ceph/src/boost/libs/mpl/doc/tutorial/placeholders.html

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  10 <title>THE BOOST MPL LIBRARY: Placeholders</title>
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  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 &quot;placeholder&quot; 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&lt;X&gt;</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 &lt;int N&gt; struct arg; // forward declarations
  43 struct void_;
  44
  45 template &lt;&gt;
  46 struct arg&lt;<strong>1</strong>&gt;
  47 {
  48     template &lt;
  49       class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_&gt;
  50     struct apply
  51     {
  52         typedef <strong>A1</strong> type; // return the first argument
  53     };
  54 };
  55 typedef <strong>arg&lt;1&gt; _1</strong>;
  56
  57 template &lt;&gt;
  58 struct arg&lt;<strong>2</strong>&gt;
  59 {
  60     template &lt;
  61       class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_
  62     &gt;
  63     struct apply
  64     {
  65         typedef <strong>A2</strong> type; // return the second argument
  66     };
  67 };
  68 typedef <strong>arg&lt;2&gt; _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&lt;-1&gt; _;</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&lt;_,_&gt;
 114 </pre>
 115 </td>
 116 <td><pre class="first last literal-block">
 117 mpl::plus&lt;_1,_2&gt;
 118 </pre>
 119 </td>
 120 </tr>
 121 <tr><td><pre class="first last literal-block">
 122 boost::is_same&lt;
 123     _
 124   , boost::add_pointer&lt;_&gt;
 125 &gt;
 126 </pre>
 127 </td>
 128 <td><pre class="first last literal-block">
 129 boost::is_same&lt;
 130     _1
 131   , boost::add_pointer&lt;_1&gt;
 132 &gt;
 133 </pre>
 134 </td>
 135 </tr>
 136 <tr><td><pre class="first last literal-block">
 137 mpl::multiplies&lt;
 138    mpl::plus&lt;_,_&gt;
 139  , mpl::minus&lt;_,_&gt;
 140 &gt;
 141 </pre>
 142 </td>
 143 <td><pre class="first last literal-block">
 144 mpl::multiplies&lt;
 145    mpl::plus&lt;_1,_2&gt;
 146  , mpl::minus&lt;_1,_2&gt;
 147 &gt;
 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 &quot;noise&quot; to
 158 significantly improve readability.</p>
 159 </div>
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