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https://github.com/triqs/dft_tools
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67 lines
2.9 KiB
ReStructuredText
67 lines
2.9 KiB
ReStructuredText
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.. highlight:: c
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View classes and their constructors
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==============================================================================
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* A view class (array_view, matrix_view, vector_view) is used to present a
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partial (or complete) view of an array, matrix, vector.
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Basically it should be understood as a reference to the data of the viewed object,
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dressed to model the :ref:`MutableArray` concept.
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* A view class behaves like the value class when called, put in expression, ...
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but differs from it by its behaviour under copy and assignment.
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It has a reference semantics : like a pointer or a reference, it does not make a deep copy of the data
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when copied.
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* There are two ways to make a view :
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* explicit construction, by calling a constructor or the make_view function
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* automatic construction : views are the results of slicing the array
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* In the following, we will discuss only array_view, since matrix_view and vector_view
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behaves exactly in a similar way.
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Explicit construction of a view
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----------------------------------------
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====================================================================== =====================================================================================================
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Constructors of array_view Comments
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====================================================================== =====================================================================================================
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array_view(const array_view &) Copy construction (makes a shallow copy)
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array_view(const T & X) T is any type such that X.indexmap() and X.storage() can be used to construct a view.
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REF to concept here ....
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====================================================================== =====================================================================================================
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The function make_view can also be used to make a view.
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Example ::
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array<int,2> A(2,2);
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array_view<int,2> V(A);
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auto V2 = make_view(A);
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struct view_keeper {
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array_view<int,2> V;
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Memory management
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------------------------
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View classes contains a reference counting system to the memory block they view
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(i.e. a std::shared_ptr).
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This guarantees that memory will not be dellocated before the destruction of the view.
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The memory block will be dellocated when its array and all array_view
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pointing to it or to a portion of it will be destroyed, and only at that moment.
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Example::
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array<int,2> *p = new array<int,2> (2,3); // create an array p
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array_view<int,2> B(*p); // making a view
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delete p; // p is gone...
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B(0,0) = 314; cout<<B<<endl; // B (and the data) is still alive
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NB : This means that constructing a new view is cheap but still : document here the question of proxy ??
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