ceph/src/boost/libs/flyweight/doc/tutorial/lambda_expressions.html

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  15 <h1><img src="../../../../boost.png" alt="Boost logo" align=
  16 "middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1>
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  30 <p>This short introduction to lambda expressions is meant for readers unfamiliar
  31 with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who
  32 want to rapidly acquire a working knowledge of the basic concepts for the purposes
  33 of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation
  34 for further information beyond these introductory notes.
  35 </p>
  36
  37 <p>
  38 The specifiers defined by Boost.Flyweight rely heavily on the
  39 <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda
  40 Expression</code></a> concept defined by the
  41 <a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda
  42 expression can be thought of as a compile-time "type function", an entity (a
  43 concrete type, actually) that can be invoked with a list of types and returns
  44 some associated type in its turn. Consider for instance an arbitrary class
  45 template:
  46 </p>
  47
  48 <blockquote><pre>
  49 <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>T</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Q</span><span class=special>&gt;</span>
  50 <span class=keyword>class</span> <span class=identifier>foo</span>
  51 <span class=special>{</span>
  52   <span class=special>...</span>
  53 <span class=special>};</span>
  54 </pre></blockquote>
  55
  56 <p>
  57 and suppose we want to have a lambda expression that, when invoked
  58 with some generic types <code>Arg1</code> and <code>Arg2</code>,
  59 returns <code>foo&lt;Arg1,Arg2&gt;</code>. Such a lambda expression
  60 can be implemented in two ways
  61 <ol>
  62   <li>As a
  63     <a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL
  64     Metafunction Class</code></a>, a type with a special nested class template
  65     named <code>apply</code>:
  66 <blockquote><pre>
  67 <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
  68 <span class=special>{</span>
  69   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
  70   <span class=keyword>struct</span> <span class=identifier>apply</span>
  71   <span class=special>{</span>
  72     <span class=comment>// this is the &quot;return type&quot; of foo_specifier</span>
  73     <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>Arg2</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;</span>
  74   <span class=special>};</span>
  75 <span class=special>};</span>
  76 </pre></blockquote>
  77   </li>
  78   <li>
  79     As a
  80     <a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL
  81     Placeholder Expression</code></a>, a class template instantiated with one or
  82     more <i>placeholders</i>:
  83 <blockquote><pre>
  84 <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
  85 </pre></blockquote>
  86     Note that, in this case, <code>foo_specifier</code> is a concrete type, much
  87     as <code>int</code> or <code>std::set&lt;std::string&gt;</code> are; yet,
  88     MPL internal mechanisms are able to detect that this type has been gotten
  89     from instantiating a class template with placeholders <code>boost::mpl::_1</code>
  90     and <code>boost::mpl::_2</code> and take these placeholders as slots to
  91     be substituted for actual types (the first and second type supplied,
  92     respectively) when <code>foo_specifier</code> is
  93     invoked. So, an instantiation of <code>foo</code> can be used
  94     to refer back to the <code>foo</code> class template itself! The net
  95     effect is the same as with metafunctions, but placeholder expressions spare
  96     us the need to write boilerplate metafunction classes
  97     --and the kind of metaprogramming magic they depend on has an undeniable
  98     beauty to it.
  99   </li>
 100 </ol>
 101 So far the examples shown just forward the arguments <code>Arg1</code> and
 102 <code>Arg2</code> directly to a class template without further elaboration,
 103 but there is nothing preventing us from doing some argument manipulation,
 104 like, for instance, switching their places:
 105 </p>
 106
 107 <blockquote><pre>
 108 <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
 109 <span class=special>{</span>
 110   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
 111   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg1</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
 112 <span class=special>};</span>
 113
 114 <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
 115 </pre></blockquote>
 116
 117 <p>
 118 passing placeholder subexpressions as arguments to the overall expression:
 119 </p>
 120
 121 <blockquote><pre>
 122 <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
 123 <span class=special>{</span>
 124   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>&gt;</span>
 125   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>&gt;,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
 126 <span class=special>};</span>
 127
 128 <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span>
 129   <span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;,</span>
 130   <span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>&gt;</span>
 131 <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
 132 </pre></blockquote>
 133
 134 <p>
 135 or accepting less or more arguments than the class template itself
 136 (the number of parameters of a lambda expression is called its <i>arity</i>):
 137 </p>
 138
 139 <blockquote><pre>
 140 <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
 141 <span class=special>{</span>
 142   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>&gt;</span>
 143   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
 144 <span class=special>};</span>
 145
 146 <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
 147
 148 <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
 149 <span class=special>{</span>
 150   <span class=keyword>template</span><span class=special>&lt;</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg2</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Arg3</span><span class=special>&gt;</span>
 151   <span class=keyword>struct</span> <span class=identifier>apply</span><span class=special>{</span><span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg3</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>type</span><span class=special>;};</span>
 152 <span class=special>};</span>
 153
 154 <span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_1</span><span class=special>,</span><span class=identifier>foo</span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_2</span><span class=special>,</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>mpl</span><span class=special>::</span><span class=identifier>_3</span><span class=special>&gt;</span> <span class=special>&gt;</span> <span class=identifier>foo_specifier</span><span class=special>;</span>
 155 </pre></blockquote>
 156
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 169 <br>
 170
 171 <p>Revised August 13th 2008</p>
 172
 173 <p>&copy; Copyright 2006-2008 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
 174 Distributed under the Boost Software
 175 License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
 176 LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
 177 http://www.boost.org/LICENSE_1_0.txt</a>)
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