166 lines
7.9 KiB
HTML
166 lines
7.9 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: Placeholders</title>
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</head>
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<body class="docframe">
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Prev</a> <a href="./placeholder-expression.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./placeholder-expression.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./lambda-details.html" class="navigation-link">Lambda Details</a> / <a href="./placeholders.html" class="navigation-link">Placeholders</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="placeholders">
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<h1><a class="toc-backref" href="./lambda-details.html#id55" name="placeholders">Placeholders</a></h1>
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<p>The definition of "placeholder" may surprise you:</p>
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<div class="admonition-definition admonition">
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<p class="admonition-title first">Definition</p>
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<p>A <strong>placeholder</strong> is a metafunction class of the form <tt class="literal"><span class="pre">mpl::arg<X></span></tt>.</p>
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</div>
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<div class="section" id="implementation">
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<h2><a name="implementation">Implementation</a></h2>
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<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
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<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
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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
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implementation of placeholders looks something like this:</p>
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<table class="footnote" frame="void" id="config" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<tr><td class="label"><a class="fn-backref" href="#id12" name="config">[6]</a></td><td>MPL provides five placeholders by default. See
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the Configuration Macros section of <a class="reference" href="./reference-manual.html">the MPL reference manual</a> for a
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description of how to change the number of placeholders
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provided.</td></tr>
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</tbody>
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</table>
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<pre class="literal-block">
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namespace boost { namespace mpl { namespace placeholders {
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template <int N> struct arg; // forward declarations
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struct void_;
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template <>
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struct arg<<strong>1</strong>>
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{
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template <
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class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_>
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struct apply
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{
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typedef <strong>A1</strong> type; // return the first argument
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};
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};
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typedef <strong>arg<1> _1</strong>;
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template <>
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struct arg<<strong>2</strong>>
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{
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template <
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class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_
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>
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struct apply
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{
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typedef <strong>A2</strong> type; // return the second argument
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};
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};
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typedef <strong>arg<2> _2</strong>;
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<em>more specializations and typedefs...</em>
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}}}
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</pre>
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<!-- @example.replace('...','') -->
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<p>Remember that invoking a metafunction class is the same as invoking
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its nested <tt class="literal"><span class="pre">apply</span></tt> metafunction. When a placeholder in a lambda
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expression is evaluated, it is invoked on the expression's actual
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arguments, returning just one of them. The results are then
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substituted back into the lambda expression and the evaluation
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process continues.</p>
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</div>
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<div class="section" id="the-unnamed-placeholder">
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<h2><a name="the-unnamed-placeholder">The Unnamed Placeholder</a></h2>
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<p>There's one special placeholder, known as the <strong>unnamed
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placeholder</strong>, that we haven't yet defined:</p>
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<pre class="literal-block">
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namespace boost { namespace mpl { namespace placeholders {
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<strong>typedef arg<-1> _;</strong> // the unnamed placeholder
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}}}
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</pre>
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<!-- @ stack[-2].prepend('namespace shield {')
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example.append('}') # so we don't conflict with the prefix
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compile('all') -->
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<p>The details of its implementation aren't important; all you really
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need to know about the unnamed placeholder is that it gets special
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treatment. When a lambda expression is being transformed into a
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metafunction class by <tt class="literal"><span class="pre">mpl::lambda</span></tt>,</p>
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<blockquote>
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the <em>n</em>th appearance of the unnamed placeholder <em>in a given
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template specialization</em> is replaced with <tt class="literal"><span class="pre">_</span></tt><em>n</em>.</blockquote>
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<p>So, for example, every row of Table 1.1
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below contains two equivalent lambda expressions.</p>
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<table border="1" class="table">
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<caption>Unnamed Placeholder Semantics</caption>
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<colgroup>
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<col width="48%" />
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<col width="52%" />
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</colgroup>
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<tbody valign="top">
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<tr><td><pre class="first last literal-block">
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mpl::plus<_,_>
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</pre>
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</td>
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<td><pre class="first last literal-block">
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mpl::plus<_1,_2>
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</pre>
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</td>
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</tr>
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<tr><td><pre class="first last literal-block">
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boost::is_same<
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_
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, boost::add_pointer<_>
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>
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</pre>
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</td>
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<td><pre class="first last literal-block">
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boost::is_same<
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_1
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, boost::add_pointer<_1>
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>
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</pre>
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</td>
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</tr>
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<tr><td><pre class="first last literal-block">
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mpl::multiplies<
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mpl::plus<_,_>
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, mpl::minus<_,_>
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>
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</pre>
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</td>
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<td><pre class="first last literal-block">
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mpl::multiplies<
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mpl::plus<_1,_2>
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, mpl::minus<_1,_2>
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>
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</pre>
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</td>
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</tr>
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</tbody>
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</table>
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<!-- @ for n in range(len(stack)):
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stack[n].wrap('typedef ', 'type%d;' % n)
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compile('all') -->
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<p>Especially when used in simple lambda expressions, the unnamed
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placeholder often eliminates just enough syntactic "noise" to
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significantly improve readability.</p>
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</div>
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</div>
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