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1 | <?xml version="1.0" encoding="utf-8" ?> |
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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 6 | <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> | |
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| 9 | <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> | |
| 10 | <title>THE BOOST MPL LIBRARY: Higher-Order Metafunctions</title> | |
| 11 | <link rel="stylesheet" href="../style.css" type="text/css" /> | |
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| 13 | <body class="docframe"> | |
| 14 | <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./implementing-division.html" class="navigation-link">Prev</a> <a href="./handling-placeholders.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./dimensional-analysis.html" class="navigation-link">Back</a> <a href="./handling-placeholders.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial-metafunctions.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> | |
| 15 | <td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./higher-order.html" class="navigation-link">Higher-Order Metafunctions</a></td> | |
| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="higher-order"> | |
| 18 | <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id47" name="higher-order">Higher-Order Metafunctions</a></h1> | |
| 19 | <p>In the previous section we used two different forms — | |
| 20 | metafunction classes and placeholder expressions — | |
| 21 | to pass and return metafunctions just like any other metadata. | |
| 22 | Bundling metafunctions into "first class metadata" allows | |
| 23 | <tt class="literal"><span class="pre">transform</span></tt> to perform an infinite variety of different | |
| 24 | operations: in our case, multiplication and division of dimensions. | |
| 25 | Though the idea of using functions to manipulate other functions | |
| 26 | may seem simple, its great power and flexibility <a class="citation-reference" href="#hudak89" id="id9" name="id9">[Hudak89]</a> has | |
| 27 | earned it a fancy title: <strong>higher-order functional programming</strong>. | |
| 28 | A function that operates on another function is known as a | |
| 29 | <strong>higher-order function</strong>. It follows that <tt class="literal"><span class="pre">transform</span></tt> is a | |
| 30 | higher-order | |
| 31 | metafunction: a metafunction that operates on another metafunction.</p> | |
| 32 | <table class="citation" frame="void" id="hudak89" rules="none"> | |
| 33 | <colgroup><col class="label" /><col /></colgroup> | |
| 34 | <tbody valign="top"> | |
| 35 | <tr><td class="label"><a class="fn-backref" href="#id9" name="hudak89">[Hudak89]</a></td><td>Paul Hudak. "Conception, Evolution, and Application of | |
| 36 | Functional Programming Languages," ACM Computing Surveys 21, | |
| 37 | no. 3 Pages: 359 - 411. New York: ACM Press. 1989. | |
| 38 | ISSN:0360-0300. http://doi.acm.org/10.1145/72551.72554.</td></tr> | |
| 39 | </tbody> | |
| 40 | </table> | |
| 41 | <p>Now that we've seen the power of higher-order metafunctions at | |
| 42 | work, it would be good to be able to create new ones. In order to | |
| 43 | explore the basic mechanisms, let's try a simple example. Our task | |
| 44 | is to write a metafunction called <tt class="literal"><span class="pre">twice</span></tt>, which — given a unary | |
| 45 | metafunction <em>f</em> and arbitrary metadata <em>x</em> — computes:</p> | |
| 46 | <blockquote> | |
| 47 | <em>twice</em>(<em>f</em>, <em>x</em>) := <em>f</em>(<em>f</em>(<em>x</em>))</blockquote> | |
| 48 | <p>This might seem like a trivial example, and in fact it is. You | |
| 49 | won't find much use for <tt class="literal"><span class="pre">twice</span></tt> in real code. We hope you'll | |
| 50 | bear with us anyway: Because it doesn't do much more than accept | |
| 51 | and invoke a metafunction, <tt class="literal"><span class="pre">twice</span></tt> captures all the essential | |
| 52 | elements of "higher-orderness" without any distracting details.</p> | |
| 53 | <p>If <em>f</em> is a metafunction class, the definition of <tt class="literal"><span class="pre">twice</span></tt> is | |
| 54 | straightforward:</p> | |
| 55 | <pre class="literal-block"> | |
| 56 | template <class F, class X> | |
| 57 | struct twice | |
| 58 | { | |
| 59 | typedef typename F::template apply<X>::type once; // f(x) | |
| 60 | typedef typename F::template apply<once>::type type; // f(f(x)) | |
| 61 | }; | |
| 62 | </pre> | |
| 63 | <!-- @ prefix.append( | |
| 64 | '''#include <boost/type_traits/add_pointer.hpp> | |
| 65 | #include <boost/static_assert.hpp> | |
| 66 | #include <boost/type_traits/is_same.hpp>''') | |
| 67 | ||
| 68 | twice_test = ''' | |
| 69 | #include <boost/mpl/assert.hpp> | |
| 70 | struct add_pointer_f | |
| 71 | { | |
| 72 | template <class T> struct apply : boost::add_pointer<T> | |
| 73 | {}; | |
| 74 | }; | |
| 75 | BOOST_MPL_ASSERT((boost::is_same<twice<add_pointer_f,int>::type,int**>)); | |
| 76 | ''' | |
| 77 | example.append(twice_test) | |
| 78 | compile() --> | |
| 79 | <!-- @litre_translator.line_offset -= 7 --> | |
| 80 | <p>Or, applying metafunction forwarding:</p> | |
| 81 | <pre class="literal-block"> | |
| 82 | template <class F, class X> | |
| 83 | struct twice | |
| 84 | : F::template apply< | |
| 85 | typename F::template apply<X>::type | |
| 86 | > | |
| 87 | {}; | |
| 88 | </pre> | |
| 89 | <!-- @ example.append(twice_test) | |
| 90 | compile() --> | |
| 91 | <div class="admonition-c-language-note admonition"> | |
| 92 | <p class="admonition-title first">C++ Language Note</p> | |
| 93 | <p>The C++ standard requires the <tt class="literal"><span class="pre">template</span></tt> keyword when we use a | |
| 94 | <strong>dependent name</strong> that refers to a member template. | |
| 95 | <tt class="literal"><span class="pre">F::apply</span></tt> may or may not name a template, <em>depending</em> on the | |
| 96 | particular <tt class="literal"><span class="pre">F</span></tt> that is passed. See <a class="reference" href="./resources.html">the book's</a> Appendix B for more | |
| 97 | information about <tt class="literal"><span class="pre">template</span></tt>.</p> | |
| 98 | </div> | |
| 99 | <p>Given the need to sprinkle our code with the <tt class="literal"><span class="pre">template</span></tt> keyword, | |
| 100 | it would be nice to reduce the syntactic burden of invoking | |
| 101 | metafunction classes. As usual, the solution is to factor the | |
| 102 | pattern into a metafunction:</p> | |
| 103 | <pre class="literal-block"> | |
| 104 | template <class UnaryMetaFunctionClass, class Arg> | |
| 105 | struct apply1 | |
| 106 | : UnaryMetaFunctionClass::template apply<Arg> | |
| 107 | {}; | |
| 108 | </pre> | |
| 109 | <p>Now <tt class="literal"><span class="pre">twice</span></tt> is just:</p> | |
| 110 | <pre class="literal-block"> | |
| 111 | template <class F, class X> | |
| 112 | struct twice | |
| 113 | : apply1<F, typename apply1<F,X>::type> | |
| 114 | {}; | |
| 115 | </pre> | |
| 116 | <p>To see <tt class="literal"><span class="pre">twice</span></tt> at work, we can apply it to a little metafunction | |
| 117 | class built around the <tt class="literal"><span class="pre">add_pointer</span></tt> metafunction:</p> | |
| 118 | <pre class="literal-block"> | |
| 119 | struct add_pointer_f | |
| 120 | { | |
| 121 | template <class T> | |
| 122 | struct apply : boost::add_pointer<T> {}; | |
| 123 | }; | |
| 124 | </pre> | |
| 125 | <!-- @litre_translator.line_offset -= 7 --> | |
| 126 | <p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with <tt class="literal"><span class="pre">add_pointer_f</span></tt> to build | |
| 127 | pointers-to-pointers:</p> | |
| 128 | <pre class="literal-block"> | |
| 129 | BOOST_STATIC_ASSERT(( | |
| 130 | boost::is_same< | |
| 131 | twice<add_pointer_f, int>::type | |
| 132 | , int** | |
| 133 | >::value | |
| 134 | )); | |
| 135 | </pre> | |
| 136 | <!-- @ apply1 = stack[-4] | |
| 137 | add_pointer_f = stack[-2] | |
| 138 | compile('all', pop = 0) --> | |
| 139 | </div> | |
| 140 | ||
| 141 | <div class="footer-separator"></div> | |
| 142 | <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./implementing-division.html" class="navigation-link">Prev</a> <a href="./handling-placeholders.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./dimensional-analysis.html" class="navigation-link">Back</a> <a href="./handling-placeholders.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial-metafunctions.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> | |
| 143 | </tr></table></body> | |
| 144 | </html> |