The number
n
must equal the number of arguments. - When mocking a const method, one must use
MOCK_CONST_METHODn
. - When mocking a class template, the macro name must be suffixed with
_T
. - In order to specify the call type, the macro name must be suffixed
with
_WITH_CALLTYPE
, and the call type is the first macro argument.
Old macros and their new equivalents:
Simple | |
---|---|
Old |
MOCK_METHOD1(Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int))
|
Const Method | |
Old |
MOCK_CONST_METHOD1(Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (const))
|
Method in a Class Template | |
Old |
MOCK_METHOD1_T(Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int))
|
Const Method in a Class Template | |
Old |
MOCK_CONST_METHOD1_T(Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (const))
|
Method with Call Type | |
Old |
MOCK_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))
|
Const Method with Call Type | |
Old |
MOCK_CONST_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))
|
Method with Call Type in a Class Template | |
Old |
MOCK_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))
|
Const Method with Call Type in a Class Template | |
Old |
MOCK_CONST_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))
|
New |
MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))
|
The Nice, the Strict, and the Naggy
If a mock method has no EXPECT_CALL
spec but is called,
we say that it’s an “uninteresting call”, and the default action (which
can be specified using ON_CALL()
) of the method will be
taken. Currently, an uninteresting call will also by default cause gMock
to print a warning. (In the future, we might remove this warning by
default.)
However, sometimes you may want to ignore these uninteresting calls, and sometimes you may want to treat them as errors. gMock lets you make the decision on a per-mock-object basis.
Suppose your test uses a mock class MockFoo
:
(...) {
TEST;
MockFoo mock_foo(mock_foo, DoThis());
EXPECT_CALL... code that uses mock_foo ...
}
If a method of mock_foo
other than DoThis()
is called, you will get a warning. However, if you rewrite your test to
use NiceMock<MockFoo>
instead, you can suppress the
warning:
using ::testing::NiceMock;
(...) {
TEST<MockFoo> mock_foo;
NiceMock(mock_foo, DoThis());
EXPECT_CALL... code that uses mock_foo ...
}
NiceMock<MockFoo>
is a subclass of
MockFoo
, so it can be used wherever MockFoo
is
accepted.
It also works if MockFoo
’s constructor takes some
arguments, as NiceMock<MockFoo>
“inherits”
MockFoo
’s constructors:
using ::testing::NiceMock;
(...) {
TEST<MockFoo> mock_foo(5, "hi"); // Calls MockFoo(5, "hi").
NiceMock(mock_foo, DoThis());
EXPECT_CALL... code that uses mock_foo ...
}
The usage of StrictMock
is similar, except that it makes
all uninteresting calls failures:
using ::testing::StrictMock;
(...) {
TEST<MockFoo> mock_foo;
StrictMock(mock_foo, DoThis());
EXPECT_CALL... code that uses mock_foo ...
// The test will fail if a method of mock_foo other than DoThis()
// is called.
}
NOTE: NiceMock
and StrictMock
only affects
uninteresting calls (calls of methods with no
expectations); they do not affect unexpected calls (calls of
methods with expectations, but they don’t match). See Understanding Uninteresting vs
Unexpected Calls.
There are some caveats though (I dislike them just as much as the next guy, but sadly they are side effects of C++’s limitations):
NiceMock<MockFoo>
andStrictMock<MockFoo>
only work for mock methods defined using theMOCK_METHOD
macro directly in theMockFoo
class. If a mock method is defined in a base class ofMockFoo
, the “nice” or “strict” modifier may not affect it, depending on the compiler. In particular, nestingNiceMock
andStrictMock
(e.g.NiceMock<StrictMock<MockFoo> >
) is not supported.NiceMock<MockFoo>
andStrictMock<MockFoo>
may not work correctly if the destructor ofMockFoo
is not virtual. We would like to fix this, but it requires cleaning up existing tests. http://b/28934720 tracks the issue.- During the constructor or destructor of
MockFoo
, the mock object is not nice or strict. This may cause surprises if the constructor or destructor calls a mock method onthis
object. (This behavior, however, is consistent with C++’s general rule: if a constructor or destructor calls a virtual method ofthis
object, that method is treated as non-virtual. In other words, to the base class’s constructor or destructor,this
object behaves like an instance of the base class, not the derived class. This rule is required for safety. Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)
Finally, you should be very cautious about when to use naggy or strict mocks, as they tend to make tests more brittle and harder to maintain. When you refactor your code without changing its externally visible behavior, ideally you shouldn’t need to update any tests. If your code interacts with a naggy mock, however, you may start to get spammed with warnings as the result of your change. Worse, if your code interacts with a strict mock, your tests may start to fail and you’ll be forced to fix them. Our general recommendation is to use nice mocks (not yet the default) most of the time, use naggy mocks (the current default) when developing or debugging tests, and use strict mocks only as the last resort.
Simplifying the Interface without Breaking Existing Code
Sometimes a method has a long list of arguments that is mostly uninteresting. For example:
class LogSink {
public:
...
virtual void send(LogSeverity severity, const char* full_filename,
const char* base_filename, int line,
const struct tm* tm_time,
const char* message, size_t message_len) = 0;
};
This method’s argument list is lengthy and hard to work with (the
message
argument is not even 0-terminated). If we mock it
as is, using the mock will be awkward. If, however, we try to simplify
this interface, we’ll need to fix all clients depending on it, which is
often infeasible.
The trick is to redispatch the method in the mock class:
class ScopedMockLog : public LogSink {
public:
...
virtual void send(LogSeverity severity, const char* full_filename,
const char* base_filename, int line, const tm* tm_time,
const char* message, size_t message_len) {
// We are only interested in the log severity, full file name, and
// log message.
(severity, full_filename, std::string(message, message_len));
Log}
// Implements the mock method:
//
// void Log(LogSeverity severity,
// const string& file_path,
// const string& message);
(void, Log,
MOCK_METHOD(LogSeverity severity, const string& file_path,
const string& message));
};
By defining a new mock method with a trimmed argument list, we make the mock class more user-friendly.
This technique may also be applied to make overloaded methods more amenable to mocking. For example, when overloads have been used to implement default arguments:
class MockTurtleFactory : public TurtleFactory {
public:
* MakeTurtle(int length, int weight) override { ... }
Turtle* MakeTurtle(int length, int weight, int speed) override { ... }
Turtle
// the above methods delegate to this one:
(Turtle*, DoMakeTurtle, ());
MOCK_METHOD};
This allows tests that don’t care which overload was invoked to avoid specifying argument matchers:
(factory, DoMakeTurtle)
ON_CALL.WillByDefault(MakeMockTurtle());
Alternative to Mocking Concrete Classes
Often you may find yourself using classes that don’t implement
interfaces. In order to test your code that uses such a class (let’s
call it Concrete
), you may be tempted to make the methods
of Concrete
virtual and then mock it.
Try not to do that.
Making a non-virtual function virtual is a big decision. It creates an extension point where subclasses can tweak your class’ behavior. This weakens your control on the class because now it’s harder to maintain the class invariants. You should make a function virtual only when there is a valid reason for a subclass to override it.
Mocking concrete classes directly is problematic as it creates a tight coupling between the class and the tests - any small change in the class may invalidate your tests and make test maintenance a pain.
To avoid such problems, many programmers have been practicing “coding
to interfaces”: instead of talking to the Concrete
class,
your code would define an interface and talk to it. Then you implement
that interface as an adaptor on top of Concrete
. In tests,
you can easily mock that interface to observe how your code is
doing.
This technique incurs some overhead:
- You pay the cost of virtual function calls (usually not a problem).
- There is more abstraction for the programmers to learn.
However, it can also bring significant benefits in addition to better testability:
Concrete
’s API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you’ll be more productive.- If
Concrete
’s implementation ever has to change, you don’t have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.
Some people worry that if everyone is practicing this technique, they will end up writing lots of redundant code. This concern is totally understandable. However, there are two reasons why it may not be the case:
- Different projects may need to use
Concrete
in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top ofConcrete
, and they will not be the same code. - If enough projects want to use the same interface, they can always
share it, just like they have been sharing
Concrete
. You can check in the interface and the adaptor somewhere nearConcrete
(perhaps in acontrib
sub-directory) and let many projects use it.
You need to weigh the pros and cons carefully for your particular problem, but I’d like to assure you that the Java community has been practicing this for a long time and it’s a proven effective technique applicable in a wide variety of situations. :-)
Delegating Calls to a Fake
Some times you have a non-trivial fake implementation of an interface. For example:
class Foo {
public:
virtual ~Foo() {}
virtual char DoThis(int n) = 0;
virtual void DoThat(const char* s, int* p) = 0;
};
class FakeFoo : public Foo {
public:
char DoThis(int n) override {
return (n > 0) ? '+' :
(n < 0) ? '-' : '0';
}
void DoThat(const char* s, int* p) override {
*p = strlen(s);
}
};
Now you want to mock this interface such that you can set
expectations on it. However, you also want to use FakeFoo
for the default behavior, as duplicating it in the mock object is, well,
a lot of work.
When you define the mock class using gMock, you can have it delegate its default action to a fake class you already have, using this pattern:
class MockFoo : public Foo {
public:
// Normal mock method definitions using gMock.
(char, DoThis, (int n), (override));
MOCK_METHOD(void, DoThat, (const char* s, int* p), (override));
MOCK_METHOD
// Delegates the default actions of the methods to a FakeFoo object.
// This must be called *before* the custom ON_CALL() statements.
void DelegateToFake() {
(*this, DoThis).WillByDefault([this](int n) {
ON_CALLreturn fake_.DoThis(n);
});
(*this, DoThat).WillByDefault([this](const char* s, int* p) {
ON_CALLfake_.DoThat(s, p);
});
}
private:
fake_; // Keeps an instance of the fake in the mock.
FakeFoo };
With that, you can use MockFoo
in your tests as usual.
Just remember that if you don’t explicitly set an action in an
ON_CALL()
or EXPECT_CALL()
, the fake will be
called upon to do it.:
using ::testing::_;
(AbcTest, Xyz) {
TEST;
MockFoo foo
.DelegateToFake(); // Enables the fake for delegation.
foo
// Put your ON_CALL(foo, ...)s here, if any.
// No action specified, meaning to use the default action.
(foo, DoThis(5));
EXPECT_CALL(foo, DoThat(_, _));
EXPECT_CALL
int n = 0;
('+', foo.DoThis(5)); // FakeFoo::DoThis() is invoked.
EXPECT_EQ.DoThat("Hi", &n); // FakeFoo::DoThat() is invoked.
foo(2, n);
EXPECT_EQ}
Some tips:
If you want, you can still override the default action by providing your own
ON_CALL()
or using.WillOnce()
/.WillRepeatedly()
inEXPECT_CALL()
.In
DelegateToFake()
, you only need to delegate the methods whose fake implementation you intend to use.The general technique discussed here works for overloaded methods, but you’ll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of
ON_CALL()
), use this technique; to disambiguate a fake function (the one you place insideInvoke()
), use astatic_cast
to specify the function’s type. For instance, if classFoo
has methodschar DoThis(int n)
andbool DoThis(double x) const
, and you want to invoke the latter, you need to writeInvoke(&fake_, static_cast<bool (FakeFoo::*)(double) const>(&FakeFoo::DoThis))
instead ofInvoke(&fake_, &FakeFoo::DoThis)
(The strange-looking thing inside the angled brackets ofstatic_cast
is the type of a function pointer to the secondDoThis()
method.).Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven’t got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, don’t abuse this. We would only recommend to do it as an intermediate step when you are refactoring your code.
Regarding the tip on mixing a mock and a fake, here’s an example on
why it may be a bad sign: Suppose you have a class System
for low-level system operations. In particular, it does file and I/O
operations. And suppose you want to test how your code uses
System
to do I/O, and you just want the file operations to
work normally. If you mock out the entire System
class,
you’ll have to provide a fake implementation for the file operation
part, which suggests that System
is taking on too many
roles.
Instead, you can define a FileOps
interface and an
IOOps
interface and split System
’s
functionalities into the two. Then you can mock IOOps
without mocking FileOps
.
Delegating Calls to a Real Object
When using testing doubles (mocks, fakes, stubs, and etc), sometimes their behaviors will differ from those of the real objects. This difference could be either intentional (as in simulating an error such that you can test the error handling code) or unintentional. If your mocks have different behaviors than the real objects by mistake, you could end up with code that passes the tests but fails in production.
You can use the delegating-to-real technique to ensure that your mock has the same behavior as the real object while retaining the ability to validate calls. This technique is very similar to the delegating-to-fake technique, the difference being that we use a real object instead of a fake. Here’s an example:
using ::testing::AtLeast;
class MockFoo : public Foo {
public:
() {
MockFoo// By default, all calls are delegated to the real object.
(*this, DoThis).WillByDefault([this](int n) {
ON_CALLreturn real_.DoThis(n);
});
(*this, DoThat).WillByDefault([this](const char* s, int* p) {
ON_CALLreal_.DoThat(s, p);
});
...
}
(char, DoThis, ...);
MOCK_METHOD(void, DoThat, ...);
MOCK_METHOD...
private:
real_;
Foo };
...
;
MockFoo mock(mock, DoThis())
EXPECT_CALL.Times(3);
(mock, DoThat("Hi"))
EXPECT_CALL.Times(AtLeast(1));
... use mock in test ...
With this, gMock will verify that your code made the right calls (with the right arguments, in the right order, called the right number of times, etc), and a real object will answer the calls (so the behavior will be the same as in production). This gives you the best of both worlds.
Delegating Calls to a Parent Class
Ideally, you should code to interfaces, whose methods are all pure virtual. In reality, sometimes you do need to mock a virtual method that is not pure (i.e, it already has an implementation). For example:
class Foo {
public:
virtual ~Foo();
virtual void Pure(int n) = 0;
virtual int Concrete(const char* str) { ... }
};
class MockFoo : public Foo {
public:
// Mocking a pure method.
(void, Pure, (int n), (override));
MOCK_METHOD// Mocking a concrete method. Foo::Concrete() is shadowed.
(int, Concrete, (const char* str), (override));
MOCK_METHOD};
Sometimes you may want to call Foo::Concrete()
instead
of MockFoo::Concrete()
. Perhaps you want to do it as part
of a stub action, or perhaps your test doesn’t need to mock
Concrete()
at all (but it would be oh-so painful to have to
define a new mock class whenever you don’t need to mock one of its
methods).
The trick is to leave a back door in your mock class for accessing the real methods in the base class:
class MockFoo : public Foo {
public:
// Mocking a pure method.
(void, Pure, (int n), (override));
MOCK_METHOD// Mocking a concrete method. Foo::Concrete() is shadowed.
(int, Concrete, (const char* str), (override));
MOCK_METHOD
// Use this to call Concrete() defined in Foo.
int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
Now, you can call Foo::Concrete()
inside an action
by:
...
(foo, Concrete).WillOnce([&foo](const char* str) {
EXPECT_CALLreturn foo.FooConcrete(str);
});
or tell the mock object that you don’t want to mock
Concrete()
:
...
(foo, Concrete).WillByDefault([&foo](const char* str) {
ON_CALLreturn foo.FooConcrete(str);
});
(Why don’t we just write { return foo.Concrete(str); }
?
If you do that, MockFoo::Concrete()
will be called (and
cause an infinite recursion) since Foo::Concrete()
is
virtual. That’s just how C++ works.)
Using Matchers
Matching Argument Values Exactly
You can specify exactly which arguments a mock method is expecting:
using ::testing::Return;
...
(foo, DoThis(5))
EXPECT_CALL.WillOnce(Return('a'));
(foo, DoThat("Hello", bar)); EXPECT_CALL
Using Simple Matchers
You can use matchers to match arguments that have a certain property:
using ::testing::NotNull;
using ::testing::Return;
...
(foo, DoThis(Ge(5))) // The argument must be >= 5.
EXPECT_CALL.WillOnce(Return('a'));
(foo, DoThat("Hello", NotNull()));
EXPECT_CALL// The second argument must not be NULL.
A frequently used matcher is _
, which matches
anything:
(foo, DoThat(_, NotNull())); EXPECT_CALL
Combining Matchers
You can build complex matchers from existing ones using
AllOf()
, AllOfArray()
, AnyOf()
,
AnyOfArray()
and Not()
:
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
// The argument must be > 5 and != 10.
(foo, DoThis(AllOf(Gt(5),
EXPECT_CALL(10))));
Ne
// The first argument must not contain sub-string "blah".
(foo, DoThat(Not(HasSubstr("blah")),
EXPECT_CALL)); NULL
Casting Matchers
gMock matchers are statically typed, meaning that the compiler can
catch your mistake if you use a matcher of the wrong type (for example,
if you use Eq(5)
to match a string
argument).
Good for you!
Sometimes, however, you know what you’re doing and want the compiler
to give you some slack. One example is that you have a matcher for
long
and the argument you want to match is
int
. While the two types aren’t exactly the same, there is
nothing really wrong with using a Matcher<long>
to
match an int
- after all, we can first convert the
int
argument to a long
losslessly before
giving it to the matcher.
To support this need, gMock gives you the
SafeMatcherCast<T>(m)
function. It casts a matcher
m
to type Matcher<T>
. To ensure safety,
gMock checks that (let U
be the type m
accepts
:
- Type
T
can be implicitly cast to typeU
; - When both
T
andU
are built-in arithmetic types (bool
, integers, and floating-point numbers), the conversion fromT
toU
is not lossy (in other words, any value representable byT
can also be represented byU
); and - When
U
is a reference,T
must also be a reference (as the underlying matcher may be interested in the address of theU
value).
The code won’t compile if any of these conditions isn’t met.
Here’s one example:
using ::testing::SafeMatcherCast;
// A base class and a child class.
class Base { ... };
class Derived : public Base { ... };
class MockFoo : public Foo {
public:
(void, DoThis, (Derived* derived), (override));
MOCK_METHOD};
...
;
MockFoo foo// m is a Matcher<Base*> we got from somewhere.
(foo, DoThis(SafeMatcherCast<Derived*>(m))); EXPECT_CALL
If you find SafeMatcherCast<T>(m)
too limiting,
you can use a similar function MatcherCast<T>(m)
. The
difference is that MatcherCast
works as long as you can
static_cast
type T
to type U
.
MatcherCast
essentially lets you bypass C++’s type
system (static_cast
isn’t always safe as it could throw
away information, for example), so be careful not to misuse/abuse
it.
Selecting Between Overloaded Functions
If you expect an overloaded function to be called, the compiler may need some help on which overloaded version it is.
To disambiguate functions overloaded on the const-ness of this
object, use the Const()
argument wrapper.
using ::testing::ReturnRef;
class MockFoo : public Foo {
...
(Bar&, GetBar, (), (override));
MOCK_METHOD(const Bar&, GetBar, (), (const, override));
MOCK_METHOD};
...
;
MockFoo foo, bar2;
Bar bar1(foo, GetBar()) // The non-const GetBar().
EXPECT_CALL.WillOnce(ReturnRef(bar1));
(Const(foo), GetBar()) // The const GetBar().
EXPECT_CALL.WillOnce(ReturnRef(bar2));
(Const()
is defined by gMock and returns a
const
reference to its argument.)
To disambiguate overloaded functions with the same number of
arguments but different argument types, you may need to specify the
exact type of a matcher, either by wrapping your matcher in
Matcher<type>()
, or using a matcher whose type is
fixed (TypedEq<type>
, An<type>()
,
etc):
using ::testing::An;
using ::testing::Matcher;
using ::testing::TypedEq;
class MockPrinter : public Printer {
public:
(void, Print, (int n), (override));
MOCK_METHOD(void, Print, (char c), (override));
MOCK_METHOD};
(PrinterTest, Print) {
TEST;
MockPrinter printer
(printer, Print(An<int>())); // void Print(int);
EXPECT_CALL(printer, Print(Matcher<int>(Lt(5)))); // void Print(int);
EXPECT_CALL(printer, Print(TypedEq<char>('a'))); // void Print(char);
EXPECT_CALL
.Print(3);
printer.Print(6);
printer.Print('a');
printer}
Performing Different Actions Based on the Arguments
When a mock method is called, the last matching expectation that’s still active will be selected (think “newer overrides older”). So, you can make a method do different things depending on its argument values like this:
using ::testing::_;
using ::testing::Lt;
using ::testing::Return;
...
// The default case.
(foo, DoThis(_))
EXPECT_CALL.WillRepeatedly(Return('b'));
// The more specific case.
(foo, DoThis(Lt(5)))
EXPECT_CALL.WillRepeatedly(Return('a'));
Now, if foo.DoThis()
is called with a value less than 5,
'a'
will be returned; otherwise 'b'
will be
returned.
Matching Multiple Arguments as a Whole
Sometimes it’s not enough to match the arguments individually. For
example, we may want to say that the first argument must be less than
the second argument. The With()
clause allows us to match
all arguments of a mock function as a whole. For example,
using ::testing::_;
using ::testing::Ne;
using ::testing::Lt;
...
(foo, InRange(Ne(0), _))
EXPECT_CALL.With(Lt());
says that the first argument of InRange()
must not be 0,
and must be less than the second argument.
The expression inside With()
must be a matcher of type
Matcher< ::std::tuple<A1, ..., An> >
, where
A1
, …, An
are the types of the function
arguments.
You can also write AllArgs(m)
instead of m
inside .With()
. The two forms are equivalent, but
.With(AllArgs(Lt()))
is more readable than
.With(Lt())
.
You can use Args<k1, ..., kn>(m)
to match the
n
selected arguments (as a tuple) against m
.
For example,
using ::testing::_;
using ::testing::AllOf;
using ::testing::Args;
using ::testing::Lt;
...
(foo, Blah)
EXPECT_CALL.With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));
says that Blah
will be called with arguments
x
, y
, and z
where
x < y < z
. Note that in this example, it wasn’t
necessary specify the positional matchers.
As a convenience and example, gMock provides some matchers for
2-tuples, including the Lt()
matcher above. See here for the complete list.
Note that if you want to pass the arguments to a predicate of your
own (e.g.
.With(Args<0, 1>(Truly(&MyPredicate)))
), that
predicate MUST be written to take a ::std::tuple
as its
argument; gMock will pass the n
selected arguments as
one single tuple to the predicate.
Using Matchers as Predicates
Have you noticed that a matcher is just a fancy predicate that also
knows how to describe itself? Many existing algorithms take predicates
as arguments (e.g. those defined in STL’s <algorithm>
header), and it would be a shame if gMock matchers were not allowed to
participate.
Luckily, you can use a matcher where a unary predicate functor is
expected by wrapping it inside the Matches()
function. For
example,
#include <algorithm>
#include <vector>
using ::testing::Matches;
using ::testing::Ge;
<int> v;
vector...
// How many elements in v are >= 10?
const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));
Since you can build complex matchers from simpler ones easily using
gMock, this gives you a way to conveniently construct composite
predicates (doing the same using STL’s <functional>
header is just painful). For example, here’s a predicate that’s
satisfied by any number that is >= 0, <= 100, and != 50:
using testing::AllOf;
using testing::Ge;
using testing::Le;
using testing::Matches;
using testing::Ne;
...
(AllOf(Ge(0), Le(100), Ne(50))) Matches
Using Matchers in googletest Assertions
Since matchers are basically predicates that also know how to
describe themselves, there is a way to take advantage of them in
googletest assertions. It’s called ASSERT_THAT
and
EXPECT_THAT
:
(value, matcher); // Asserts that value matches matcher.
ASSERT_THAT(value, matcher); // The non-fatal version. EXPECT_THAT
For example, in a googletest test you can write:
#include "gmock/gmock.h"
using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::MatchesRegex;
using ::testing::StartsWith;
...
(Foo(), StartsWith("Hello"));
EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
EXPECT_THAT(Baz(), AllOf(Ge(5), Le(10))); ASSERT_THAT
which (as you can probably guess) executes Foo()
,
Bar()
, and Baz()
, and verifies that:
Foo()
returns a string that starts with"Hello"
.Bar()
returns a string that matches regular expression"Line \\d+"
.Baz()
returns a number in the range [5, 10].
The nice thing about these macros is that they read like
English. They generate informative messages too. For example, if
the first EXPECT_THAT()
above fails, the message will be
something like:
: Foo()
Value of: "Hi, world!"
Actual: starts with "Hello" Expected
Credit: The idea of
(ASSERT|EXPECT)_THAT
was borrowed from Joe Walnes’ Hamcrest
project, which adds assertThat()
to JUnit.
Using Predicates as Matchers
gMock provides a built-in set of matchers.
In case you find them lacking, you can use an arbitrary unary predicate
function or functor as a matcher - as long as the predicate accepts a
value of the type you want. You do this by wrapping the predicate inside
the Truly()
function, for example:
using ::testing::Truly;
int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
...
// Bar() must be called with an even number.
(foo, Bar(Truly(IsEven))); EXPECT_CALL
Note that the predicate function / functor doesn’t have to return
bool
. It works as long as the return value can be used as
the condition in in statement if (condition) ...
.
Matching Arguments that Are Not Copyable
When you do an EXPECT_CALL(mock_obj, Foo(bar))
, gMock
saves away a copy of bar
. When Foo()
is called
later, gMock compares the argument to Foo()
with the saved
copy of bar
. This way, you don’t need to worry about
bar
being modified or destroyed after the
EXPECT_CALL()
is executed. The same is true when you use
matchers like Eq(bar)
, Le(bar)
, and so on.
But what if bar
cannot be copied (i.e. has no copy
constructor)? You could define your own matcher function or callback and
use it with Truly()
, as the previous couple of recipes have
shown. Or, you may be able to get away from it if you can guarantee that
bar
won’t be changed after the EXPECT_CALL()
is executed. Just tell gMock that it should save a reference to
bar
, instead of a copy of it. Here’s how:
using ::testing::ByRef;
using ::testing::Eq;
using ::testing::Lt;
...
// Expects that Foo()'s argument == bar.
(mock_obj, Foo(Eq(ByRef(bar))));
EXPECT_CALL
// Expects that Foo()'s argument < bar.
(mock_obj, Foo(Lt(ByRef(bar)))); EXPECT_CALL
Remember: if you do this, don’t change bar
after the
EXPECT_CALL()
, or the result is undefined.
Validating a Member of an Object
Often a mock function takes a reference to object as an argument.
When matching the argument, you may not want to compare the entire
object against a fixed object, as that may be over-specification.
Instead, you may need to validate a certain member variable or the
result of a certain getter method of the object. You can do this with
Field()
and Property()
. More specifically,
(&Foo::bar, m) Field
is a matcher that matches a Foo
object whose
bar
member variable satisfies matcher m
.
(&Foo::baz, m) Property
is a matcher that matches a Foo
object whose
baz()
method returns a value that satisfies matcher
m
.
For example:
Expression | Description |
---|---|
Field(&Foo::number, Ge(3)) |
Matches x where
x.number >= 3 . |
Property(&Foo::name, StartsWith("John ")) |
Matches x where
x.name() starts with "John " . |
Note that in Property(&Foo::baz, ...)
, method
baz()
must take no argument and be declared as
const
.
BTW, Field()
and Property()
can also match
plain pointers to objects. For instance,
using ::testing::Field;
using ::testing::Ge;
...
(&Foo::number, Ge(3)) Field
matches a plain pointer p
where
p->number >= 3
. If p
is
NULL
, the match will always fail regardless of the inner
matcher.
What if you want to validate more than one members at the same time?
Remember that there are AllOf()
and
AllOfArray()
.
Finally Field()
and Property()
provide
overloads that take the field or property names as the first argument to
include it in the error message. This can be useful when creating
combined matchers.
using ::testing::AllOf;
using ::testing::Field;
using ::testing::Matcher;
using ::testing::SafeMatcherCast;
<Foo> IsFoo(const Foo& foo) {
Matcherreturn AllOf(Field("some_field", &Foo::some_field, foo.some_field),
("other_field", &Foo::other_field, foo.other_field),
Field("last_field", &Foo::last_field, foo.last_field));
Field}
Validating the Value Pointed to by a Pointer Argument
C++ functions often take pointers as arguments. You can use matchers
like IsNull()
, NotNull()
, and other comparison
matchers to match a pointer, but what if you want to make sure the value
pointed to by the pointer, instead of the pointer itself, has a
certain property? Well, you can use the Pointee(m)
matcher.
Pointee(m)
matches a pointer if m
matches
the value the pointer points to. For example:
using ::testing::Ge;
using ::testing::Pointee;
...
(foo, Bar(Pointee(Ge(3)))); EXPECT_CALL
expects foo.Bar()
to be called with a pointer that
points to a value greater than or equal to 3.
One nice thing about Pointee()
is that it treats a
NULL
pointer as a match failure, so you can write
Pointee(m)
instead of
using ::testing::AllOf;
using ::testing::NotNull;
using ::testing::Pointee;
...
(NotNull(), Pointee(m)) AllOf
without worrying that a NULL
pointer will crash your
test.
Also, did we tell you that Pointee()
works with both raw
pointers and smart pointers
(std::unique_ptr
, std::shared_ptr
, etc)?
What if you have a pointer to pointer? You guessed it - you can use
nested Pointee()
to probe deeper inside the value. For
example, Pointee(Pointee(Lt(3)))
matches a pointer that
points to a pointer that points to a number less than 3 (what a
mouthful…).
Testing a Certain Property of an Object
Sometimes you want to specify that an object argument has a certain property, but there is no existing matcher that does this. If you want good error messages, you should define a matcher. If you want to do it quick and dirty, you could get away with writing an ordinary function.
Let’s say you have a mock function that takes an object of type
Foo
, which has an int bar()
method and an
int baz()
method, and you want to constrain that the
argument’s bar()
value plus its baz()
value is
a given number. Here’s how you can define a matcher to do it:
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;
class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
public:
explicit BarPlusBazEqMatcher(int expected_sum)
: expected_sum_(expected_sum) {}
bool MatchAndExplain(const Foo& foo,
* /* listener */) const override {
MatchResultListenerreturn (foo.bar() + foo.baz()) == expected_sum_;
}
void DescribeTo(::std::ostream* os) const override {
*os << "bar() + baz() equals " << expected_sum_;
}
void DescribeNegationTo(::std::ostream* os) const override {
*os << "bar() + baz() does not equal " << expected_sum_;
}
private:
const int expected_sum_;
};
<const Foo&> BarPlusBazEq(int expected_sum) {
Matcherreturn MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
}
...
(..., DoThis(BarPlusBazEq(5)))...; EXPECT_CALL
Matching Containers
Sometimes an STL container (e.g. list, vector, map, …) is passed to a
mock function and you may want to validate it. Since most STL containers
support the ==
operator, you can write
Eq(expected_container)
or simply
expected_container
to match a container exactly.
Sometimes, though, you may want to be more flexible (for example, the first element must be an exact match, but the second element can be any positive number, and so on). Also, containers used in tests often have a small number of elements, and having to define the expected container out-of-line is a bit of a hassle.
You can use the ElementsAre()
or
UnorderedElementsAre()
matcher in such cases:
using ::testing::_;
using ::testing::ElementsAre;
using ::testing::Gt;
...
(void, Foo, (const vector<int>& numbers), (override));
MOCK_METHOD...
(mock, Foo(ElementsAre(1, Gt(0), _, 5))); EXPECT_CALL
The above matcher says that the container must have 4 elements, which must be 1, greater than 0, anything, and 5 respectively.
If you instead write:
using ::testing::_;
using ::testing::Gt;
using ::testing::UnorderedElementsAre;
...
(void, Foo, (const vector<int>& numbers), (override));
MOCK_METHOD...
(mock, Foo(UnorderedElementsAre(1, Gt(0), _, 5))); EXPECT_CALL
It means that the container must have 4 elements, which (under some permutation) must be 1, greater than 0, anything, and 5 respectively.
As an alternative you can place the arguments in a C-style array and
use ElementsAreArray()
or
UnorderedElementsAreArray()
instead:
using ::testing::ElementsAreArray;
...
// ElementsAreArray accepts an array of element values.
const int expected_vector1[] = {1, 5, 2, 4, ...};
(mock, Foo(ElementsAreArray(expected_vector1)));
EXPECT_CALL
// Or, an array of element matchers.
<int> expected_vector2[] = {1, Gt(2), _, 3, ...};
Matcher(mock, Foo(ElementsAreArray(expected_vector2))); EXPECT_CALL
In case the array needs to be dynamically created (and therefore the
array size cannot be inferred by the compiler), you can give
ElementsAreArray()
an additional argument to specify the
array size:
using ::testing::ElementsAreArray;
...
int* const expected_vector3 = new int[count];
... fill expected_vector3 with values ...
(mock, Foo(ElementsAreArray(expected_vector3, count))); EXPECT_CALL
Use Pair
when comparing maps or other associative
containers.
using testing::ElementsAre;
using testing::Pair;
...
std::map<string, int> m = {{"a", 1}, {"b", 2}, {"c", 3}};
(m, ElementsAre(Pair("a", 1), Pair("b", 2), Pair("c", 3))); EXPECT_THAT
Tips:
ElementsAre*()
can be used to match any container that implements the STL iterator pattern (i.e. it has aconst_iterator
type and supportsbegin()/end()
), not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.- You can use nested
ElementsAre*()
to match nested (multi-dimensional) containers. - If the container is passed by pointer instead of by reference, just
write
Pointee(ElementsAre*(...))
. - The order of elements matters for
ElementsAre*()
. If you are using it with containers whose element order are undefined (e.g.hash_map
) you should useWhenSorted
aroundElementsAre
.
Sharing Matchers
Under the hood, a gMock matcher object consists of a pointer to a ref-counted implementation object. Copying matchers is allowed and very efficient, as only the pointer is copied. When the last matcher that references the implementation object dies, the implementation object will be deleted.
Therefore, if you have some complex matcher that you want to use again and again, there is no need to build it everytime. Just assign it to a matcher variable and use that variable repeatedly! For example,
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::Le;
using ::testing::Matcher;
...
<int> in_range = AllOf(Gt(5), Le(10));
Matcher... use in_range as a matcher in multiple EXPECT_CALLs ...
Matchers must have no side-effects
WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be purely functional: they cannot have any side effects, and the match result must not depend on anything other than the matcher’s parameters and the value being matched.
This requirement must be satisfied no matter how a matcher is defined (e.g., if it is one of the standard matchers, or a custom matcher). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and gMock.
Setting Expectations
Knowing When to Expect
ON_CALL
is likely the single most
under-utilized construct in gMock.
There are basically two constructs for defining the behavior of a
mock object: ON_CALL
and EXPECT_CALL
. The
difference? ON_CALL
defines what happens when a mock method
is called, but doesn’t imply any expectation on the method being
called. EXPECT_CALL
not only defines the behavior, but
also sets an expectation that the method will be called with the
given arguments, for the given number of times (and in the
given order when you specify the order too).
Since EXPECT_CALL
does more, isn’t it better than
ON_CALL
? Not really. Every EXPECT_CALL
adds a
constraint on the behavior of the code under test. Having more
constraints than necessary is baaad - even worse than not
having enough constraints.
This may be counter-intuitive. How could tests that verify more be worse than tests that verify less? Isn’t verification the whole point of tests?
The answer lies in what a test should verify. A good test verifies the contract of the code. If a test over-specifies, it doesn’t leave enough freedom to the implementation. As a result, changing the implementation without breaking the contract (e.g. refactoring and optimization), which should be perfectly fine to do, can break such tests. Then you have to spend time fixing them, only to see them broken again the next time the implementation is changed.
Keep in mind that one doesn’t have to verify more than one property in one test. In fact, it’s a good style to verify only one thing in one test. If you do that, a bug will likely break only one or two tests instead of dozens (which case would you rather debug?). If you are also in the habit of giving tests descriptive names that tell what they verify, you can often easily guess what’s wrong just from the test log itself.
So use ON_CALL
by default, and only use
EXPECT_CALL
when you actually intend to verify that the
call is made. For example, you may have a bunch of ON_CALL
s
in your test fixture to set the common mock behavior shared by all tests
in the same group, and write (scarcely) different
EXPECT_CALL
s in different TEST_F
s to verify
different aspects of the code’s behavior. Compared with the style where
each TEST
has many EXPECT_CALL
s, this leads to
tests that are more resilient to implementational changes (and thus less
likely to require maintenance) and makes the intent of the tests more
obvious (so they are easier to maintain when you do need to maintain
them).
If you are bothered by the “Uninteresting mock function call” message
printed when a mock method without an EXPECT_CALL
is
called, you may use a NiceMock
instead to suppress all such
messages for the mock object, or suppress the message for specific
methods by adding EXPECT_CALL(...).Times(AnyNumber())
. DO
NOT suppress it by blindly adding an EXPECT_CALL(...)
, or
you’ll have a test that’s a pain to maintain.
Ignoring Uninteresting Calls
If you are not interested in how a mock method is called, just don’t
say anything about it. In this case, if the method is ever called, gMock
will perform its default action to allow the test program to continue.
If you are not happy with the default action taken by gMock, you can
override it using DefaultValue<T>::Set()
(described
here) or ON_CALL()
.
Please note that once you expressed interest in a particular mock
method (via EXPECT_CALL()
), all invocations to it must
match some expectation. If this function is called but the arguments
don’t match any EXPECT_CALL()
statement, it will be an
error.
Disallowing Unexpected Calls
If a mock method shouldn’t be called at all, explicitly say so:
using ::testing::_;
...
(foo, Bar(_))
EXPECT_CALL.Times(0);
If some calls to the method are allowed, but the rest are not, just list all the expected calls:
using ::testing::AnyNumber;
using ::testing::Gt;
...
(foo, Bar(5));
EXPECT_CALL(foo, Bar(Gt(10)))
EXPECT_CALL.Times(AnyNumber());
A call to foo.Bar()
that doesn’t match any of the
EXPECT_CALL()
statements will be an error.
Understanding Uninteresting vs Unexpected Calls
Uninteresting calls and unexpected calls are different concepts in gMock. Very different.
A call x.Y(...)
is uninteresting if
there’s not even a single EXPECT_CALL(x, Y(...))
set. In other words, the test isn’t interested in the x.Y()
method at all, as evident in that the test doesn’t care to say anything
about it.
A call x.Y(...)
is unexpected if there
are some EXPECT_CALL(x, Y(...))
s set, but none of
them matches the call. Put another way, the test is interested in the
x.Y()
method (therefore it explicitly sets some
EXPECT_CALL
to verify how it’s called); however, the
verification fails as the test doesn’t expect this particular call to
happen.
An unexpected call is always an error, as the code under test doesn’t behave the way the test expects it to behave.
By default, an uninteresting call is not an error, as it violates no constraint specified by the test. (gMock’s philosophy is that saying nothing means there is no constraint.) However, it leads to a warning, as it might indicate a problem (e.g. the test author might have forgotten to specify a constraint).
In gMock, NiceMock
and StrictMock
can be
used to make a mock class “nice” or “strict”. How does this affect
uninteresting calls and unexpected calls?
A nice mock suppresses uninteresting call warnings. It is less chatty than the default mock, but otherwise is the same. If a test fails with a default mock, it will also fail using a nice mock instead. And vice versa. Don’t expect making a mock nice to change the test’s result.
A strict mock turns uninteresting call warnings into errors. So making a mock strict may change the test’s result.
Let’s look at an example:
(...) {
TEST<MockDomainRegistry> mock_registry;
NiceMock(mock_registry, GetDomainOwner("google.com"))
EXPECT_CALL.WillRepeatedly(Return("Larry Page"));
// Use mock_registry in code under test.
... &mock_registry ...
}
The sole EXPECT_CALL
here says that all calls to
GetDomainOwner()
must have "google.com"
as the
argument. If GetDomainOwner("yahoo.com")
is called, it will
be an unexpected call, and thus an error. Having a nice mock doesn’t
change the severity of an unexpected call.
So how do we tell gMock that GetDomainOwner()
can be
called with some other arguments as well? The standard technique is to
add a “catch all” EXPECT_CALL
:
(mock_registry, GetDomainOwner(_))
EXPECT_CALL.Times(AnyNumber()); // catches all other calls to this method.
(mock_registry, GetDomainOwner("google.com"))
EXPECT_CALL.WillRepeatedly(Return("Larry Page"));
Remember that _
is the wildcard matcher that matches
anything. With this, if GetDomainOwner("google.com")
is
called, it will do what the second EXPECT_CALL
says; if it
is called with a different argument, it will do what the first
EXPECT_CALL
says.
Note that the order of the two EXPECT_CALL
s is
important, as a newer EXPECT_CALL
takes precedence over an
older one.
For more on uninteresting calls, nice mocks, and strict mocks, read “The Nice, the Strict, and the Naggy”.
Ignoring Uninteresting Arguments
If your test doesn’t care about the parameters (it only cares about the number or order of calls), you can often simply omit the parameter list:
// Expect foo.Bar( ... ) twice with any arguments.
(foo, Bar).Times(2);
EXPECT_CALL
// Delegate to the given method whenever the factory is invoked.
(foo_factory, MakeFoo)
ON_CALL.WillByDefault(&BuildFooForTest);
This functionality is only available when a method is not overloaded; to prevent unexpected behavior it is a compilation error to try to set an expectation on a method where the specific overload is ambiguous. You can work around this by supplying a simpler mock interface than the mocked class provides.
This pattern is also useful when the arguments are interesting, but match logic is substantially complex. You can leave the argument list unspecified and use SaveArg actions to save the values for later verification. If you do that, you can easily differentiate calling the method the wrong number of times from calling it with the wrong arguments.
Expecting Ordered Calls
Although an EXPECT_CALL()
statement defined earlier
takes precedence when gMock tries to match a function call with an
expectation, by default calls don’t have to happen in the order
EXPECT_CALL()
statements are written. For example, if the
arguments match the matchers in the third EXPECT_CALL()
,
but not those in the first two, then the third expectation will be
used.
If you would rather have all calls occur in the order of the
expectations, put the EXPECT_CALL()
statements in a block
where you define a variable of type InSequence
:
using ::testing::_;
using ::testing::InSequence;
{
;
InSequence s
(foo, DoThis(5));
EXPECT_CALL(bar, DoThat(_))
EXPECT_CALL.Times(2);
(foo, DoThis(6));
EXPECT_CALL}
In this example, we expect a call to foo.DoThis(5)
,
followed by two calls to bar.DoThat()
where the argument
can be anything, which are in turn followed by a call to
foo.DoThis(6)
. If a call occurred out-of-order, gMock will
report an error.
Expecting Partially Ordered Calls
Sometimes requiring everything to occur in a predetermined order can
lead to brittle tests. For example, we may care about A
occurring before both B
and C
, but aren’t
interested in the relative order of B
and C
.
In this case, the test should reflect our real intent, instead of being
overly constraining.
gMock allows you to impose an arbitrary DAG (directed acyclic graph)
on the calls. One way to express the DAG is to use the After clause of EXPECT_CALL
.
Another way is via the InSequence()
clause (not the same
as the InSequence
class), which we borrowed from jMock 2.
It’s less flexible than After()
, but more convenient when
you have long chains of sequential calls, as it doesn’t require you to
come up with different names for the expectations in the chains. Here’s
how it works:
If we view EXPECT_CALL()
statements as nodes in a graph,
and add an edge from node A to node B wherever A must occur before B, we
can get a DAG. We use the term “sequence” to mean a directed path in
this DAG. Now, if we decompose the DAG into sequences, we just need to
know which sequences each EXPECT_CALL()
belongs to in order
to be able to reconstruct the original DAG.
So, to specify the partial order on the expectations we need to do
two things: first to define some Sequence
objects, and then
for each EXPECT_CALL()
say which Sequence
objects it is part of.
Expectations in the same sequence must occur in the order they are written. For example,
using ::testing::Sequence;
...
, s2;
Sequence s1
(foo, A())
EXPECT_CALL.InSequence(s1, s2);
(bar, B())
EXPECT_CALL.InSequence(s1);
(bar, C())
EXPECT_CALL.InSequence(s2);
(foo, D())
EXPECT_CALL.InSequence(s2);
specifies the following DAG (where s1
is
A -> B
, and s2
is
A -> C -> D
):
+---> B
|
A ---|
|
+---> C ---> D
This means that A must occur before B and C, and C must occur before D. There’s no restriction about the order other than these.
Controlling When an Expectation Retires
When a mock method is called, gMock only considers expectations that are still active. An expectation is active when created, and becomes inactive (aka retires) when a call that has to occur later has occurred. For example, in
using ::testing::_;
using ::testing::Sequence;
...
, s2;
Sequence s1
(log, Log(WARNING, _, "File too large.")) // #1
EXPECT_CALL.Times(AnyNumber())
.InSequence(s1, s2);
(log, Log(WARNING, _, "Data set is empty.")) // #2
EXPECT_CALL.InSequence(s1);
(log, Log(WARNING, _, "User not found.")) // #3
EXPECT_CALL.InSequence(s2);
as soon as either #2 or #3 is matched, #1 will retire. If a warning
"File too large."
is logged after this, it will be an
error.
Note that an expectation doesn’t retire automatically when it’s saturated. For example,
using ::testing::_;
...
(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")); // #2 EXPECT_CALL
says that there will be exactly one warning with the message
"File too large."
. If the second warning contains this
message too, #2 will match again and result in an upper-bound-violated
error.
If this is not what you want, you can ask an expectation to retire as soon as it becomes saturated:
using ::testing::_;
...
(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #2
EXPECT_CALL.RetiresOnSaturation();
Here #2 can be used only once, so if you have two warnings with the
message "File too large."
, the first will match #2 and the
second will match #1 - there will be no error.
Using Actions
Returning References from Mock Methods
If a mock function’s return type is a reference, you need to use
ReturnRef()
instead of Return()
to return a
result:
using ::testing::ReturnRef;
class MockFoo : public Foo {
public:
(Bar&, GetBar, (), (override));
MOCK_METHOD};
...
;
MockFoo foo;
Bar bar(foo, GetBar())
EXPECT_CALL.WillOnce(ReturnRef(bar));
...
Returning Live Values from Mock Methods
The Return(x)
action saves a copy of x
when
the action is created, and always returns the same value whenever it’s
executed. Sometimes you may want to instead return the live
value of x
(i.e. its value at the time when the action is
executed.). Use either ReturnRef()
or
ReturnPointee()
for this purpose.
If the mock function’s return type is a reference, you can do it
using ReturnRef(x)
, as shown in the previous recipe
(“Returning References from Mock Methods”). However, gMock doesn’t let
you use ReturnRef()
in a mock function whose return type is
not a reference, as doing that usually indicates a user error. So, what
shall you do?
Though you may be tempted, DO NOT use ByRef()
:
using testing::ByRef;
using testing::Return;
class MockFoo : public Foo {
public:
(int, GetValue, (), (override));
MOCK_METHOD};
...
int x = 0;
;
MockFoo foo(foo, GetValue())
EXPECT_CALL.WillRepeatedly(Return(ByRef(x))); // Wrong!
= 42;
x (42, foo.GetValue()); EXPECT_EQ
Unfortunately, it doesn’t work here. The above code will fail with error:
Value of: foo.GetValue()
Actual: 0
Expected: 42
The reason is that Return(*value*)
converts
value
to the actual return type of the mock function at the
time when the action is created, not when it is
executed. (This behavior was chosen for the action to be safe
when value
is a proxy object that references some temporary
objects.) As a result, ByRef(x)
is converted to an
int
value (instead of a const int&
) when
the expectation is set, and Return(ByRef(x))
will always
return 0.
ReturnPointee(pointer)
was provided to solve this
problem specifically. It returns the value pointed to by
pointer
at the time the action is executed:
using testing::ReturnPointee;
...
int x = 0;
;
MockFoo foo(foo, GetValue())
EXPECT_CALL.WillRepeatedly(ReturnPointee(&x)); // Note the & here.
= 42;
x (42, foo.GetValue()); // This will succeed now. EXPECT_EQ
Combining Actions
Want to do more than one thing when a function is called? That’s
fine. DoAll()
allow you to do sequence of actions every
time. Only the return value of the last action in the sequence will be
used.
using ::testing::_;
using ::testing::DoAll;
class MockFoo : public Foo {
public:
(bool, Bar, (int n), (override));
MOCK_METHOD};
...
(foo, Bar(_))
EXPECT_CALL.WillOnce(DoAll(action_1,
,
action_2...
)); action_n
Verifying Complex Arguments
If you want to verify that a method is called with a particular argument but the match criteria is complex, it can be difficult to distinguish between cardinality failures (calling the method the wrong number of times) and argument match failures. Similarly, if you are matching multiple parameters, it may not be easy to distinguishing which argument failed to match. For example:
// Not ideal: this could fail because of a problem with arg1 or arg2, or maybe
// just the method wasn't called.
(foo, SendValues(_, ElementsAre(1, 4, 4, 7), EqualsProto( ... ))); EXPECT_CALL
You can instead save the arguments and test them individually:
(foo, SendValues)
EXPECT_CALL.WillOnce(DoAll(SaveArg<1>(&actual_array), SaveArg<2>(&actual_proto)));
... run the test
(actual_array, ElementsAre(1, 4, 4, 7));
EXPECT_THAT(actual_proto, EqualsProto( ... )); EXPECT_THAT
Mocking Side Effects
Sometimes a method exhibits its effect not via returning a value but
via side effects. For example, it may change some global state or modify
an output argument. To mock side effects, in general you can define your
own action by implementing ::testing::ActionInterface
.
If all you need to do is to change an output argument, the built-in
SetArgPointee()
action is convenient:
using ::testing::_;
using ::testing::SetArgPointee;
class MockMutator : public Mutator {
public:
(void, Mutate, (bool mutate, int* value), (override));
MOCK_METHOD...
}
...
;
MockMutator mutator(mutator, Mutate(true, _))
EXPECT_CALL.WillOnce(SetArgPointee<1>(5));
In this example, when mutator.Mutate()
is called, we
will assign 5 to the int
variable pointed to by argument #1
(0-based).
SetArgPointee()
conveniently makes an internal copy of
the value you pass to it, removing the need to keep the value in scope
and alive. The implication however is that the value must have a copy
constructor and assignment operator.
If the mock method also needs to return a value as well, you can
chain SetArgPointee()
with Return()
using
DoAll()
, remembering to put the Return()
statement last:
using ::testing::_;
using ::testing::Return;
using ::testing::SetArgPointee;
class MockMutator : public Mutator {
public:
...
(bool, MutateInt, (int* value), (override));
MOCK_METHOD}
...
;
MockMutator mutator(mutator, MutateInt(_))
EXPECT_CALL.WillOnce(DoAll(SetArgPointee<0>(5),
(true))); Return
Note, however, that if you use the ReturnOKWith()
method, it will override the values provided by
SetArgPointee()
in the response parameters of your function
call.
If the output argument is an array, use the
SetArrayArgument<N>(first, last)
action instead. It
copies the elements in source range [first, last)
to the
array pointed to by the N
-th (0-based) argument:
using ::testing::NotNull;
using ::testing::SetArrayArgument;
class MockArrayMutator : public ArrayMutator {
public:
(void, Mutate, (int* values, int num_values), (override));
MOCK_METHOD...
}
...
;
MockArrayMutator mutatorint values[5] = {1, 2, 3, 4, 5};
(mutator, Mutate(NotNull(), 5))
EXPECT_CALL.WillOnce(SetArrayArgument<0>(values, values + 5));
This also works when the argument is an output iterator:
using ::testing::_;
using ::testing::SetArrayArgument;
class MockRolodex : public Rolodex {
public:
(void, GetNames, (std::back_insert_iterator<vector<string>>),
MOCK_METHOD(override));
...
}
...
;
MockRolodex rolodex<string> names;
vector.push_back("George");
names.push_back("John");
names.push_back("Thomas");
names(rolodex, GetNames(_))
EXPECT_CALL.WillOnce(SetArrayArgument<0>(names.begin(), names.end()));
Changing a Mock Object’s Behavior Based on the State
If you expect a call to change the behavior of a mock object, you can
use ::testing::InSequence
to specify different behaviors
before and after the call:
using ::testing::InSequence;
using ::testing::Return;
...
{
;
InSequence seq(my_mock, IsDirty())
EXPECT_CALL.WillRepeatedly(Return(true));
(my_mock, Flush());
EXPECT_CALL(my_mock, IsDirty())
EXPECT_CALL.WillRepeatedly(Return(false));
}
.FlushIfDirty(); my_mock
This makes my_mock.IsDirty()
return true
before my_mock.Flush()
is called and return
false
afterwards.
If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:
using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;
(ReturnPointee, p) { return *p; }
ACTION_P...
int previous_value = 0;
(my_mock, GetPrevValue)
EXPECT_CALL.WillRepeatedly(ReturnPointee(&previous_value));
(my_mock, UpdateValue)
EXPECT_CALL.WillRepeatedly(SaveArg<0>(&previous_value));
.DoSomethingToUpdateValue(); my_mock
Here my_mock.GetPrevValue()
will always return the
argument of the last UpdateValue()
call.
Setting the Default Value for a Return Type
If a mock method’s return type is a built-in C++ type or pointer, by default it will return 0 when invoked. Also, in C++ 11 and above, a mock method whose return type has a default constructor will return a default-constructed value by default. You only need to specify an action if this default value doesn’t work for you.
Sometimes, you may want to change this default value, or you may want
to specify a default value for types gMock doesn’t know about. You can
do this using the ::testing::DefaultValue
class
template:
using ::testing::DefaultValue;
class MockFoo : public Foo {
public:
(Bar, CalculateBar, (), (override));
MOCK_METHOD};
...
;
Bar default_bar// Sets the default return value for type Bar.
<Bar>::Set(default_bar);
DefaultValue
;
MockFoo foo
// We don't need to specify an action here, as the default
// return value works for us.
(foo, CalculateBar());
EXPECT_CALL
.CalculateBar(); // This should return default_bar.
foo
// Unsets the default return value.
<Bar>::Clear(); DefaultValue
Please note that changing the default value for a type can make you
tests hard to understand. We recommend you to use this feature
judiciously. For example, you may want to make sure the
Set()
and Clear()
calls are right next to the
code that uses your mock.
Setting the Default Actions for a Mock Method
You’ve learned how to change the default value of a given type.
However, this may be too coarse for your purpose: perhaps you have two
mock methods with the same return type and you want them to have
different behaviors. The ON_CALL()
macro allows you to
customize your mock’s behavior at the method level:
using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
(foo, Sign(_))
ON_CALL.WillByDefault(Return(-1));
(foo, Sign(0))
ON_CALL.WillByDefault(Return(0));
(foo, Sign(Gt(0)))
ON_CALL.WillByDefault(Return(1));
(foo, Sign(_))
EXPECT_CALL.Times(AnyNumber());
.Sign(5); // This should return 1.
foo.Sign(-9); // This should return -1.
foo.Sign(0); // This should return 0. foo
As you may have guessed, when there are more than one
ON_CALL()
statements, the newer ones in the order take
precedence over the older ones. In other words, the
last one that matches the function arguments will be
used. This matching order allows you to set up the common behavior in a
mock object’s constructor or the test fixture’s set-up phase and
specialize the mock’s behavior later.
Note that both ON_CALL
and EXPECT_CALL
have
the same “later statements take precedence” rule, but they don’t
interact. That is, EXPECT_CALL
s have their own precedence
order distinct from the ON_CALL
precedence order.
Using Functions/Methods/Functors/Lambdas as Actions
If the built-in actions don’t suit you, you can use an existing
callable (function, std::function
, method, functor, lambda
as an action.
using ::testing::_; using ::testing::Invoke;
class MockFoo : public Foo {
public:
(int, Sum, (int x, int y), (override));
MOCK_METHOD(bool, ComplexJob, (int x), (override));
MOCK_METHOD};
int CalculateSum(int x, int y) { return x + y; }
int Sum3(int x, int y, int z) { return x + y + z; }
class Helper {
public:
bool ComplexJob(int x);
};
...
;
MockFoo foo;
Helper helper(foo, Sum(_, _))
EXPECT_CALL.WillOnce(&CalculateSum)
.WillRepeatedly(Invoke(NewPermanentCallback(Sum3, 1)));
(foo, ComplexJob(_))
EXPECT_CALL.WillOnce(Invoke(&helper, &Helper::ComplexJob));
.WillRepeatedly([](int x) { return x > 0; });
.Sum(5, 6); // Invokes CalculateSum(5, 6).
foo.Sum(2, 3); // Invokes Sum3(1, 2, 3).
foo.ComplexJob(10); // Invokes helper.ComplexJob(10).
foo.ComplexJob(-1); // Invokes the inline lambda. foo
The only requirement is that the type of the function, etc must be compatible with the signature of the mock function, meaning that the latter’s arguments can be implicitly converted to the corresponding arguments of the former, and the former’s return type can be implicitly converted to that of the latter. So, you can invoke something whose type is not exactly the same as the mock function, as long as it’s safe to do so - nice, huh?
Note:
The action takes ownership of the callback and will delete it when the action itself is destructed.
If the type of a callback is derived from a base callback type
C
, you need to implicitly cast it toC
to resolve the overloading, e.g.using ::testing::Invoke; ... <bool>* is_ok = ...; ResultCallback... Invoke(is_ok) ...; // This works. * done = new BlockingClosure; BlockingClosure... Invoke(implicit_cast<Closure*>(done)) ...; // The cast is necessary.
Using Functions with Extra Info as Actions
The function or functor you call using Invoke()
must
have the same number of arguments as the mock function you use it for.
Sometimes you may have a function that takes more arguments, and you are
willing to pass in the extra arguments yourself to fill the gap. You can
do this in gMock using callbacks with pre-bound arguments. Here’s an
example:
using ::testing::Invoke;
class MockFoo : public Foo {
public:
(char, DoThis, (int n), (override));
MOCK_METHOD};
char SignOfSum(int x, int y) {
const int sum = x + y;
return (sum > 0) ? '+' : (sum < 0) ? '-' : '0';
}
(FooTest, Test) {
TEST_F;
MockFoo foo
(foo, DoThis(2))
EXPECT_CALL.WillOnce(Invoke(NewPermanentCallback(SignOfSum, 5)));
('+', foo.DoThis(2)); // Invokes SignOfSum(5, 2).
EXPECT_EQ}
Invoking a Function/Method/Functor/Lambda/Callback Without Arguments
Invoke()
is very useful for doing actions that are more
complex. It passes the mock function’s arguments to the function, etc
being invoked such that the callee has the full context of the call to
work with. If the invoked function is not interested in some or all of
the arguments, it can simply ignore them.
Yet, a common pattern is that a test author wants to invoke a
function without the arguments of the mock function.
Invoke()
allows her to do that using a wrapper function
that throws away the arguments before invoking an underlining nullary
function. Needless to say, this can be tedious and obscures the intent
of the test.
InvokeWithoutArgs()
solves this problem. It’s like
Invoke()
except that it doesn’t pass the mock function’s
arguments to the callee. Here’s an example:
using ::testing::_;
using ::testing::InvokeWithoutArgs;
class MockFoo : public Foo {
public:
(bool, ComplexJob, (int n), (override));
MOCK_METHOD};
bool Job1() { ... }
bool Job2(int n, char c) { ... }
...
;
MockFoo foo(foo, ComplexJob(_))
EXPECT_CALL.WillOnce(InvokeWithoutArgs(Job1))
.WillOnce(InvokeWithoutArgs(NewPermanentCallback(Job2, 5, 'a')));
.ComplexJob(10); // Invokes Job1().
foo.ComplexJob(20); // Invokes Job2(5, 'a'). foo
Note:
The action takes ownership of the callback and will delete it when the action itself is destructed.
If the type of a callback is derived from a base callback type
C
, you need to implicitly cast it toC
to resolve the overloading, e.g.using ::testing::InvokeWithoutArgs; ... <bool>* is_ok = ...; ResultCallback... InvokeWithoutArgs(is_ok) ...; // This works. * done = ...; BlockingClosure... InvokeWithoutArgs(implicit_cast<Closure*>(done)) ...; // The cast is necessary.
Invoking an Argument of the Mock Function
Sometimes a mock function will receive a function pointer, a functor (in other words, a “callable”) as an argument, e.g.
class MockFoo : public Foo {
public:
(bool, DoThis, (int n, (ResultCallback1<bool, int>* callback)),
MOCK_METHOD(override));
};
and you may want to invoke this callable argument:
using ::testing::_;
...
;
MockFoo foo(foo, DoThis(_, _))
EXPECT_CALL.WillOnce(...);
// Will execute callback->Run(5), where callback is the
// second argument DoThis() receives.
NOTE: The section below is legacy documentation from before C++ had lambdas:
Arghh, you need to refer to a mock function argument but C++ has no lambda (yet), so you have to define your own action. :-( Or do you really?
Well, gMock has an action to solve exactly this problem:
<N>(arg_1, arg_2, ..., arg_m) InvokeArgument
will invoke the N
-th (0-based) argument the mock
function receives, with arg_1
, arg_2
, …, and
arg_m
. No matter if the argument is a function pointer, a
functor, or a callback. gMock handles them all.
With that, you could write:
using ::testing::_;
using ::testing::InvokeArgument;
...
(foo, DoThis(_, _))
EXPECT_CALL.WillOnce(InvokeArgument<1>(5));
// Will execute callback->Run(5), where callback is the
// second argument DoThis() receives.
What if the callable takes an argument by reference? No problem -
just wrap it inside ByRef()
:
...
(bool, Bar,
MOCK_METHOD((ResultCallback2<bool, int, const Helper&>* callback)),
(override));
...
using ::testing::_;
using ::testing::ByRef;
using ::testing::InvokeArgument;
...
;
MockFoo foo;
Helper helper...
(foo, Bar(_))
EXPECT_CALL.WillOnce(InvokeArgument<0>(5, ByRef(helper)));
// ByRef(helper) guarantees that a reference to helper, not a copy of it,
// will be passed to the callback.
What if the callable takes an argument by reference and we do
not wrap the argument in ByRef()
? Then
InvokeArgument()
will make a copy of the argument,
and pass a reference to the copy, instead of a reference to the
original value, to the callable. This is especially handy when the
argument is a temporary value:
...
(bool, DoThat, (bool (*f)(const double& x, const string& s)),
MOCK_METHOD(override));
...
using ::testing::_;
using ::testing::InvokeArgument;
...
;
MockFoo foo...
(foo, DoThat(_))
EXPECT_CALL.WillOnce(InvokeArgument<0>(5.0, string("Hi")));
// Will execute (*f)(5.0, string("Hi")), where f is the function pointer
// DoThat() receives. Note that the values 5.0 and string("Hi") are
// temporary and dead once the EXPECT_CALL() statement finishes. Yet
// it's fine to perform this action later, since a copy of the values
// are kept inside the InvokeArgument action.
Ignoring an Action’s Result
Sometimes you have an action that returns something, but you
need an action that returns void
(perhaps you want to use
it in a mock function that returns void
, or perhaps it
needs to be used in DoAll()
and it’s not the last in the
list). IgnoreResult()
lets you do that. For example:
using ::testing::_;
using ::testing::DoAll;
using ::testing::IgnoreResult;
using ::testing::Return;
int Process(const MyData& data);
();
string DoSomething
class MockFoo : public Foo {
public:
(void, Abc, (const MyData& data), (override));
MOCK_METHOD(bool, Xyz, (), (override));
MOCK_METHOD};
...
;
MockFoo foo(foo, Abc(_))
EXPECT_CALL// .WillOnce(Invoke(Process));
// The above line won't compile as Process() returns int but Abc() needs
// to return void.
.WillOnce(IgnoreResult(Process));
(foo, Xyz())
EXPECT_CALL.WillOnce(DoAll(IgnoreResult(DoSomething),
// Ignores the string DoSomething() returns.
(true))); Return
Note that you cannot use IgnoreResult()
on an action that already returns void
. Doing so will lead
to ugly compiler errors.
Selecting an Action’s Arguments
Say you have a mock function Foo()
that takes seven
arguments, and you have a custom action that you want to invoke when
Foo()
is called. Trouble is, the custom action only wants
three arguments:
using ::testing::_;
using ::testing::Invoke;
...
(bool, Foo,
MOCK_METHOD(bool visible, const string& name, int x, int y,
(const map<pair<int, int>>), double& weight, double min_weight,
double max_wight));
...
bool IsVisibleInQuadrant1(bool visible, int x, int y) {
return visible && x >= 0 && y >= 0;
}
...
(mock, Foo)
EXPECT_CALL.WillOnce(Invoke(IsVisibleInQuadrant1)); // Uh, won't compile. :-(
To please the compiler God, you need to define an “adaptor” that has
the same signature as Foo()
and calls the custom action
with the right arguments:
using ::testing::_;
using ::testing::Invoke;
...
bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
const map<pair<int, int>, double>& weight,
double min_weight, double max_wight) {
return IsVisibleInQuadrant1(visible, x, y);
}
...
(mock, Foo)
EXPECT_CALL.WillOnce(Invoke(MyIsVisibleInQuadrant1)); // Now it works.
But isn’t this awkward?
gMock provides a generic action adaptor, so you can spend your time minding more important business than writing your own adaptors. Here’s the syntax:
<N1, N2, ..., Nk>(action) WithArgs
creates an action that passes the arguments of the mock function at
the given indices (0-based) to the inner action
and
performs it. Using WithArgs
, our original example can be
written as:
using ::testing::_;
using ::testing::Invoke;
using ::testing::WithArgs;
...
(mock, Foo)
EXPECT_CALL.WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1))); // No need to define your own adaptor.
For better readability, gMock also gives you:
WithoutArgs(action)
when the inneraction
takes no argument, andWithArg<N>(action)
(nos
afterArg
) when the inneraction
takes one argument.
As you may have realized, InvokeWithoutArgs(...)
is just
syntactic sugar for WithoutArgs(Invoke(...))
.
Here are more tips:
- The inner action used in
WithArgs
and friends does not have to beInvoke()
– it can be anything. - You can repeat an argument in the argument list if necessary, e.g.
WithArgs<2, 3, 3, 5>(...)
. - You can change the order of the arguments,
e.g.
WithArgs<3, 2, 1>(...)
. - The types of the selected arguments do not have to match
the signature of the inner action exactly. It works as long as they can
be implicitly converted to the corresponding arguments of the inner
action. For example, if the 4-th argument of the mock function is an
int
andmy_action
takes adouble
,WithArg<4>(my_action)
will work.
Ignoring Arguments in Action Functions
The selecting-an-action’s-arguments
recipe showed us one way to make a mock function and an action with
incompatible argument lists fit together. The downside is that wrapping
the action in WithArgs<...>()
can get tedious for
people writing the tests.
If you are defining a function (or method, functor, lambda, callback)
to be used with Invoke*()
, and you are not interested in
some of its arguments, an alternative to WithArgs
is to
declare the uninteresting arguments as Unused
. This makes
the definition less cluttered and less fragile in case the types of the
uninteresting arguments change. It could also increase the chance the
action function can be reused. For example, given
public:
(double, Foo, double(const string& label, double x, double y),
MOCK_METHOD(override));
(double, Bar, (int index, double x, double y), (override)); MOCK_METHOD
instead of
using ::testing::_;
using ::testing::Invoke;
double DistanceToOriginWithLabel(const string& label, double x, double y) {
return sqrt(x*x + y*y);
}
double DistanceToOriginWithIndex(int index, double x, double y) {
return sqrt(x*x + y*y);
}
...
(mock, Foo("abc", _, _))
EXPECT_CALL.WillOnce(Invoke(DistanceToOriginWithLabel));
(mock, Bar(5, _, _))
EXPECT_CALL.WillOnce(Invoke(DistanceToOriginWithIndex));
you could write
using ::testing::_;
using ::testing::Invoke;
using ::testing::Unused;
double DistanceToOrigin(Unused, double x, double y) {
return sqrt(x*x + y*y);
}
...
(mock, Foo("abc", _, _))
EXPECT_CALL.WillOnce(Invoke(DistanceToOrigin));
(mock, Bar(5, _, _))
EXPECT_CALL.WillOnce(Invoke(DistanceToOrigin));
Sharing Actions
Just like matchers, a gMock action object consists of a pointer to a ref-counted implementation object. Therefore copying actions is also allowed and very efficient. When the last action that references the implementation object dies, the implementation object will be deleted.
If you have some complex action that you want to use again and again, you may not have to build it from scratch everytime. If the action doesn’t have an internal state (i.e. if it always does the same thing no matter how many times it has been called), you can assign it to an action variable and use that variable repeatedly. For example:
using ::testing::Action;
using ::testing::DoAll;
using ::testing::Return;
using ::testing::SetArgPointee;
...
<bool(int*)> set_flag = DoAll(SetArgPointee<0>(5),
Action(true));
Return... use set_flag in .WillOnce() and .WillRepeatedly() ...
However, if the action has its own state, you may be surprised if you
share the action object. Suppose you have an action factory
IncrementCounter(init)
which creates an action that
increments and returns a counter whose initial value is
init
, using two actions created from the same expression
and using a shared action will exhibit different behaviors. Example:
(foo, DoThis())
EXPECT_CALL.WillRepeatedly(IncrementCounter(0));
(foo, DoThat())
EXPECT_CALL.WillRepeatedly(IncrementCounter(0));
.DoThis(); // Returns 1.
foo.DoThis(); // Returns 2.
foo.DoThat(); // Returns 1 - Blah() uses a different
foo// counter than Bar()'s.
versus
using ::testing::Action;
...
<int()> increment = IncrementCounter(0);
Action(foo, DoThis())
EXPECT_CALL.WillRepeatedly(increment);
(foo, DoThat())
EXPECT_CALL.WillRepeatedly(increment);
.DoThis(); // Returns 1.
foo.DoThis(); // Returns 2.
foo.DoThat(); // Returns 3 - the counter is shared. foo
Testing Asynchronous Behavior
One oft-encountered problem with gMock is that it can be hard to test
asynchronous behavior. Suppose you had a EventQueue
class
that you wanted to test, and you created a separate
EventDispatcher
interface so that you could easily mock it
out. However, the implementation of the class fired all the events on a
background thread, which made test timings difficult. You could just
insert sleep()
statements and hope for the best, but that
makes your test behavior nondeterministic. A better way is to use gMock
actions and Notification
objects to force your asynchronous
test to behave synchronously.
using ::testing::DoAll;
using ::testing::InvokeWithoutArgs;
using ::testing::Return;
class MockEventDispatcher : public EventDispatcher {
(bool, DispatchEvent, (int32), (override));
MOCK_METHOD};
(Notify, notification) {
ACTION_P->Notify();
notification}
(EventQueueTest, EnqueueEventTest) {
TEST;
MockEventDispatcher mock_event_dispatcher(&mock_event_dispatcher);
EventQueue event_queue
const int32 kEventId = 321;
;
Notification done(mock_event_dispatcher, DispatchEvent(kEventId))
EXPECT_CALL.WillOnce(Notify(&done));
.EnqueueEvent(kEventId);
event_queue.WaitForNotification();
done}
In the example above, we set our normal gMock expectations, but then
add an additional action to notify the Notification
object.
Now we can just call Notification::WaitForNotification()
in
the main thread to wait for the asynchronous call to finish. After that,
our test suite is complete and we can safely exit.
Note: this example has a downside: namely, if the expectation is not
satisfied, our test will run forever. It will eventually time-out and
fail, but it will take longer and be slightly harder to debug. To
alleviate this problem, you can use
WaitForNotificationWithTimeout(ms)
instead of
WaitForNotification()
.
Misc Recipes on Using gMock
Mocking Methods That Use Move-Only Types
C++11 introduced move-only types. A move-only-typed value
can be moved from one object to another, but cannot be copied.
std::unique_ptr<T>
is probably the most commonly used
move-only type.
Mocking a method that takes and/or returns move-only types presents some challenges, but nothing insurmountable. This recipe shows you how you can do it. Note that the support for move-only method arguments was only introduced to gMock in April 2017; in older code, you may find more complex workarounds for lack of this feature.
Let’s say we are working on a fictional project that lets one post and share snippets called “buzzes”. Your code uses these types:
enum class AccessLevel { kInternal, kPublic };
class Buzz {
public:
explicit Buzz(AccessLevel access) { ... }
...
};
class Buzzer {
public:
virtual ~Buzzer() {}
virtual std::unique_ptr<Buzz> MakeBuzz(StringPiece text) = 0;
virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, int64_t timestamp) = 0;
...
};
A Buzz
object represents a snippet being posted. A class
that implements the Buzzer
interface is capable of creating
and sharing Buzz
es. Methods in Buzzer
may
return a unique_ptr<Buzz>
or take a
unique_ptr<Buzz>
. Now we need to mock
Buzzer
in our tests.
To mock a method that accepts or returns move-only types, you just
use the familiar MOCK_METHOD
syntax as usual:
class MockBuzzer : public Buzzer {
public:
(std::unique_ptr<Buzz>, MakeBuzz, (StringPiece text), (override));
MOCK_METHOD(bool, ShareBuzz, (std::unique_ptr<Buzz> buzz, int64_t timestamp),
MOCK_METHOD(override));
};
Now that we have the mock class defined, we can use it in tests. In
the following code examples, we assume that we have defined a
MockBuzzer
object named mock_buzzer_
:
mock_buzzer_; MockBuzzer
First let’s see how we can set expectations on the
MakeBuzz()
method, which returns a
unique_ptr<Buzz>
.
As usual, if you set an expectation without an action (i.e. the
.WillOnce()
or .WillRepeatedly()
clause), when
that expectation fires, the default action for that method will be
taken. Since unique_ptr<>
has a default constructor
that returns a null unique_ptr
, that’s what you’ll get if
you don’t specify an action:
// Use the default action.
(mock_buzzer_, MakeBuzz("hello"));
EXPECT_CALL
// Triggers the previous EXPECT_CALL.
(nullptr, mock_buzzer_.MakeBuzz("hello")); EXPECT_EQ
If you are not happy with the default action, you can tweak it as usual; see Setting Default Actions.
If you just need to return a pre-defined move-only value, you can use
the Return(ByMove(...))
action:
// When this fires, the unique_ptr<> specified by ByMove(...) will
// be returned.
(mock_buzzer_, MakeBuzz("world"))
EXPECT_CALL.WillOnce(Return(ByMove(MakeUnique<Buzz>(AccessLevel::kInternal))));
(nullptr, mock_buzzer_.MakeBuzz("world")); EXPECT_NE
Note that ByMove()
is essential here - if you drop it,
the code won’t compile.
Quiz time! What do you think will happen if a
Return(ByMove(...))
action is performed more than once
(e.g. you write ... .WillRepeatedly(Return(ByMove(...)));
)?
Come think of it, after the first time the action runs, the source value
will be consumed (since it’s a move-only value), so the next time
around, there’s no value to move from – you’ll get a run-time error that
Return(ByMove(...))
can only be run once.
If you need your mock method to do more than just moving a pre-defined value, remember that you can always use a lambda or a callable object, which can do pretty much anything you want:
(mock_buzzer_, MakeBuzz("x"))
EXPECT_CALL.WillRepeatedly([](StringPiece text) {
return MakeUnique<Buzz>(AccessLevel::kInternal);
});
(nullptr, mock_buzzer_.MakeBuzz("x"));
EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x")); EXPECT_NE
Every time this EXPECT_CALL
fires, a new
unique_ptr<Buzz>
will be created and returned. You
cannot do this with Return(ByMove(...))
.
That covers returning move-only values; but how do we work with
methods accepting move-only arguments? The answer is that they work
normally, although some actions will not compile when any of method’s
arguments are move-only. You can always use Return
, or a lambda or functor:
using ::testing::Unused;
(mock_buzzer_, ShareBuzz(NotNull(), _)).WillOnce(Return(true));
EXPECT_CALL(mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal)),
EXPECT_TRUE0);
(mock_buzzer_, ShareBuzz(_, _)).WillOnce(
EXPECT_CALL[](std::unique_ptr<Buzz> buzz, Unused) { return buzz != nullptr; });
(mock_buzzer_.ShareBuzz(nullptr, 0)); EXPECT_FALSE
Many built-in actions (WithArgs
,
WithoutArgs
,DeleteArg
, SaveArg
,
…) could in principle support move-only arguments, but the support for
this is not implemented yet. If this is blocking you, please file a
bug.
A few actions (e.g. DoAll
) copy their arguments
internally, so they can never work with non-copyable objects; you’ll
have to use functors instead.
Legacy workarounds for move-only types
Support for move-only function arguments was only introduced to gMock in April 2017. In older code, you may encounter the following workaround for the lack of this feature (it is no longer necessary - we’re including it just for reference):
class MockBuzzer : public Buzzer {
public:
(bool, DoShareBuzz, (Buzz* buzz, Time timestamp));
MOCK_METHODbool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) override {
return DoShareBuzz(buzz.get(), timestamp);
}
};
The trick is to delegate the ShareBuzz()
method to a
mock method (let’s call it DoShareBuzz()
) that does not
take move-only parameters. Then, instead of setting expectations on
ShareBuzz()
, you set them on the DoShareBuzz()
mock method:
mock_buzzer_;
MockBuzzer (mock_buzzer_, DoShareBuzz(NotNull(), _));
EXPECT_CALL
// When one calls ShareBuzz() on the MockBuzzer like this, the call is
// forwarded to DoShareBuzz(), which is mocked. Therefore this statement
// will trigger the above EXPECT_CALL.
mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal), 0);
Making the Compilation Faster
Believe it or not, the vast majority of the time spent on compiling a mock class is in generating its constructor and destructor, as they perform non-trivial tasks (e.g. verification of the expectations). What’s more, mock methods with different signatures have different types and thus their constructors/destructors need to be generated by the compiler separately. As a result, if you mock many different types of methods, compiling your mock class can get really slow.
If you are experiencing slow compilation, you can move the definition
of your mock class’ constructor and destructor out of the class body and
into a .cc
file. This way, even if you
#include
your mock class in N files, the compiler only
needs to generate its constructor and destructor once, resulting in a
much faster compilation.
Let’s illustrate the idea using an example. Here’s the definition of a mock class before applying this recipe:
// File mock_foo.h.
...
class MockFoo : public Foo {
public:
// Since we don't declare the constructor or the destructor,
// the compiler will generate them in every translation unit
// where this mock class is used.
(int, DoThis, (), (override));
MOCK_METHOD(bool, DoThat, (const char* str), (override));
MOCK_METHOD... more mock methods ...
};
After the change, it would look like:
// File mock_foo.h.
...
class MockFoo : public Foo {
public:
// The constructor and destructor are declared, but not defined, here.
();
MockFoovirtual ~MockFoo();
(int, DoThis, (), (override));
MOCK_METHOD(bool, DoThat, (const char* str), (override));
MOCK_METHOD... more mock methods ...
};
and
// File mock_foo.cc.
#include "path/to/mock_foo.h"
// The definitions may appear trivial, but the functions actually do a
// lot of things through the constructors/destructors of the member
// variables used to implement the mock methods.
::MockFoo() {}
MockFoo::~MockFoo() {} MockFoo
Forcing a Verification
When it’s being destroyed, your friendly mock object will automatically verify that all expectations on it have been satisfied, and will generate googletest failures if not. This is convenient as it leaves you with one less thing to worry about. That is, unless you are not sure if your mock object will be destroyed.
How could it be that your mock object won’t eventually be destroyed? Well, it might be created on the heap and owned by the code you are testing. Suppose there’s a bug in that code and it doesn’t delete the mock object properly - you could end up with a passing test when there’s actually a bug.
Using a heap checker is a good idea and can alleviate the concern,
but its implementation is not 100% reliable. So, sometimes you do want
to force gMock to verify a mock object before it is (hopefully)
destructed. You can do this with
Mock::VerifyAndClearExpectations(&mock_object)
:
(MyServerTest, ProcessesRequest) {
TESTusing ::testing::Mock;
* const foo = new MockFoo;
MockFoo(*foo, ...)...;
EXPECT_CALL// ... other expectations ...
// server now owns foo.
(foo);
MyServer server.ProcessRequest(...);
server
// In case that server's destructor will forget to delete foo,
// this will verify the expectations anyway.
::VerifyAndClearExpectations(foo);
Mock} // server is destroyed when it goes out of scope here.
Tip: The
Mock::VerifyAndClearExpectations()
function returns a
bool
to indicate whether the verification was successful
(true
for yes), so you can wrap that function call inside a
ASSERT_TRUE()
if there is no point going further when the
verification has failed.
Using Check Points
Sometimes you may want to “reset” a mock object at various check points in your test: at each check point, you verify that all existing expectations on the mock object have been satisfied, and then you set some new expectations on it as if it’s newly created. This allows you to work with a mock object in “phases” whose sizes are each manageable.
One such scenario is that in your test’s SetUp()
function, you may want to put the object you are testing into a certain
state, with the help from a mock object. Once in the desired state, you
want to clear all expectations on the mock, such that in the
TEST_F
body you can set fresh expectations on it.
As you may have figured out, the
Mock::VerifyAndClearExpectations()
function we saw in the
previous recipe can help you here. Or, if you are using
ON_CALL()
to set default actions on the mock object and
want to clear the default actions as well, use
Mock::VerifyAndClear(&mock_object)
instead. This
function does what
Mock::VerifyAndClearExpectations(&mock_object)
does and
returns the same bool
, plus it clears the
ON_CALL()
statements on mock_object
too.
Another trick you can use to achieve the same effect is to put the expectations in sequences and insert calls to a dummy “check-point” function at specific places. Then you can verify that the mock function calls do happen at the right time. For example, if you are exercising code:
(1);
Foo(2);
Foo(3); Foo
and want to verify that Foo(1)
and Foo(3)
both invoke mock.Bar("a")
, but Foo(2)
doesn’t
invoke anything. You can write:
using ::testing::MockFunction;
(FooTest, InvokesBarCorrectly) {
TEST;
MyMock mock// Class MockFunction<F> has exactly one mock method. It is named
// Call() and has type F.
<void(string check_point_name)> check;
MockFunction{
;
InSequence s
(mock, Bar("a"));
EXPECT_CALL(check, Call("1"));
EXPECT_CALL(check, Call("2"));
EXPECT_CALL(mock, Bar("a"));
EXPECT_CALL}
(1);
Foo.Call("1");
check(2);
Foo.Call("2");
check(3);
Foo}
The expectation spec says that the first Bar("a")
must
happen before check point “1”, the second Bar("a")
must
happen after check point “2”, and nothing should happen between the two
check points. The explicit check points make it easy to tell which
Bar("a")
is called by which call to Foo()
.
Mocking Destructors
Sometimes you want to make sure a mock object is destructed at the
right time, e.g. after bar->A()
is called but before
bar->B()
is called. We already know that you can specify
constraints on the order of mock function
calls, so all we need to do is to mock the destructor of the mock
function.
This sounds simple, except for one problem: a destructor is a special
function with special syntax and special semantics, and the
MOCK_METHOD
macro doesn’t work for it:
(void, ~MockFoo, ()); // Won't compile! MOCK_METHOD
The good news is that you can use a simple pattern to achieve the
same effect. First, add a mock function Die()
to your mock
class and call it in the destructor, like this:
class MockFoo : public Foo {
...
// Add the following two lines to the mock class.
(void, Die, ());
MOCK_METHODvirtual ~MockFoo() { Die(); }
};
(If the name Die()
clashes with an existing symbol,
choose another name.) Now, we have translated the problem of testing
when a MockFoo
object dies to testing when its
Die()
method is called:
* foo = new MockFoo;
MockFoo* bar = new MockBar;
MockBar...
{
;
InSequence s
// Expects *foo to die after bar->A() and before bar->B().
(*bar, A());
EXPECT_CALL(*foo, Die());
EXPECT_CALL(*bar, B());
EXPECT_CALL}
And that’s that.
Using gMock and Threads
In a unit test, it’s best if you could isolate and test a piece of code in a single-threaded context. That avoids race conditions and dead locks, and makes debugging your test much easier.
Yet most programs are multi-threaded, and sometimes to test something we need to pound on it from more than one thread. gMock works for this purpose too.
Remember the steps for using a mock:
- Create a mock object
foo
. - Set its default actions and expectations using
ON_CALL()
andEXPECT_CALL()
. - The code under test calls methods of
foo
. - Optionally, verify and reset the mock.
- Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.
If you follow the following simple rules, your mocks and threads can live happily together:
- Execute your test code (as opposed to the code being tested) in one thread. This makes your test easy to follow.
- Obviously, you can do step #1 without locking.
- When doing step #2 and #5, make sure no other thread is accessing
foo
. Obvious too, huh? - #3 and #4 can be done either in one thread or in multiple threads - anyway you want. gMock takes care of the locking, so you don’t have to do any - unless required by your test logic.
If you violate the rules (for example, if you set expectations on a mock while another thread is calling its methods), you get undefined behavior. That’s not fun, so don’t do it.
gMock guarantees that the action for a mock function is done in the same thread that called the mock function. For example, in
(mock, Foo(1))
EXPECT_CALL.WillOnce(action1);
(mock, Foo(2))
EXPECT_CALL.WillOnce(action2);
if Foo(1)
is called in thread 1 and Foo(2)
is called in thread 2, gMock will execute action1
in thread
1 and action2
in thread 2.
gMock does not impose a sequence on actions performed in
different threads (doing so may create deadlocks as the actions may need
to cooperate). This means that the execution of action1
and
action2
in the above example may interleave. If
this is a problem, you should add proper synchronization logic to
action1
and action2
to make the test
thread-safe.
Also, remember that DefaultValue<T>
is a global
resource that potentially affects all living mock objects in
your program. Naturally, you won’t want to mess with it from multiple
threads or when there still are mocks in action.
Controlling How Much Information gMock Prints
When gMock sees something that has the potential of being an error (e.g. a mock function with no expectation is called, a.k.a. an uninteresting call, which is allowed but perhaps you forgot to explicitly ban the call), it prints some warning messages, including the arguments of the function, the return value, and the stack trace. Hopefully this will remind you to take a look and see if there is indeed a problem.
Sometimes you are confident that your tests are correct and may not appreciate such friendly messages. Some other times, you are debugging your tests or learning about the behavior of the code you are testing, and wish you could observe every mock call that happens (including argument values, the return value, and the stack trace). Clearly, one size doesn’t fit all.
You can control how much gMock tells you using the
--gmock_verbose=LEVEL
command-line flag, where
LEVEL
is a string with three possible values:
info
: gMock will print all informational messages, warnings, and errors (most verbose). At this setting, gMock will also log any calls to theON_CALL/EXPECT_CALL
macros. It will include a stack trace in “uninteresting call” warnings.warning
: gMock will print both warnings and errors (less verbose); it will omit the stack traces in “uninteresting call” warnings. This is the default.error
: gMock will print errors only (least verbose).
Alternatively, you can adjust the value of that flag from within your tests like so:
::testing::FLAGS_gmock_verbose = "error";
If you find gMock printing too many stack frames with its
informational or warning messages, remember that you can control their
amount with the --gtest_stack_trace_depth=max_depth
flag.
Now, judiciously use the right flag to enable gMock serve you better!
Gaining Super Vision into Mock Calls
You have a test using gMock. It fails: gMock tells you some
expectations aren’t satisfied. However, you aren’t sure why: Is there a
typo somewhere in the matchers? Did you mess up the order of the
EXPECT_CALL
s? Or is the code under test doing something
wrong? How can you find out the cause?
Won’t it be nice if you have X-ray vision and can actually see the
trace of all EXPECT_CALL
s and mock method calls as they are
made? For each call, would you like to see its actual argument values
and which EXPECT_CALL
gMock thinks it matches? If you still
need some help to figure out who made these calls, how about being able
to see the complete stack trace at each mock call?
You can unlock this power by running your test with the
--gmock_verbose=info
flag. For example, given the test
program:
#include "gmock/gmock.h"
using testing::_;
using testing::HasSubstr;
using testing::Return;
class MockFoo {
public:
(void, F, (const string& x, const string& y));
MOCK_METHOD};
(Foo, Bar) {
TEST;
MockFoo mock(mock, F(_, _)).WillRepeatedly(Return());
EXPECT_CALL(mock, F("a", "b"));
EXPECT_CALL(mock, F("c", HasSubstr("d")));
EXPECT_CALL
.F("a", "good");
mock.F("a", "b");
mock}
if you run it with --gmock_verbose=info
, you will see
this output:
[ RUN ] Foo.Bar
foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
Stack trace: ...
foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
Stack trace: ...
foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
Stack trace: ...
foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
Function call: F(@0x7fff7c8dad40"a",@0x7fff7c8dad10"good")
Stack trace: ...
foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
Function call: F(@0x7fff7c8dada0"a",@0x7fff7c8dad70"b")
Stack trace: ...
foo_test.cc:16: Failure
Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
Expected: to be called once
Actual: never called - unsatisfied and active
[ FAILED ] Foo.Bar
Suppose the bug is that the "c"
in the third
EXPECT_CALL
is a typo and should actually be
"a"
. With the above message, you should see that the actual
F("a", "good")
call is matched by the first
EXPECT_CALL
, not the third as you thought. From that it
should be obvious that the third EXPECT_CALL
is written
wrong. Case solved.
If you are interested in the mock call trace but not the stack
traces, you can combine --gmock_verbose=info
with
--gtest_stack_trace_depth=0
on the test command line.
Running Tests in Emacs
If you build and run your tests in Emacs using the
M-x google-compile
command (as many googletest users do),
the source file locations of gMock and googletest errors will be
highlighted. Just press <Enter>
on one of them and
you’ll be taken to the offending line. Or, you can just type `C-x`` to
jump to the next error.
To make it even easier, you can add the following lines to your
~/.emacs
file:
(global-set-key "\M-m" 'google-compile) ; m is for make
(global-set-key [M-down] 'next-error)
(global-set-key [M-up] '(lambda () (interactive) (next-error -1)))
Then you can type M-m
to start a build (if you want to
run the test as well, just make sure foo_test.run
or
runtests
is in the build command you supply after typing
M-m
), or M-up
/M-down
to move back
and forth between errors.
Extending gMock
Writing New Matchers Quickly
WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be functionally pure. See this section for more details.
The MATCHER*
family of macros can be used to define
custom matchers easily. The syntax:
(name, description_string_expression) { statements; } MATCHER
will define a matcher with the given name that executes the
statements, which must return a bool
to indicate if the
match succeeds. Inside the statements, you can refer to the value being
matched by arg
, and refer to its type by
arg_type
.
The description string is a string
-typed
expression that documents what the matcher does, and is used to generate
the failure message when the match fails. It can (and should) reference
the special bool
variable negation
, and should
evaluate to the description of the matcher when negation
is
false
, or that of the matcher’s negation when
negation
is true
.
For convenience, we allow the description string to be empty
(""
), in which case gMock will use the sequence of words in
the matcher name as the description.
For example:
(IsDivisibleBy7, "") { return (arg % 7) == 0; } MATCHER
allows you to write
// Expects mock_foo.Bar(n) to be called where n is divisible by 7.
(mock_foo, Bar(IsDivisibleBy7())); EXPECT_CALL
or,
using ::testing::Not;
...
// Verifies that two values are divisible by 7.
(some_expression, IsDivisibleBy7());
EXPECT_THAT(some_other_expression, Not(IsDivisibleBy7())); EXPECT_THAT
If the above assertions fail, they will print something like:
Value of: some_expression
Expected: is divisible by 7
Actual: 27
...
Value of: some_other_expression
Expected: not (is divisible by 7)
Actual: 21
where the descriptions "is divisible by 7"
and
"not (is divisible by 7)"
are automatically calculated from
the matcher name IsDivisibleBy7
.
As you may have noticed, the auto-generated descriptions (especially
those for the negation) may not be so great. You can always override
them with a string
expression of your own:
(IsDivisibleBy7,
MATCHER::StrCat(negation ? "isn't" : "is", " divisible by 7")) {
abslreturn (arg % 7) == 0;
}
Optionally, you can stream additional information to a hidden
argument named result_listener
to explain the match result.
For example, a better definition of IsDivisibleBy7
is:
(IsDivisibleBy7, "") {
MATCHERif ((arg % 7) == 0)
return true;
*result_listener << "the remainder is " << (arg % 7);
return false;
}
With this definition, the above assertion will give a better message:
Value of: some_expression
Expected: is divisible by 7
Actual: 27 (the remainder is 6)
You should let MatchAndExplain()
print any
additional information that can help a user understand the match
result. Note that it should explain why the match succeeds in case of a
success (unless it’s obvious) - this is useful when the matcher is used
inside Not()
. There is no need to print the argument value
itself, as gMock already prints it for you.
NOTE: The type of the value being matched (arg_type
) is
determined by the context in which you use the matcher and is supplied
to you by the compiler, so you don’t need to worry about declaring it
(nor can you). This allows the matcher to be polymorphic. For example,
IsDivisibleBy7()
can be used to match any type where the
value of (arg % 7) == 0
can be implicitly converted to a
bool
. In the Bar(IsDivisibleBy7())
example
above, if method Bar()
takes an int
,
arg_type
will be int
; if it takes an
unsigned long
, arg_type
will be
unsigned long
; and so on.
Writing New Parameterized Matchers Quickly
Sometimes you’ll want to define a matcher that has parameters. For that you can use the macro:
(name, param_name, description_string) { statements; } MATCHER_P
where the description string can be either ""
or a
string
expression that references negation
and
param_name
.
For example:
(HasAbsoluteValue, value, "") { return abs(arg) == value; } MATCHER_P
will allow you to write:
(Blah("a"), HasAbsoluteValue(n)); EXPECT_THAT
which may lead to this message (assuming n
is 10):
Value of: Blah("a")
Expected: has absolute value 10
Actual: -9
Note that both the matcher description and its parameter are printed, making the message human-friendly.
In the matcher definition body, you can write foo_type
to reference the type of a parameter named foo
. For
example, in the body of MATCHER_P(HasAbsoluteValue, value)
above, you can write value_type
to refer to the type of
value
.
gMock also provides MATCHER_P2
, MATCHER_P3
,
…, up to MATCHER_P10
to support multi-parameter
matchers:
(name, param_1, ..., param_k, description_string) { statements; } MATCHER_Pk
Please note that the custom description string is for a particular instance of the matcher, where the parameters have been bound to actual values. Therefore usually you’ll want the parameter values to be part of the description. gMock lets you do that by referencing the matcher parameters in the description string expression.
For example,
using ::testing::PrintToString;
(InClosedRange, low, hi,
MATCHER_P2::StrFormat("%s in range [%s, %s]", negation ? "isn't" : "is",
absl(low), PrintToString(hi))) {
PrintToStringreturn low <= arg && arg <= hi;
}
...
(3, InClosedRange(4, 6)); EXPECT_THAT
would generate a failure that contains the message:
Expected: is in range [4, 6]
If you specify ""
as the description, the failure
message will contain the sequence of words in the matcher name followed
by the parameter values printed as a tuple. For example,
(InClosedRange, low, hi, "") { ... }
MATCHER_P2...
(3, InClosedRange(4, 6)); EXPECT_THAT
would generate a failure that contains the text:
Expected: in closed range (4, 6)
For the purpose of typing, you can view
(Foo, p1, ..., pk, description_string) { ... } MATCHER_Pk
as shorthand for
template <typename p1_type, ..., typename pk_type>
<p1_type, ..., pk_type>
FooMatcherPk(p1_type p1, ..., pk_type pk) { ... } Foo
When you write Foo(v1, ..., vk)
, the compiler infers the
types of the parameters v1
, …, and vk
for you.
If you are not happy with the result of the type inference, you can
specify the types by explicitly instantiating the template, as in
Foo<long, bool>(5, false)
. As said earlier, you don’t
get to (or need to) specify arg_type
as that’s determined
by the context in which the matcher is used.
You can assign the result of expression Foo(p1, ..., pk)
to a variable of type
FooMatcherPk<p1_type, ..., pk_type>
. This can be
useful when composing matchers. Matchers that don’t have a parameter or
have only one parameter have special types: you can assign
Foo()
to a FooMatcher
-typed variable, and
assign Foo(p)
to a
FooMatcherP<p_type>
-typed variable.
While you can instantiate a matcher template with reference types, passing the parameters by pointer usually makes your code more readable. If, however, you still want to pass a parameter by reference, be aware that in the failure message generated by the matcher you will see the value of the referenced object but not its address.
You can overload matchers with different numbers of parameters:
(Blah, a, description_string_1) { ... }
MATCHER_P(Blah, a, b, description_string_2) { ... } MATCHER_P2
While it’s tempting to always use the MATCHER*
macros
when defining a new matcher, you should also consider implementing
MatcherInterface
or using
MakePolymorphicMatcher()
instead (see the recipes that
follow), especially if you need to use the matcher a lot. While these
approaches require more work, they give you more control on the types of
the value being matched and the matcher parameters, which in general
leads to better compiler error messages that pay off in the long run.
They also allow overloading matchers based on parameter types (as
opposed to just based on the number of parameters).
Writing New Monomorphic Matchers
A matcher of argument type T
implements
::testing::MatcherInterface<T>
and does two things:
it tests whether a value of type T
matches the matcher, and
can describe what kind of values it matches. The latter ability is used
for generating readable error messages when expectations are
violated.
The interface looks like this:
class MatchResultListener {
public:
...
// Streams x to the underlying ostream; does nothing if the ostream
// is NULL.
template <typename T>
& operator<<(const T& x);
MatchResultListener
// Returns the underlying ostream.
::std::ostream* stream();
};
template <typename T>
class MatcherInterface {
public:
virtual ~MatcherInterface();
// Returns true if the matcher matches x; also explains the match
// result to 'listener'.
virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;
// Describes this matcher to an ostream.
virtual void DescribeTo(::std::ostream* os) const = 0;
// Describes the negation of this matcher to an ostream.
virtual void DescribeNegationTo(::std::ostream* os) const;
};
If you need a custom matcher but Truly()
is not a good
option (for example, you may not be happy with the way
Truly(predicate)
describes itself, or you may want your
matcher to be polymorphic as Eq(value)
is), you can define
a matcher to do whatever you want in two steps: first implement the
matcher interface, and then define a factory function to create a
matcher instance. The second step is not strictly needed but it makes
the syntax of using the matcher nicer.
For example, you can define a matcher to test whether an
int
is divisible by 7 and then use it like this:
using ::testing::MakeMatcher;
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;
class DivisibleBy7Matcher : public MatcherInterface<int> {
public:
bool MatchAndExplain(int n,
* /* listener */) const override {
MatchResultListenerreturn (n % 7) == 0;
}
void DescribeTo(::std::ostream* os) const override {
*os << "is divisible by 7";
}
void DescribeNegationTo(::std::ostream* os) const override {
*os << "is not divisible by 7";
}
};
<int> DivisibleBy7() {
Matcherreturn MakeMatcher(new DivisibleBy7Matcher);
}
...
(foo, Bar(DivisibleBy7())); EXPECT_CALL
You may improve the matcher message by streaming additional
information to the listener
argument in
MatchAndExplain()
:
class DivisibleBy7Matcher : public MatcherInterface<int> {
public:
bool MatchAndExplain(int n,
* listener) const override {
MatchResultListenerconst int remainder = n % 7;
if (remainder != 0) {
*listener << "the remainder is " << remainder;
}
return remainder == 0;
}
...
};
Then, EXPECT_THAT(x, DivisibleBy7());
may generate a
message like this:
Value of: x
Expected: is divisible by 7
Actual: 23 (the remainder is 2)
Writing New Polymorphic Matchers
You’ve learned how to write your own matchers in the previous recipe.
Just one problem: a matcher created using MakeMatcher()
only works for one particular type of arguments. If you want a
polymorphic matcher that works with arguments of several types
(for instance, Eq(x)
can be used to match a
value
as long as value == x
compiles
– value
and x
don’t have to share the
same type), you can learn the trick from
testing/base/public/gmock-matchers.h
but it’s a bit
involved.
Fortunately, most of the time you can define a polymorphic matcher
easily with the help of MakePolymorphicMatcher()
. Here’s
how you can define NotNull()
as an example:
using ::testing::MakePolymorphicMatcher;
using ::testing::MatchResultListener;
using ::testing::PolymorphicMatcher;
class NotNullMatcher {
public:
// To implement a polymorphic matcher, first define a COPYABLE class
// that has three members MatchAndExplain(), DescribeTo(), and
// DescribeNegationTo(), like the following.
// In this example, we want to use NotNull() with any pointer, so
// MatchAndExplain() accepts a pointer of any type as its first argument.
// In general, you can define MatchAndExplain() as an ordinary method or
// a method template, or even overload it.
template <typename T>
bool MatchAndExplain(T* p,
* /* listener */) const {
MatchResultListenerreturn p != NULL;
}
// Describes the property of a value matching this matcher.
void DescribeTo(::std::ostream* os) const { *os << "is not NULL"; }
// Describes the property of a value NOT matching this matcher.
void DescribeNegationTo(::std::ostream* os) const { *os << "is NULL"; }
};
// To construct a polymorphic matcher, pass an instance of the class
// to MakePolymorphicMatcher(). Note the return type.
<NotNullMatcher> NotNull() {
PolymorphicMatcherreturn MakePolymorphicMatcher(NotNullMatcher());
}
...
(foo, Bar(NotNull())); // The argument must be a non-NULL pointer. EXPECT_CALL
Note: Your polymorphic matcher class does
not need to inherit from MatcherInterface
or any other class, and its methods do not need to be
virtual.
Like in a monomorphic matcher, you may explain the match result by
streaming additional information to the listener
argument
in MatchAndExplain()
.
Writing New Cardinalities
A cardinality is used in Times()
to tell gMock how many
times you expect a call to occur. It doesn’t have to be exact. For
example, you can say AtLeast(5)
or
Between(2, 4)
.
If the built-in set of
cardinalities doesn’t suit you, you are free to define your own by
implementing the following interface (in namespace
testing
):
class CardinalityInterface {
public:
virtual ~CardinalityInterface();
// Returns true if call_count calls will satisfy this cardinality.
virtual bool IsSatisfiedByCallCount(int call_count) const = 0;
// Returns true if call_count calls will saturate this cardinality.
virtual bool IsSaturatedByCallCount(int call_count) const = 0;
// Describes self to an ostream.
virtual void DescribeTo(::std::ostream* os) const = 0;
};
For example, to specify that a call must occur even number of times, you can write
using ::testing::Cardinality;
using ::testing::CardinalityInterface;
using ::testing::MakeCardinality;
class EvenNumberCardinality : public CardinalityInterface {
public:
bool IsSatisfiedByCallCount(int call_count) const override {
return (call_count % 2) == 0;
}
bool IsSaturatedByCallCount(int call_count) const override {
return false;
}
void DescribeTo(::std::ostream* os) const {
*os << "called even number of times";
}
};
() {
Cardinality EvenNumberreturn MakeCardinality(new EvenNumberCardinality);
}
...
(foo, Bar(3))
EXPECT_CALL.Times(EvenNumber());
Writing New Actions Quickly
If the built-in actions don’t work for you, you can easily define your own one. Just define a functor class with a (possibly templated) call operator, matching the signature of your action.
struct Increment {
template <typename T>
operator()(T* arg) {
T return ++(*arg);
}
}
The same approach works with stateful functors (or any callable, really):
struct MultiplyBy {
template <typename T>
T operator()(T arg) { return arg * multiplier; }
int multiplier;
}
// Then use:
// EXPECT_CALL(...).WillOnce(MultiplyBy{7});
Legacy macro-based Actions
Before C++11, the functor-based actions were not supported; the old
way of writing actions was through a set of ACTION*
macros.
We suggest to avoid them in new code; they hide a lot of logic behind
the macro, potentially leading to harder-to-understand compiler errors.
Nevertheless, we cover them here for completeness.
By writing
(name) { statements; } ACTION
in a namespace scope (i.e. not inside a class or function), you will
define an action with the given name that executes the statements. The
value returned by statements
will be used as the return
value of the action. Inside the statements, you can refer to the K-th
(0-based) argument of the mock function as argK
. For
example:
(IncrementArg1) { return ++(*arg1); } ACTION
allows you to write
... WillOnce(IncrementArg1());
Note that you don’t need to specify the types of the mock function
arguments. Rest assured that your code is type-safe though: you’ll get a
compiler error if *arg1
doesn’t support the ++
operator, or if the type of ++(*arg1)
isn’t compatible with
the mock function’s return type.
Another example:
(Foo) {
ACTION(*arg2)(5);
();
Blah*arg1 = 0;
return arg0;
}
defines an action Foo()
that invokes argument #2 (a
function pointer) with 5, calls function Blah()
, sets the
value pointed to by argument #1 to 0, and returns argument #0.
For more convenience and flexibility, you can also use the following
pre-defined symbols in the body of ACTION
:
argK_type |
The type of the K-th (0-based) argument of the mock function |
---|---|
args |
All arguments of the mock function as a tuple |
args_type |
The type of all arguments of the mock function as a tuple |
return_type |
The return type of the mock function |
function_type |
The type of the mock function |
For example, when using an ACTION
as a stub action for
mock function:
int DoSomething(bool flag, int* ptr);
we have:
Pre-defined Symbol | Is Bound To |
---|---|
arg0 |
the value of flag |
arg0_type |
the type bool |
arg1 |
the value of ptr |
arg1_type |
the type int* |
args |
the tuple (flag, ptr) |
args_type |
the type std::tuple<bool, int*> |
return_type |
the type int |
function_type |
the type int(bool, int*) |
Legacy macro-based parameterized Actions
Sometimes you’ll want to parameterize an action you define. For that we have another macro
(name, param) { statements; } ACTION_P
For example,
(Add, n) { return arg0 + n; } ACTION_P
will allow you to write
// Returns argument #0 + 5.
... WillOnce(Add(5));
For convenience, we use the term arguments for the values used to invoke the mock function, and the term parameters for the values used to instantiate an action.
Note that you don’t need to provide the type of the parameter either.
Suppose the parameter is named param
, you can also use the
gMock-defined symbol param_type
to refer to the type of the
parameter as inferred by the compiler. For example, in the body of
ACTION_P(Add, n)
above, you can write n_type
for the type of n
.
gMock also provides ACTION_P2
, ACTION_P3
,
and etc to support multi-parameter actions. For example,
(ReturnDistanceTo, x, y) {
ACTION_P2double dx = arg0 - x;
double dy = arg1 - y;
return sqrt(dx*dx + dy*dy);
}
lets you write
... WillOnce(ReturnDistanceTo(5.0, 26.5));
You can view ACTION
as a degenerated parameterized
action where the number of parameters is 0.
You can also easily define actions overloaded on the number of parameters:
(Plus, a) { ... }
ACTION_P(Plus, a, b) { ... } ACTION_P2
Restricting the Type of an Argument or Parameter in an ACTION
For maximum brevity and reusability, the ACTION*
macros
don’t ask you to provide the types of the mock function arguments and
the action parameters. Instead, we let the compiler infer the types for
us.
Sometimes, however, we may want to be more explicit about the types. There are several tricks to do that. For example:
(Foo) {
ACTION// Makes sure arg0 can be converted to int.
int n = arg0;
... use n instead of arg0 here ...
}
(Bar, param) {
ACTION_P// Makes sure the type of arg1 is const char*.
::testing::StaticAssertTypeEq<const char*, arg1_type>();
// Makes sure param can be converted to bool.
bool flag = param;
}
where StaticAssertTypeEq
is a compile-time assertion in
googletest that verifies two types are the same.
Writing New Action Templates Quickly
Sometimes you want to give an action explicit template parameters
that cannot be inferred from its value parameters.
ACTION_TEMPLATE()
supports that and can be viewed as an
extension to ACTION()
and ACTION_P*()
.
The syntax:
(ActionName,
ACTION_TEMPLATE(kind1, name1, ..., kind_m, name_m),
HAS_m_TEMPLATE_PARAMS(p1, ..., p_n)) { statements; } AND_n_VALUE_PARAMS
defines an action template that takes m explicit template
parameters and n value parameters, where m is in [1,
10] and n is in [0, 10]. name_i
is the name of the
i-th template parameter, and kind_i
specifies
whether it’s a typename
, an integral constant, or a
template. p_i
is the name of the i-th value
parameter.
Example:
// DuplicateArg<k, T>(output) converts the k-th argument of the mock
// function to type T and copies it to *output.
(DuplicateArg,
ACTION_TEMPLATE// Note the comma between int and k:
(int, k, typename, T),
HAS_2_TEMPLATE_PARAMS(output)) {
AND_1_VALUE_PARAMS*output = T(::std::get<k>(args));
}
To create an instance of an action template, write:
<t1, ..., t_m>(v1, ..., v_n) ActionName
where the t
s are the template arguments and the
v
s are the value arguments. The value argument types are
inferred by the compiler. For example:
using ::testing::_;
...
int n;
(mock, Foo).WillOnce(DuplicateArg<1, unsigned char>(&n)); EXPECT_CALL
If you want to explicitly specify the value argument types, you can provide additional template arguments:
<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n) ActionName
where u_i
is the desired type of v_i
.
ACTION_TEMPLATE
and
ACTION
/ACTION_P*
can be overloaded on the
number of value parameters, but not on the number of template
parameters. Without the restriction, the meaning of the following is
unclear:
<int, bool>(x); OverloadedAction
Are we using a single-template-parameter action where
bool
refers to the type of x
, or a
two-template-parameter action where the compiler is asked to infer the
type of x
?
Using the ACTION Object’s Type
If you are writing a function that returns an ACTION
object, you’ll need to know its type. The type depends on the macro used
to define the action and the parameter types. The rule is relatively
simple:
Given Definition | Expression | Has Type |
---|---|---|
ACTION(Foo) |
Foo() |
FooAction |
ACTION_TEMPLATE(Foo, |
Foo<t1, ..., | FooAction<t1, …, |
Note that we have to pick different suffixes (Action
,
ActionP
, ActionP2
, and etc) for actions with
different numbers of value parameters, or the action definitions cannot
be overloaded on the number of them.
Writing New Monomorphic Actions
While the ACTION*
macros are very convenient, sometimes
they are inappropriate. For example, despite the tricks shown in the
previous recipes, they don’t let you directly specify the types of the
mock function arguments and the action parameters, which in general
leads to unoptimized compiler error messages that can baffle unfamiliar
users. They also don’t allow overloading actions based on parameter
types without jumping through some hoops.
An alternative to the ACTION*
macros is to implement
::testing::ActionInterface<F>
, where F
is the type of the mock function in which the action will be used. For
example:
template <typename F>
class ActionInterface {
public:
virtual ~ActionInterface();
// Performs the action. Result is the return type of function type
// F, and ArgumentTuple is the tuple of arguments of F.
//
// For example, if F is int(bool, const string&), then Result would
// be int, and ArgumentTuple would be ::std::tuple<bool, const string&>.
virtual Result Perform(const ArgumentTuple& args) = 0;
};
using ::testing::_;
using ::testing::Action;
using ::testing::ActionInterface;
using ::testing::MakeAction;
typedef int IncrementMethod(int*);
class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
public:
int Perform(const ::std::tuple<int*>& args) override {
int* p = ::std::get<0>(args); // Grabs the first argument.
return *p++;
}
};
<IncrementMethod> IncrementArgument() {
Actionreturn MakeAction(new IncrementArgumentAction);
}
...
(foo, Baz(_))
EXPECT_CALL.WillOnce(IncrementArgument());
int n = 5;
.Baz(&n); // Should return 5 and change n to 6. foo
Writing New Polymorphic Actions
The previous recipe showed you how to define your own action. This is
all good, except that you need to know the type of the function in which
the action will be used. Sometimes that can be a problem. For example,
if you want to use the action in functions with different types
(e.g. like Return()
and SetArgPointee()
).
If an action can be used in several types of mock functions, we say
it’s polymorphic. The MakePolymorphicAction()
function template makes it easy to define such an action:
namespace testing {
template <typename Impl>
<Impl> MakePolymorphicAction(const Impl& impl);
PolymorphicAction} // namespace testing
As an example, let’s define an action that returns the second argument in the mock function’s argument list. The first step is to define an implementation class:
class ReturnSecondArgumentAction {
public:
template <typename Result, typename ArgumentTuple>
(const ArgumentTuple& args) const {
Result Perform// To get the i-th (0-based) argument, use ::std::get(args).
return ::std::get<1>(args);
}
};
This implementation class does not need to inherit from any
particular class. What matters is that it must have a
Perform()
method template. This method template takes the
mock function’s arguments as a tuple in a single
argument, and returns the result of the action. It can be either
const
or not, but must be invokable with exactly one
template argument, which is the result type. In other words, you must be
able to call Perform<R>(args)
where R
is
the mock function’s return type and args
is its arguments
in a tuple.
Next, we use MakePolymorphicAction()
to turn an instance
of the implementation class into the polymorphic action we need. It will
be convenient to have a wrapper for this:
using ::testing::MakePolymorphicAction;
using ::testing::PolymorphicAction;
<ReturnSecondArgumentAction> ReturnSecondArgument() {
PolymorphicActionreturn MakePolymorphicAction(ReturnSecondArgumentAction());
}
Now, you can use this polymorphic action the same way you use the built-in ones:
using ::testing::_;
class MockFoo : public Foo {
public:
(int, DoThis, (bool flag, int n), (override));
MOCK_METHOD(string, DoThat, (int x, const char* str1, const char* str2),
MOCK_METHOD(override));
};
...
;
MockFoo foo(foo, DoThis).WillOnce(ReturnSecondArgument());
EXPECT_CALL(foo, DoThat).WillOnce(ReturnSecondArgument());
EXPECT_CALL...
.DoThis(true, 5); // Will return 5.
foo.DoThat(1, "Hi", "Bye"); // Will return "Hi". foo
Teaching gMock How to Print Your Values
When an uninteresting or unexpected call occurs, gMock prints the
argument values and the stack trace to help you debug. Assertion macros
like EXPECT_THAT
and EXPECT_EQ
also print the
values in question when the assertion fails. gMock and googletest do
this using googletest’s user-extensible value printer.
This printer knows how to print built-in C++ types, native arrays,
STL containers, and any type that supports the <<
operator. For other types, it prints the raw bytes in the value and
hopes that you the user can figure it out. googletest’s
advanced guide explains how to extend the printer to do a better job
at printing your particular type than to dump the bytes.
Useful Mocks Created Using gMock
Mock std::function
std::function
is a general function type introduced in
C++11. It is a preferred way of passing callbacks to new interfaces.
Functions are copiable, and are not usually passed around by pointer,
which makes them tricky to mock. But fear not -
MockFunction
can help you with that.
MockFunction<R(T1, ..., Tn)>
has a mock method
Call()
with the signature:
(T1, ..., Tn); R Call
It also has a AsStdFunction()
method, which creates a
std::function
proxy forwarding to Call:
std::function<R(T1, ..., Tn)> AsStdFunction();
To use MockFunction
, first create
MockFunction
object and set up expectations on its
Call
method. Then pass proxy obtained from
AsStdFunction()
to the code you are testing. For
example:
(FooTest, RunsCallbackWithBarArgument) {
TEST// 1. Create a mock object.
<int(string)> mock_function;
MockFunction
// 2. Set expectations on Call() method.
(mock_function, Call("bar")).WillOnce(Return(1));
EXPECT_CALL
// 3. Exercise code that uses std::function.
(mock_function.AsStdFunction());
Foo// Foo's signature can be either of:
// void Foo(const std::function<int(string)>& fun);
// void Foo(std::function<int(string)> fun);
// 4. All expectations will be verified when mock_function
// goes out of scope and is destroyed.
}
Remember that function objects created with
AsStdFunction()
are just forwarders. If you create multiple
of them, they will share the same set of expectations.
Although std::function
supports unlimited number of
arguments, MockFunction
implementation is limited to ten.
If you ever hit that limit… well, your callback has bigger problems than
being mockable. :-)