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| 7 | <title>THE BOOST MPL LIBRARY: Placeholders</title> |
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| 13 | </tr></table><div class="header-separator"></div> |
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| 14 | <div class="section" id="placeholders"> |
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| 15 | <h1><a class="toc-backref" href="./lambda-details.html#id55" name="placeholders">Placeholders</a></h1> |
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| 16 | <p>The definition of "placeholder" may surprise you:</p> |
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| 17 | <div class="admonition-definition admonition"> |
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| 18 | <p class="admonition-title first">Definition</p> |
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| 19 | <p>A <strong>placeholder</strong> is a metafunction class of the form <tt class="literal"><span class="pre">mpl::arg<X></span></tt>.</p> |
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| 20 | </div> |
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| 21 | <div class="section" id="implementation"> |
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| 22 | <h2><a name="implementation">Implementation</a></h2> |
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| 23 | <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 |
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| 24 | <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 |
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| 25 | 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 |
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| 26 | implementation of placeholders looks something like this:</p> |
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| 27 | <table class="footnote" frame="void" id="config" rules="none"> |
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| 28 | <colgroup><col class="label" /><col /></colgroup> |
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| 29 | <tbody valign="top"> |
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| 30 | <tr><td class="label"><a class="fn-backref" href="#id12" name="config">[6]</a></td><td>MPL provides five placeholders by default. See |
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| 31 | the Configuration Macros section of <a class="reference" href="./reference-manual.html">the MPL reference manual</a> for a |
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| 32 | description of how to change the number of placeholders |
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| 33 | provided.</td></tr> |
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| 34 | </tbody> |
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| 35 | </table> |
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| 36 | <pre class="literal-block"> |
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| 37 | namespace boost { namespace mpl { namespace placeholders { |
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| 38 | |
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| 39 | template <int N> struct arg; // forward declarations |
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| 40 | struct void_; |
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| 41 | |
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| 42 | template <> |
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| 43 | struct arg<<strong>1</strong>> |
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| 44 | { |
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| 45 | template < |
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| 46 | class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_> |
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| 47 | struct apply |
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| 48 | { |
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| 49 | typedef <strong>A1</strong> type; // return the first argument |
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| 50 | }; |
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| 51 | }; |
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| 52 | typedef <strong>arg<1> _1</strong>; |
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| 53 | |
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| 54 | template <> |
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| 55 | struct arg<<strong>2</strong>> |
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| 56 | { |
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| 57 | template < |
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| 58 | class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_ |
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| 59 | > |
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| 60 | struct apply |
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| 61 | { |
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| 62 | typedef <strong>A2</strong> type; // return the second argument |
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| 63 | }; |
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| 64 | }; |
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| 65 | typedef <strong>arg<2> _2</strong>; |
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| 66 | |
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| 67 | <em>more specializations and typedefs...</em> |
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| 68 | |
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| 69 | }}} |
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| 70 | </pre> |
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| 71 | <!-- @example.replace('...','') --> |
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| 72 | <p>Remember that invoking a metafunction class is the same as invoking |
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| 73 | its nested <tt class="literal"><span class="pre">apply</span></tt> metafunction. When a placeholder in a lambda |
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| 74 | expression is evaluated, it is invoked on the expression's actual |
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| 75 | arguments, returning just one of them. The results are then |
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| 76 | substituted back into the lambda expression and the evaluation |
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| 77 | process continues.</p> |
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| 78 | </div> |
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| 79 | <div class="section" id="the-unnamed-placeholder"> |
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| 80 | <h2><a name="the-unnamed-placeholder">The Unnamed Placeholder</a></h2> |
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| 81 | <p>There's one special placeholder, known as the <strong>unnamed |
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| 82 | placeholder</strong>, that we haven't yet defined:</p> |
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| 83 | <pre class="literal-block"> |
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| 84 | namespace boost { namespace mpl { namespace placeholders { |
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| 85 | |
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| 86 | <strong>typedef arg<-1> _;</strong> // the unnamed placeholder |
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| 87 | |
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| 88 | }}} |
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| 89 | </pre> |
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| 90 | <!-- @ stack[-2].prepend('namespace shield {') |
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| 91 | example.append('}') # so we don't conflict with the prefix |
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| 92 | compile('all') --> |
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| 93 | <p>The details of its implementation aren't important; all you really |
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| 94 | need to know about the unnamed placeholder is that it gets special |
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| 95 | treatment. When a lambda expression is being transformed into a |
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| 96 | metafunction class by <tt class="literal"><span class="pre">mpl::lambda</span></tt>,</p> |
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| 97 | <blockquote> |
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| 98 | the <em>n</em>th appearance of the unnamed placeholder <em>in a given |
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| 99 | template specialization</em> is replaced with <tt class="literal"><span class="pre">_</span></tt><em>n</em>.</blockquote> |
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| 100 | <p>So, for example, every row of Table 1.1 |
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| 101 | below contains two equivalent lambda expressions.</p> |
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| 102 | <table border="1" class="table"> |
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| 103 | <caption>Unnamed Placeholder Semantics</caption> |
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| 104 | <colgroup> |
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| 105 | <col width="48%" /> |
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| 106 | <col width="52%" /> |
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| 107 | </colgroup> |
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| 108 | <tbody valign="top"> |
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| 109 | <tr><td><pre class="first last literal-block"> |
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| 110 | mpl::plus<_,_> |
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| 111 | </pre> |
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| 112 | </td> |
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| 113 | <td><pre class="first last literal-block"> |
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| 114 | mpl::plus<_1,_2> |
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| 115 | </pre> |
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| 116 | </td> |
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| 117 | </tr> |
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| 118 | <tr><td><pre class="first last literal-block"> |
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| 119 | boost::is_same< |
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| 120 | _ |
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| 121 | , boost::add_pointer<_> |
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| 122 | > |
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| 123 | </pre> |
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| 124 | </td> |
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| 125 | <td><pre class="first last literal-block"> |
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| 126 | boost::is_same< |
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| 127 | _1 |
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| 128 | , boost::add_pointer<_1> |
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| 129 | > |
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| 130 | </pre> |
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| 131 | </td> |
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| 132 | </tr> |
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| 133 | <tr><td><pre class="first last literal-block"> |
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| 134 | mpl::multiplies< |
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| 135 | mpl::plus<_,_> |
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| 136 | , mpl::minus<_,_> |
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| 137 | > |
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| 138 | </pre> |
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| 139 | </td> |
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| 140 | <td><pre class="first last literal-block"> |
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| 141 | mpl::multiplies< |
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| 142 | mpl::plus<_1,_2> |
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| 143 | , mpl::minus<_1,_2> |
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| 144 | > |
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| 145 | </pre> |
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| 146 | </td> |
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| 147 | </tr> |
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| 148 | </tbody> |
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| 149 | </table> |
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| 150 | <!-- @ for n in range(len(stack)): |
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| 151 | stack[n].wrap('typedef ', 'type%d;' % n) |
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| 152 | compile('all') --> |
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| 153 | <p>Especially when used in simple lambda expressions, the unnamed |
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| 154 | placeholder often eliminates just enough syntactic "noise" to |
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| 155 | significantly improve readability.</p> |
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| 156 | </div> |
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| 157 | </div> |
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| 158 | |
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