123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181 |
- <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN">
- <html>
- <head>
- <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
- <title>Boost.Flyweight Documentation - Tutorial - Annex - MPL lambda expressions</title>
- <link rel="stylesheet" href="../style.css" type="text/css">
- <link rel="start" href="../index.html">
- <link rel="prev" href="technical.html">
- <link rel="up" href="index.html">
- <link rel="next" href="../reference/index.html">
- </head>
- <body>
- <h1><img src="../../../../boost.png" alt="Boost logo" align=
- "middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1>
- <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
- Technical issues
- </a></div>
- <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br>
- Boost.Flyweight tutorial
- </a></div>
- <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br>
- Boost.Flyweight reference
- </a></div><br clear="all" style="clear: all;">
- <hr>
- <p>This short introduction to lambda expressions is meant for readers unfamiliar
- with the <a href="../../../mpl/doc/index.html">Boost MPL Library</a> who
- want to rapidly acquire a working knowledge of the basic concepts for the purposes
- of using them in Boost.Flyweight. Please refer to the Boost.MPL documentation
- for further information beyond these introductory notes.
- </p>
- <p>
- The specifiers defined by Boost.Flyweight rely heavily on the
- <a href="../../../mpl/doc/refmanual/lambda-expression.html"><code>Lambda
- Expression</code></a> concept defined by the
- <a href="../../../mpl/doc/index.html">Boost MPL Library</a>. A lambda
- expression can be thought of as a compile-time "type function", an entity (a
- concrete type, actually) that can be invoked with a list of types and returns
- some associated type in its turn. Consider for instance an arbitrary class
- template:
- </p>
- <blockquote><pre>
- <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>
- <span class=keyword>class</span> <span class=identifier>foo</span>
- <span class=special>{</span>
- <span class=special>...</span>
- <span class=special>};</span>
- </pre></blockquote>
- <p>
- and suppose we want to have a lambda expression that, when invoked
- with some generic types <code>Arg1</code> and <code>Arg2</code>,
- returns <code>foo<Arg1,Arg2></code>. Such a lambda expression
- can be implemented in two ways
- <ol>
- <li>As a
- <a href="../../../mpl/doc/refmanual/metafunction-class.html"><code>MPL
- Metafunction Class</code></a>, a type with a special nested class template
- named <code>apply</code>:
- <blockquote><pre>
- <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
- <span class=special>{</span>
- <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>struct</span> <span class=identifier>apply</span>
- <span class=special>{</span>
- <span class=comment>// this is the "return type" of foo_specifier</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>Arg2</span><span class=special>></span> <span class=identifier>type</span><span class=special>;</span>
- <span class=special>};</span>
- <span class=special>};</span>
- </pre></blockquote>
- </li>
- <li>
- As a
- <a href="../../../mpl/doc/refmanual/placeholder-expression.html"><code>MPL
- Placeholder Expression</code></a>, a class template instantiated with one or
- more <i>placeholders</i>:
- <blockquote><pre>
- <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>
- </pre></blockquote>
- Note that, in this case, <code>foo_specifier</code> is a concrete type, much
- as <code>int</code> or <code>std::set<std::string></code> are; yet,
- MPL internal mechanisms are able to detect that this type has been gotten
- from instantiating a class template with placeholders <code>boost::mpl::_1</code>
- and <code>boost::mpl::_2</code> and take these placeholders as slots to
- be substituted for actual types (the first and second type supplied,
- respectively) when <code>foo_specifier</code> is
- invoked. So, an instantiation of <code>foo</code> can be used
- to refer back to the <code>foo</code> class template itself! The net
- effect is the same as with metafunctions, but placeholder expressions spare
- us the need to write boilerplate metafunction classes
- --and the kind of metaprogramming magic they depend on has an undeniable
- beauty to it.
- </li>
- </ol>
- So far the examples shown just forward the arguments <code>Arg1</code> and
- <code>Arg2</code> directly to a class template without further elaboration,
- but there is nothing preventing us from doing some argument manipulation,
- like, for instance, switching their places:
- </p>
- <blockquote><pre>
- <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
- <span class=special>{</span>
- <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>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>
- <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>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>
- </pre></blockquote>
- <p>
- passing placeholder subexpressions as arguments to the overall expression:
- </p>
- <blockquote><pre>
- <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
- <span class=special>{</span>
- <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>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>
- <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>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>_2</span><span class=special>></span>
- <span class=special>></span> <span class=identifier>foo_specifier</span><span class=special>;</span>
- </pre></blockquote>
- <p>
- or accepting less or more arguments than the class template itself
- (the number of parameters of a lambda expression is called its <i>arity</i>):
- </p>
- <blockquote><pre>
- <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
- <span class=special>{</span>
- <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>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>
- <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>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>
- <span class=keyword>struct</span> <span class=identifier>foo_specifier</span>
- <span class=special>{</span>
- <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>
- <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>
- <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>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>
- </pre></blockquote>
- <hr>
- <div class="prev_link"><a href="technical.html"><img src="../prev.gif" alt="technical issues" border="0"><br>
- Technical issues
- </a></div>
- <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br>
- Boost.Flyweight tutorial
- </a></div>
- <div class="next_link"><a href="../reference/index.html"><img src="../next.gif" alt="Boost.Flyweight reference" border="0"><br>
- Boost.Flyweight reference
- </a></div><br clear="all" style="clear: all;">
- <br>
- <p>Revised August 13th 2008</p>
- <p>© Copyright 2006-2008 Joaquín M López Muñoz.
- Distributed under the Boost Software
- License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt">
- LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
- http://www.boost.org/LICENSE_1_0.txt</a>)
- </p>
- </body>
- </html>
|