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| <h1><img src="../../../../boost.png" alt="Boost logo" align= |
| "middle" width="277" height="86">Boost.Flyweight Tutorial Annex: MPL lambda expressions</h1> |
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| <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> |
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| <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>) |
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