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<title>Boost.Flyweight Documentation - Tutorial - Annex - MPL lambda expressions</title>
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<h1><img src="../../../../boost.png" alt="Boost logo" align=
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"middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1>
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<div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
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Technical issues
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Boost.Flyweight reference
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</a></div><br clear="all" style="clear: all;">
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<hr>
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<p>This short introduction to lambda expressions is meant for readers unfamiliar
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with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who
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want to rapidly acquire a working knowledge of the basic concepts for the purposes
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of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation
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for further information beyond these introductory notes.
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</p>
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<p>
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The specifiers defined by Boost.Flyweight rely heavily on the
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<a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda
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Expression</code></a> concept defined by the
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<a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda
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expression can be thought of as a compile-time "type function", an entity (a
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concrete type, actually) that can be invoked with a list of types and returns
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some associated type in its turn. Consider for instance an arbitrary class
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template:
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</p>
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<blockquote><pre>
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<span class=keyword>template</span><span class=special><</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>></span>
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<span class=keyword>class</span> <span class=identifier>foo</span>
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<span class=special>{</span>
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<span class=special>...</span>
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<span class=special>};</span>
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</pre></blockquote>
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<p>
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and suppose we want to have a lambda expression that, when invoked
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with some generic types <code>Arg1</code> and <code>Arg2</code>,
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returns <code>foo<Arg1,Arg2></code>. Such a lambda expression
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can be implemented in two ways
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<ol>
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<li>As a
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<a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL
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Metafunction Class</code></a>, a type with a special nested class template
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named <code>apply</code>:
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
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<span class=special>{</span>
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<span class=keyword>template</span><span class=special><</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>
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<span class=keyword>struct</span> <span class=identifier>apply</span>
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<span class=special>{</span>
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<span class=comment>// this is the "return type" of foo_specifier</span>
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<span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>Arg2</span><span class=special>></span> <span class=identifier>type</span><span class=special>;</span>
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<span class=special>};</span>
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<span class=special>};</span>
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</pre></blockquote>
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</li>
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<li>
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As a
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<a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL
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Placeholder Expression</code></a>, a class template instantiated with one or
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more <i>placeholders</i>:
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<blockquote><pre>
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<span class=keyword>typedef</span> <span class=identifier>foo</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>,</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>foo_specifier</span><span class=special>;</span>
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</pre></blockquote>
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Note that, in this case, <code>foo_specifier</code> is a concrete type, much
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as <code>int</code> or <code>std::set<std::string></code> are; yet,
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MPL internal mechanisms are able to detect that this type has been gotten
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from instantiating a class template with placeholders <code>boost::mpl::_1</code>
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and <code>boost::mpl::_2</code> and take these placeholders as slots to
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be substituted for actual types (the first and second type supplied,
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respectively) when <code>foo_specifier</code> is
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invoked. So, an instantiation of <code>foo</code> can be used
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to refer back to the <code>foo</code> class template itself! The net
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effect is the same as with metafunctions, but placeholder expressions spare
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us the need to write boilerplate metafunction classes
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--and the kind of metaprogramming magic they depend on has an undeniable
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beauty to it.
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</li>
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</ol>
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So far the examples shown just forward the arguments <code>Arg1</code> and
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<code>Arg2</code> directly to a class template without further elaboration,
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but there is nothing preventing us from doing some argument manipulation,
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like, for instance, switching their places:
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
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<span class=special>{</span>
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<span class=keyword>template</span><span class=special><</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>
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<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><</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg1</span><span class=special>></span> <span class=identifier>type</span><span class=special>;};</span>
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<span class=special>};</span>
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<span class=keyword>typedef</span> <span class=identifier>foo</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>,</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_specifier</span><span class=special>;</span>
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</pre></blockquote>
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<p>
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passing placeholder subexpressions as arguments to the overall expression:
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
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<span class=special>{</span>
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<span class=keyword>template</span><span class=special><</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>
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<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><</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</span><span class=special><</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><</span><span class=identifier>Arg2</span><span class=special>></span> <span class=special>></span> <span class=identifier>type</span><span class=special>;};</span>
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<span class=special>};</span>
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<span class=keyword>typedef</span> <span class=identifier>foo</span><span class=special><</span>
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<span class=identifier>boost</span><span class=special>::</span><span class=identifier>shared_ptr</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>>,</span>
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<span class=identifier>std</span><span class=special>::</span><span class=identifier>less</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>></span>
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<span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span>
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</pre></blockquote>
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<p>
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or accepting less or more arguments than the class template itself
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(the number of parameters of a lambda expression is called its <i>arity</i>):
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</p>
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<blockquote><pre>
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<span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
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<span class=special>{</span>
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<span class=keyword>template</span><span class=special><</span><span class=keyword>typename</span> <span class=identifier>Arg1</span><span class=special>></span>
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<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><</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><</span><span class=identifier>Arg1</span><span class=special>></span> <span class=identifier>type</span><span class=special>;};</span>
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<span class=special>};</span>
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<span class=keyword>typedef</span> <span class=identifier>foo</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>,</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>less</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>></span> <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span>
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<span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
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<span class=special>{</span>
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<span class=keyword>template</span><span class=special><</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>></span>
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<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><</span><span class=identifier>Arg1</span><span class=special>,</span><span class=identifier>foo</span><span class=special><</span><span class=identifier>Arg2</span><span class=special>,</span><span class=identifier>Arg3</span><span class=special>></span> <span class=special>></span> <span class=identifier>type</span><span class=special>;};</span>
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<span class=special>};</span>
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<span class=keyword>typedef</span> <span class=identifier>foo</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>,</span><span class=identifier>foo</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>,</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>></span> <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span>
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</pre></blockquote>
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<hr>
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<div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
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Technical issues
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</a></div>
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Boost.Flyweight tutorial
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</a></div>
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Boost.Flyweight reference
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</a></div><br clear="all" style="clear: all;">
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<br>
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<p>Revised August 13th 2008</p>
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<p>© Copyright 2006-2008 Joaquín M López Muñoz.
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Distributed under the Boost Software
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License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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