Code generation facilities</> <para> There are cases, especially in the domain of numeric computation, when one wants to perform some part of the calculations at compile-time, and then pass the results to a run-time part of the program for further processing. For example, suppose one has implemented a complex compile-time algorithm that works with fixed-point arithmetic: </> <programlisting> <![CDATA[ // fixed-point algorithm input typedef mpl::vector< mpl::fixed_c<-1,2345678> , mpl::fixed_c<9,0001> // .. , mpl::fixed_c<3,14159> > input_data; /* complex compile-time algorithm */ typedef /*...*/ result_data; ]]> </> <para> Suppose the <literal>result_data</> here is a sequence of <literal>mpl::fixed_c</> types that keeps the results of the algorithm, and now one wishes to feed that result to the run-time part of the algorithm. With &MPL; she can do this: </> <programlisting> <![CDATA[ double my_algorithm() { // passing the results to the run-time part of the program std::vector<double> results; results.reserve(mpl::size<result_data>::value); mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) ); // ... } ]]> </> <para> The <literal>for_each<numbers,_>(...)</> call is what actually transfers the compile-time <literal>result_data</> into run-time <literal>results</>. <literal>for_each</> is a function template declared as: </> <programlisting> <![CDATA[ template< typename Seq , typename TransformOp , typename F > void for_each(F f) { // ... } ]]> </> <para>To call the function, one is required to explicitly provide two actual template parameters, a compile-time sequence <literal>Seq</> and a unary transformation metafunction <literal>TransformOp</>, plus a run-time function argument <literal>f</> (in our example, <literal>numbers</>, <literal>_</>, and <literal>boost::bind(...)</> correspondingly). <literal>f</> is a function object which <literal>operator()</> is called for every element in the <literal>Seq</> tranfromed by <literal>TransformOp</>. </> <para> Applying this to our example, the </> <programlisting> <![CDATA[ mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) ); ]]> </> <para> call is roughly equivalent to this: </> <programlisting> <![CDATA[ f(mpl::apply< _,mpl::at_c<result_data,0>::type >::type()); f(mpl::apply< _,mpl::at_c<result_data,1>::type >::type()); // ... f(mpl::apply< _,mpl::at_c<result_data,n>::type >::type()); ]]> </> <para> where <literal>n == mpl::size<result

Code generation facilities</> <para> There are cases, especially in the domain of numeric computation, when one wants to perform some part of the calculations at compile-time, and then pass the results to a run-time part of the program for further processing. For example, suppose one has implemented a complex compile-time algorithm that works with fixed-point arithmetic: </> <programlisting> <![CDATA[ // fixed-point algorithm input typedef mpl::vector< mpl::fixed_c<-1,2345678> , mpl::fixed_c<9,0001> // .. , mpl::fixed_c<3,14159> > input_data; /* complex compile-time algorithm */ typedef /*...*/ result_data; ]]> </> <para> Suppose the <literal>result_data</> here is a sequence of <literal>mpl::fixed_c</> types that keeps the results of the algorithm, and now one wishes to feed that result to the run-time part of the algorithm. With &MPL; she can do this: </> <programlisting> <![CDATA[ double my_algorithm() { // passing the results to the run-time part of the program std::vector<double> results; results.reserve(mpl::size<result_data>::value); mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) ); // ... } ]]> </> <para> The <literal>for_each<numbers,_>(...)</> call is what actually transfers the compile-time <literal>result_data</> into run-time <literal>results</>. <literal>for_each</> is a function template declared as: </> <programlisting> <![CDATA[ template< typename Seq , typename TransformOp , typename F > void for_each(F f) { // ... } ]]> </> <para>To call the function, one is required to explicitly provide two actual template parameters, a compile-time sequence <literal>Seq</> and a unary transformation metafunction <literal>TransformOp</>, plus a run-time function argument <literal>f</> (in our example, <literal>numbers</>, <literal>_</>, and <literal>boost::bind(...)</> correspondingly). <literal>f</> is a function object which <literal>operator()</> is called for every element in the <literal>Seq</> tranfromed by <literal>TransformOp</>. </> <para> Applying this to our example, the </> <programlisting> <![CDATA[ mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) ); ]]> </> <para> call is roughly equivalent to this: </> <programlisting> <![CDATA[ f(mpl::apply< _,mpl::at_c<result_data,0>::type >::type()); f(mpl::apply< _,mpl::at_c<result_data,1>::type >::type()); // ... f(mpl::apply< _,mpl::at_c<result_data,n>::type >::type()); ]]> </> <para> where <literal>n == mpl::size<result_data>::type::value</>. </> </section>