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A first general restructuration of the doc according to the pattern [tour|tutorial|reference]. In the reference part, objects are documented per topic. In each topic, [definition|c++|python|hdf5] (not yet implemented)
78 lines
4.3 KiB
ReStructuredText
78 lines
4.3 KiB
ReStructuredText
.. highlight:: c
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.. _view_vs_regular:
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Regular type vs view type
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=================================================================
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A fundamental distinction in TRIQS is the difference between the `regular type`
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and the corresponding `view types`.
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* The **regular types**
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* Following the definition of A. Stepanov, a regular type is a *value*, much like an int, a double,
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or a std::vector. Cf http://www.stepanovpapers.com/DeSt98.pdf (I added move semantics to the definition).
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* It has `value semantics`, more precisely ::
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T a; // default constructible
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T a = b; T a (b); // copy constructible
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a = b; // assignment
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a == b; a != b; // Equality.
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with the following properties ::
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T a = b; assert(a==b);
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T a; a=b; is equivalent to T a=b;
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T a=c; T b=c; a=d; assert(b==c);
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* When copied, assigned, it makes deep copy of the data and resizes itself if necessary (like e.g. std::vector).
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* Examples:
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* std::vector<T>
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* triqs::arrays::array<T,R>, triqs::arrays::matrix<T>, triqs::arrays::vector<T>
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* The **view types**
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* They have a `reference semantics`.
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* They are just partial or complete views of the corresponding regular type.
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* They never make deep copy, either in copying, constructing, transforming to python,
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* They cannot be resized.
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* They are useful to work on arrays, matrices, or on some part thereof, without making copies
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(e.g. on a slice of an array, a column of a matrix).
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* Examples:
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* triqs::arrays::array_view<T,R>, triqs::arrays::matrix_view, triqs::arrays::vector_view<T>
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Many triqs containers (array, matrix, green functions)
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is therefore provided in two flavors, and it is crucial for the user
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to learn and understand their difference in order to use them appropriately.
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Behaviour comparison table between regular and view types
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------------------------------------------------------------
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A detailed comparison of the behaviour of the two type of types is provided in the following table.
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=================== ======================================================================= ======================================================================================
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Topics regular type (e.g. array, matrix, vector) view type (e.g. array_view, matrix_view, vector_view)
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=================== ======================================================================= ======================================================================================
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Contructors - Constructors create a new fresh memory block. - Constructors only make another view of the data.
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Default Contructor - Default constructor creates an empty array - No default constructor (what would it view ?).
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Copy contructors - Create a new fresh memory block. - Make another view of the data (shallow copy).
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- Make a *true* copy of the data. - *Never* copy data, so they are quite quick.
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Assignment (=) - The assignment operator creates a new datablock if size mismatchs. - The assignment operator just copy data into the view.
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- Hence, assignment never fail for size reason Behaviour is undefined if the size of the view is too small.
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(unless there is a memory allocation exception of course) (it throws if the array has option `Tag::BoundCheck` or if
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`TRIQS_ARRAYS_ENFORCE_BOUNDCHECK` is defined, Cf below).
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Resizing - Can be resized, invalidating all references/pointers to the data. - Can be not be resized.
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Invalidation - References/pointers to the data may become invalid after resize, - References/pointers to the data are still valid after assignment.
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or assignment.
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Data Contiguity - The data are contiguous in memory. - The data may not be contiguous in memory (e.g. a slice).
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=================== ======================================================================= ======================================================================================
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