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1 | <?xml version="1.0" encoding="utf-8" ?> |
| 2 | <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> | |
| 3 | <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> | |
| 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: Placeholders</title> | |
| 11 | <link rel="stylesheet" href="../style.css" type="text/css" /> | |
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| 14 | <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Prev</a> <a href="./placeholder-expression.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./placeholder-expression.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.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="./lambda-details.html" class="navigation-link">Lambda Details</a> / <a href="./placeholders.html" class="navigation-link">Placeholders</a></td> | |
| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="placeholders"> | |
| 18 | <h1><a class="toc-backref" href="./lambda-details.html#id55" name="placeholders">Placeholders</a></h1> | |
| 19 | <p>The definition of "placeholder" may surprise you:</p> | |
| 20 | <div class="admonition-definition admonition"> | |
| 21 | <p class="admonition-title first">Definition</p> | |
| 22 | <p>A <strong>placeholder</strong> is a metafunction class of the form <tt class="literal"><span class="pre">mpl::arg<X></span></tt>.</p> | |
| 23 | </div> | |
| 24 | <div class="section" id="implementation"> | |
| 25 | <h2><a name="implementation">Implementation</a></h2> | |
| 26 | <p>The convenient names <tt class="literal"><span class="pre">_1</span></tt>, <tt class="literal"><span class="pre">_2</span></tt>,... <tt class="literal"><span class="pre">_5</span></tt> are actually | |
| 27 | <tt class="literal"><span class="pre">typedef</span></tt>s for specializations of <tt class="literal"><span class="pre">mpl::arg</span></tt> that simply | |
| 28 | select the <em>N</em>th argument for any <em>N</em>. <a class="footnote-reference" href="#config" id="id12" name="id12">[6]</a> The | |
| 29 | implementation of placeholders looks something like this:</p> | |
| 30 | <table class="footnote" frame="void" id="config" rules="none"> | |
| 31 | <colgroup><col class="label" /><col /></colgroup> | |
| 32 | <tbody valign="top"> | |
| 33 | <tr><td class="label"><a class="fn-backref" href="#id12" name="config">[6]</a></td><td>MPL provides five placeholders by default. See | |
| 34 | the Configuration Macros section of <a class="reference" href="./reference-manual.html">the MPL reference manual</a> for a | |
| 35 | description of how to change the number of placeholders | |
| 36 | provided.</td></tr> | |
| 37 | </tbody> | |
| 38 | </table> | |
| 39 | <pre class="literal-block"> | |
| 40 | namespace boost { namespace mpl { namespace placeholders { | |
| 41 | ||
| 42 | template <int N> struct arg; // forward declarations | |
| 43 | struct void_; | |
| 44 | ||
| 45 | template <> | |
| 46 | struct arg<<strong>1</strong>> | |
| 47 | { | |
| 48 | template < | |
| 49 | class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_> | |
| 50 | struct apply | |
| 51 | { | |
| 52 | typedef <strong>A1</strong> type; // return the first argument | |
| 53 | }; | |
| 54 | }; | |
| 55 | typedef <strong>arg<1> _1</strong>; | |
| 56 | ||
| 57 | template <> | |
| 58 | struct arg<<strong>2</strong>> | |
| 59 | { | |
| 60 | template < | |
| 61 | class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_ | |
| 62 | > | |
| 63 | struct apply | |
| 64 | { | |
| 65 | typedef <strong>A2</strong> type; // return the second argument | |
| 66 | }; | |
| 67 | }; | |
| 68 | typedef <strong>arg<2> _2</strong>; | |
| 69 | ||
| 70 | <em>more specializations and typedefs...</em> | |
| 71 | ||
| 72 | }}} | |
| 73 | </pre> | |
| 74 | <!-- @example.replace('...','') --> | |
| 75 | <p>Remember that invoking a metafunction class is the same as invoking | |
| 76 | its nested <tt class="literal"><span class="pre">apply</span></tt> metafunction. When a placeholder in a lambda | |
| 77 | expression is evaluated, it is invoked on the expression's actual | |
| 78 | arguments, returning just one of them. The results are then | |
| 79 | substituted back into the lambda expression and the evaluation | |
| 80 | process continues.</p> | |
| 81 | </div> | |
| 82 | <div class="section" id="the-unnamed-placeholder"> | |
| 83 | <h2><a name="the-unnamed-placeholder">The Unnamed Placeholder</a></h2> | |
| 84 | <p>There's one special placeholder, known as the <strong>unnamed | |
| 85 | placeholder</strong>, that we haven't yet defined:</p> | |
| 86 | <pre class="literal-block"> | |
| 87 | namespace boost { namespace mpl { namespace placeholders { | |
| 88 | ||
| 89 | <strong>typedef arg<-1> _;</strong> // the unnamed placeholder | |
| 90 | ||
| 91 | }}} | |
| 92 | </pre> | |
| 93 | <!-- @ stack[-2].prepend('namespace shield {') | |
| 94 | example.append('}') # so we don't conflict with the prefix | |
| 95 | compile('all') --> | |
| 96 | <p>The details of its implementation aren't important; all you really | |
| 97 | need to know about the unnamed placeholder is that it gets special | |
| 98 | treatment. When a lambda expression is being transformed into a | |
| 99 | metafunction class by <tt class="literal"><span class="pre">mpl::lambda</span></tt>,</p> | |
| 100 | <blockquote> | |
| 101 | the <em>n</em>th appearance of the unnamed placeholder <em>in a given | |
| 102 | template specialization</em> is replaced with <tt class="literal"><span class="pre">_</span></tt><em>n</em>.</blockquote> | |
| 103 | <p>So, for example, every row of Table 1.1 | |
| 104 | below contains two equivalent lambda expressions.</p> | |
| 105 | <table border="1" class="table"> | |
| 106 | <caption>Unnamed Placeholder Semantics</caption> | |
| 107 | <colgroup> | |
| 108 | <col width="48%" /> | |
| 109 | <col width="52%" /> | |
| 110 | </colgroup> | |
| 111 | <tbody valign="top"> | |
| 112 | <tr><td><pre class="first last literal-block"> | |
| 113 | mpl::plus<_,_> | |
| 114 | </pre> | |
| 115 | </td> | |
| 116 | <td><pre class="first last literal-block"> | |
| 117 | mpl::plus<_1,_2> | |
| 118 | </pre> | |
| 119 | </td> | |
| 120 | </tr> | |
| 121 | <tr><td><pre class="first last literal-block"> | |
| 122 | boost::is_same< | |
| 123 | _ | |
| 124 | , boost::add_pointer<_> | |
| 125 | > | |
| 126 | </pre> | |
| 127 | </td> | |
| 128 | <td><pre class="first last literal-block"> | |
| 129 | boost::is_same< | |
| 130 | _1 | |
| 131 | , boost::add_pointer<_1> | |
| 132 | > | |
| 133 | </pre> | |
| 134 | </td> | |
| 135 | </tr> | |
| 136 | <tr><td><pre class="first last literal-block"> | |
| 137 | mpl::multiplies< | |
| 138 | mpl::plus<_,_> | |
| 139 | , mpl::minus<_,_> | |
| 140 | > | |
| 141 | </pre> | |
| 142 | </td> | |
| 143 | <td><pre class="first last literal-block"> | |
| 144 | mpl::multiplies< | |
| 145 | mpl::plus<_1,_2> | |
| 146 | , mpl::minus<_1,_2> | |
| 147 | > | |
| 148 | </pre> | |
| 149 | </td> | |
| 150 | </tr> | |
| 151 | </tbody> | |
| 152 | </table> | |
| 153 | <!-- @ for n in range(len(stack)): | |
| 154 | stack[n].wrap('typedef ', 'type%d;' % n) | |
| 155 | compile('all') --> | |
| 156 | <p>Especially when used in simple lambda expressions, the unnamed | |
| 157 | placeholder often eliminates just enough syntactic "noise" to | |
| 158 | significantly improve readability.</p> | |
| 159 | </div> | |
| 160 | </div> | |
| 161 | ||
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| 164 | </tr></table></body> | |
| 165 | </html> |