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https://github.com/triqs/dft_tools
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106 lines
4.5 KiB
ReStructuredText
106 lines
4.5 KiB
ReStructuredText
.. highlight:: c
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.. _partial_views:
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Views
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==============================================================
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**Synopsis**:
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.. code-block:: c
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template <typename ValueType, int Rank, ull_t OptionsFlags=0, ull_t TraversalOrder=0 > class array_view;
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template <typename ValueType, ull_t OptionsFlags=0, ull_t TraversalOrder=0 > class matrix_view;
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template <typename ValueType, ull_t OptionsFlags=0> class vector_view;
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where triqs::ull_t is the type defined by :
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.. code-block:: c
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typedef unsigned long long ull_t;
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The view classes have the same template parameters to the regular type.
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* A `view` is defined as a view of a restricted portion of the array, matrix or vector.
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* The view type of X (= array, matrix, vector) is called X_view, with the same template parameters as the regular type.
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* A partial view models the :ref:`MutableCuboidArray`, :ref:`MutableMatrix`, :ref:`MutableVector`, like the corresponding regular type.
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* Basically X_view should be understood as a reference to the data of the viewed object,
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dressed to model the "`MutableX`" 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. See :ref:`view_vs_regular` for a detailed discussion of the difference between array and array_view.
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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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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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(like a std::shared_ptr, but more sophisticated to properly interface to Python numpy).
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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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.. warning::
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TRIQS arrays, and in particular views are NOT thread-safe, contrary to std::shared_ptr.
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This is a deliberate choice for optimisation.
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Weak views [Advanced]
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^^^^^^^^^^^^^^^^^^^^^^^^
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Unfortunately this can not be the end of the story, at least on current C++11 compilers.
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It turns out that, in very performance sensitive loops, increasing this tiny
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reference counter can break a lot of optimisations in almost all compilers, including the most
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recent ones (gcc 4.8, clang 3.3).
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It was therefore decided to introduce `weak views`.
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* Weak views are completely similar to "normal" views, except that they do `not` increase the memory
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reference counter.
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* Normal views are be implicitely constructed from weak view (the reverse is also true),
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As soon as you store the weak view into a normal view (e.g. array_view<T,N>)
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the reference counting is again incremented, and the memory guarantee holds.
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* Explicit construction of weak views is intentionally not documented here.
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It is designed to be (almost) an implementation detail.
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* The () operator returns a weak view `on a modern C++11 compiler`,
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allowing therefore for better performance on such compiler, in some loops.
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(in particular, `rvalue reference for *this` must be implemented).
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* It is however necessary for the advanced user to know about this implementation detail,
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because in some convolved cases, the memory guarantee may not hold.
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Example :
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