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https://github.com/triqs/dft_tools
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118 lines
3.1 KiB
ReStructuredText
118 lines
3.1 KiB
ReStructuredText
.. highlight:: c
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.. _clef_auto_assign:
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Automatic assignment of containers
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===================================
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Another use of expression is the automatic assignment of containers.
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**Synopsis** :
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If C is a container, ::
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C(x_) << some_expression_of_x_
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C[i_] << some_expression_of_i_
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C(x_)(i_,j_) << some_expression_of_x_i_j
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C[x_](i_,j_) << some_expression_of_x_i_j
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depending of course of the operator that the container support.
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The Right Hand Side (RHS) of the << statement can be any expression,
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while Left Hand Side (LHS) of the << sign is a container supporting the operation (see below).
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This statement simply will be rewritten by the CLEF library as ::
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triqs_clef_auto_assign (C, x_ -> some_expression_of_x_); // pseudo code
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triqs_clef_auto_assign (C, [](auto x_) {return some_expression_of_x_;}); // or in C++ lambda syntax
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The function `triqs_clef_auto_assign` has to be overloaded for the container and the correct
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functional form, and it is expected to fill C by evaluating this function.
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Such a function is provided for TRIQS objects (arrays, matrix, Green function),
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and also for some STL container like std::vector.
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Example :
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.. compileblock::
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#include "triqs/clef.hpp"
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#include <iostream>
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using namespace triqs::clef;
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int main() {
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int N = 5;
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double pi = std::acos(-1);
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// automatic assignment of vector
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placeholder <0> k_;
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std::vector<double> V(N);
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make_expr(V) [k_] << cos( (2* pi* k_)/ N );
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// chaining them ...
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placeholder <1> i_;
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std::vector<std::vector<double>> W(3, std::vector<double>(N));
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make_expr(W)[i_] [k_] << i_ + cos( (2* pi* k_)/ N );
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// check result...
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for (size_t u=0; u<V.size(); ++u) if (std::abs(V[u] -cos((2*pi*u)/N))> 1.e-10) throw "error!";
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for (size_t w=0; w<W.size(); ++w)
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for (size_t u=0; u<W[w].size(); ++u)
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if (std::abs( W[w][u] - (w+cos((2*pi*u)/N)))> 1.e-10) throw "error!";
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}
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**Details**
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The synopsis of the `triqs_clef_auto_assign` functions is ::
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template<typename Fnt> void triqs_clef_auto_assign (Obj & x, Fnt f);
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The compiler will then rewrite ::
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obj(x_,y_, ...) = expression
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into ::
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triqs_clef_auto_assign (obj, make_function( expression, x_, y_, ....))
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The function must be found by ADL. It is therefore useful to implement it e.g. as a friend function.
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Similarly, for [ ], adding a function ::
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template<typename Fnt> void triqs_clef_auto_assign_subscript (Obj & x, Fnt f);
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The compiler will rewrite ::
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obj[i_] = expression
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into ::
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triqs_clef_auto_assign_subscript (obj, make_function( expression, i_))
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A complete example :
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.. compileblock::
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#include <triqs/clef.hpp>
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#include <iostream>
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using namespace triqs::clef;
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struct Obj{
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double v;
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TRIQS_CLEF_IMPLEMENT_LAZY_CALL();
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//
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template<typename Fnt> friend void triqs_clef_auto_assign (Obj & x, Fnt f) {
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std::cout<< " called triqs_clef_auto_assign "<< f(x.v++)<<std::endl;
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}
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};
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int main() {
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Obj f{2};
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placeholder<3> x_;
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std::cout<< f.v << std::endl;
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f(x_ ) << 8*x_ ;
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std::cout<< f.v << std::endl;
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}
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