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67 lines
2.8 KiB
ReStructuredText
67 lines
2.8 KiB
ReStructuredText
.. highlight:: c
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Assignment
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=========================
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The `value classes` and the `view classes` have a quite general assignment operator.
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We will illustrate it on the `array` class, it is the same for `matrix` and `vector`.
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* **Syntax** : the syntax is the same in both cases::
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template<typename RHS> array & operator=(const RHS & X);
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template<typename RHS> array_view & operator=(const RHS & X);
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* **What can be RHS ?**
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RHS can be ... anything that models the HasImmutableArrayInterface concept !
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e.g. : array, array_view, matrix, matrix_view,
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but also formal expression (See , e.g. A+B), or any custom object of your choice.
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* **Behaviour**
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================= ======================================================================= ======================================================================================
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Topic value class : array, matrix, vector view: array_view, matrix_view, vector_view
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================= ======================================================================= ======================================================================================
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RHS ... - RHS models the concept `HasImmutableArrayInterface`. - RHS models HasImmutableArrayInterface
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[or less ? : RHS can be evaluated in the domain_type::value_type, no domain needed.].
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Effect - array is resized to have a domain X.domain() - View's domain must match X's domain or behaviour is undefined.
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- array is filled with the evaluation of X. - array is filled with the evaluation of X.
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================= ======================================================================= ======================================================================================
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* By evaluation of X, we mean :
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- a copy if X is an array.
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- computing the value if X is an expression.
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* **Optimisation** `[advanced]`
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In some case, we know that optimisation is possible.
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If the RHS class derives from `Tag::has_special_assign`, then the instruction ::
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A = rhs;
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will be rewritten by compiler as ::
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rhs.assign_invoke(A);
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An example is the matmul class, which stores the ref. to the objects to be multiplied.
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If A and B are matrices, writing ::
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C = matmul(A,B); // (1)
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is rewritten as ::
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matmul(A,B).assign_invoke(C);
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which is then itself rewritten into (without all the messy details) ::
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call_lapack( A, B, C); // (2)
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The point is that :
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* the user code is (1), very simple and readable
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* but the compiler compiles (2), which eliminates temporaries...
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