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125 lines
4.6 KiB
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<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type">
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<title>4. Code generation facilities</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.50.0">
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<meta name="keywords" content="template metaprogramming, generic programming, programming languages, C++, STL, type systems, polymorphism, compile-time">
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<link rel="home" href="index.html" title="the boost c++ metaprogramming library">
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<link rel="up" href="index.html" title="the boost c++ metaprogramming library">
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<link rel="previous" href="lambda.html" title="3. lambda facility">
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<link rel="next" href="example.html" title="5. example: a compile-time fsm generator">
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">4. Code generation facilities</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="lambda.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="example.html">Next</a></td>
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<div class="section">
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<div class="titlepage">
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<div>
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<h2 class="title" style="clear: both"><a name="codegeneration"></a>4. Code generation facilities</h2>
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</div>
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</div>
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<p>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:</p>
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<pre class="programlisting">
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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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</pre>
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<p>Suppose the <tt>result_data</tt> here is a sequence of <tt>mpl::fixed_c</tt> 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:</p>
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<pre class="programlisting">
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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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</pre>
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<p>The <tt>for_each<numbers,_>(...)</tt> call is what actually transfers the compile-time <tt>result_data</tt> into run-time <tt>results</tt>. <tt>for_each</tt> is a function template declared as:</p>
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<pre class="programlisting">
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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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</pre>
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<p>To call the function, one is required to explicitly provide two actual template parameters, a compile-time sequence <tt>Seq</tt> and a unary transformation metafunction <tt>TransformOp</tt>, plus a run-time function argument <tt>f</tt> (in our example, <tt>numbers</tt>, <tt>_</tt>, and <tt>boost::bind(...)</tt> correspondingly). <tt>f</tt> is a function object which <tt>operator()</tt> is called for every element in the <tt>Seq</tt> tranfromed by <tt>TransformOp</tt>.</p>
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<p>Applying this to our example, the</p>
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<pre class="programlisting">
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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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</pre>
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<p>call is roughly equivalent to this:</p>
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<pre class="programlisting">
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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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</pre>
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<p>where <tt>n == mpl::size<result_data>::type::value</tt>.</p>
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</div>
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="lambda.html">Prev</a> </td>
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<td width="20%" align="center"><a accesskey="u" href="index.html">Up</a></td>
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<td width="40%" align="right"> <a accesskey="n" href="example.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">3. Lambda facility </td>
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<td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td>
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<td width="40%" align="right" valign="top"> 5. Example: a compile-time FSM generator</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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