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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 --> | |
| 5 | <!-- Software License, Version 1.0. (See accompanying --> | |
| 6 | <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> | |
| 7 | <head> | |
| 8 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> | |
| 9 | <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> | |
| 10 | <title>THE BOOST MPL LIBRARY: Details</title> | |
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| 12 | </head> | |
| 13 | <body class="docframe"> | |
| 14 | <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./the-importance-of-being.html" class="navigation-link">Prev</a> <a href="./exercises.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Back</a> <a href="./exercises.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="./details.html" class="navigation-link">Details</a></td> | |
| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="details"> | |
| 18 | <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id59" name="details">Details</a></h1> | |
| 19 | <p>By now you should have a fairly complete view of the fundamental | |
| 20 | concepts and language of both template metaprogramming in general | |
| 21 | and of the Boost Metaprogramming Library. This section | |
| 22 | reviews the highlights.</p> | |
| 23 | <dl> | |
| 24 | <dt>Metafunction forwarding.</dt> | |
| 25 | <dd>The technique of using public derivation to | |
| 26 | supply the nested <tt class="literal"><span class="pre">type</span></tt> of a metafunction by accessing the one | |
| 27 | provided by its base class.</dd> | |
| 28 | <dt>Metafunction class.</dt> | |
| 29 | <dd>The most basic way to formulate a compile-time | |
| 30 | function so that it can be treated as polymorphic metadata; that | |
| 31 | is, as a type. A metafunction class is a class with a nested | |
| 32 | metafunction called <tt class="literal"><span class="pre">apply</span></tt>.</dd> | |
| 33 | <dt>MPL.</dt> | |
| 34 | <dd><p class="first">Most of this book's examples will use the Boost | |
| 35 | Metaprogramming Library. Like the Boost type traits headers, | |
| 36 | MPL | |
| 37 | headers follow a simple convention:</p> | |
| 38 | <pre class="literal-block"> | |
| 39 | #include <boost/mpl/<em>component-name</em>.hpp> | |
| 40 | </pre> | |
| 41 | <p class="last">If the component's name ends in an underscore, however, the | |
| 42 | corresponding MPL header name does not include the trailing | |
| 43 | underscore. For example, <tt class="literal"><span class="pre">mpl::bool_</span></tt> can be found in | |
| 44 | <tt class="literal"><span class="pre"><boost/mpl/bool.hpp></span></tt>. Where the library deviates from this | |
| 45 | convention, we'll be sure to point it out to you.</p> | |
| 46 | </dd> | |
| 47 | </dl> | |
| 48 | <!-- @ignore() --> | |
| 49 | <dl> | |
| 50 | <dt>Higher-order function.</dt> | |
| 51 | <dd>A function that operates on or returns a function. Making | |
| 52 | metafunctions polymorphic with other metadata is a key | |
| 53 | ingredient in higher-order metaprogramming.</dd> | |
| 54 | <dt>Lambda expression.</dt> | |
| 55 | <dd>Simply put, a lambda expression is callable metadata. Without | |
| 56 | some form of callable metadata, higher-order metafunctions | |
| 57 | would be impossible. Lambda expressions have two basic forms: | |
| 58 | <em>metafunction classes</em> and <em>placeholder expressions</em>.</dd> | |
| 59 | <dt>Placeholder expression.</dt> | |
| 60 | <dd><p class="first">A kind of lambda expression that, through the use of | |
| 61 | placeholders, enables in-place <em>partial metafunction | |
| 62 | application</em> and <em>metafunction composition</em>. As you will see | |
| 63 | throughout this book, these features give us the truly amazing | |
| 64 | ability to build up almost any kind of complex type computation | |
| 65 | from more primitive metafunctions, right at its point of use:</p> | |
| 66 | <pre class="literal-block"> | |
| 67 | // find the position of a type x in some_sequence such that: | |
| 68 | // x is convertible to 'int' | |
| 69 | // && x is not 'char' | |
| 70 | // && x is not a floating type | |
| 71 | typedef mpl::find_if< | |
| 72 | some_sequence | |
| 73 | , mpl::and_< | |
| 74 | boost::is_convertible<_1,int> | |
| 75 | , mpl::not_<boost::is_same<_1,char> > | |
| 76 | , mpl::not_<boost::is_float<_1> > | |
| 77 | > | |
| 78 | >::type iter; | |
| 79 | </pre> | |
| 80 | <p class="last">Placeholder expressions make good on the promise of algorithm reuse | |
| 81 | without forcing us to write new metafunction classes. The | |
| 82 | corresponding capability is often sorely missed in the runtime | |
| 83 | world of the STL, since it is often much easier to write a loop | |
| 84 | by hand than it is to use standard algorithms, despite their | |
| 85 | correctness and efficiency advantages.</p> | |
| 86 | </dd> | |
| 87 | </dl> | |
| 88 | <!-- @ example.prepend(''' | |
| 89 | #include <boost/mpl/and.hpp> | |
| 90 | #include <boost/mpl/not.hpp> | |
| 91 | #include <boost/mpl/find_if.hpp> | |
| 92 | #include <boost/type_traits/is_convertible.hpp> | |
| 93 | #include <boost/type_traits/is_float.hpp> | |
| 94 | typedef mpl::vector<char, double, short, long> some_sequence; | |
| 95 | ''') | |
| 96 | compile() --> | |
| 97 | <dl> | |
| 98 | <dt>The <tt class="literal"><span class="pre">lambda</span></tt> metafunction.</dt> | |
| 99 | <dd>A metafunction that transforms a lambda expression into a | |
| 100 | corresponding metafunction class. For detailed information on | |
| 101 | <tt class="literal"><span class="pre">lambda</span></tt> and the lambda evaluation process, | |
| 102 | please see the <a class="reference" href="./reference-manual.html">the MPL reference manual</a>.</dd> | |
| 103 | <dt>The <tt class="literal"><span class="pre">apply</span></tt> metafunction.</dt> | |
| 104 | <dd>A metafunction that invokes its first argument, which must be a | |
| 105 | lambda expression, on its remaining arguments. In general, to | |
| 106 | invoke a lambda expression, you should always pass it to | |
| 107 | <tt class="literal"><span class="pre">mpl::apply</span></tt> along with the arguments you want to apply it | |
| 108 | to in lieu of using <tt class="literal"><span class="pre">lambda</span></tt> and invoking the result "manually."</dd> | |
| 109 | <dt>Lazy evaluation.</dt> | |
| 110 | <dd>A strategy of delaying evaluation until a result is | |
| 111 | required, thereby avoiding any unnecessary computation and any | |
| 112 | associated unnecessary errors. Metafunctions are only invoked | |
| 113 | when we access their nested <tt class="literal"><span class="pre">::type</span></tt>s, so we can supply all | |
| 114 | of their arguments without performing any computation and | |
| 115 | delay evaluation to the last possible moment.</dd> | |
| 116 | </dl> | |
| 117 | </div> | |
| 118 | ||
| 119 | <div class="footer-separator"></div> | |
| 120 | <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./the-importance-of-being.html" class="navigation-link">Prev</a> <a href="./exercises.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Back</a> <a href="./exercises.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> | |
| 121 | </tr></table></body> | |
| 122 | </html> |