96 lines
2.7 KiB
Plaintext
96 lines
2.7 KiB
Plaintext
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<!-- ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| section -->
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<section id="codegeneration">
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<title>Code generation facilities</>
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<para>
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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:
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</>
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<programlisting>
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<![CDATA[
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// fixed-point algorithm input
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typedef mpl::vector<
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mpl::fixed_c<-1,2345678>
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, mpl::fixed_c<9,0001>
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// ..
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, mpl::fixed_c<3,14159>
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> input_data;
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/*
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complex compile-time algorithm
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*/
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typedef /*...*/ result_data;
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]]>
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</>
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<para>
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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:
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</>
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<programlisting>
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<![CDATA[
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double my_algorithm()
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{
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// passing the results to the run-time part of the program
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std::vector<double> results;
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results.reserve(mpl::size<result_data>::value);
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mpl::for_each<numbers,_>(
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boost::bind(&std::vector<double>::push_back, &results, _1)
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);
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// ...
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}
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]]>
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</>
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<para>
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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:
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</>
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<programlisting>
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<![CDATA[
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template<
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typename Seq
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, typename TransformOp
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, typename F
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>
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void for_each(F f)
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{
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// ...
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}
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]]>
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</>
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<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</>.
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</>
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<para>
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Applying this to our example, the
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</>
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<programlisting>
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<![CDATA[
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mpl::for_each<numbers,_>(
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boost::bind(&std::vector<double>::push_back, &results, _1)
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);
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]]>
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</>
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<para>
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call is roughly equivalent to this:
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</>
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<programlisting>
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<![CDATA[
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f(mpl::apply< _,mpl::at_c<result_data,0>::type >::type());
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f(mpl::apply< _,mpl::at_c<result_data,1>::type >::type());
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// ...
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f(mpl::apply< _,mpl::at_c<result_data,n>::type >::type());
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]]>
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</>
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<para>
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where <literal>n == mpl::size<result_data>::type::value</>.
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</>
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</section>
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