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<h2><span class="refentrytitle">Struct placeholder</span></h2>
<p>boost::type_erasure::placeholder</p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../boost_typeerasure/reference.html#header.boost.type_erasure.placeholder_hpp" title="Header <boost/type_erasure/placeholder.hpp>">boost/type_erasure/placeholder.hpp</a>>
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<span class="keyword">struct</span> <a class="link" href="placeholder.html" title="Struct placeholder">placeholder</a> <span class="special">{</span>
<span class="special">}</span><span class="special">;</span></pre></div>
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<a name="idm45555076097984"></a><h2>Description</h2>
<p>Placeholders are used heavily throughout the library. Every placeholder must derive from <a class="link" href="placeholder.html" title="Struct placeholder">placeholder</a>. The library provides a number of placeholders, out of the box, but you are welcome to define your own, if you want more descriptive names. The placeholder <a class="link" href="_self.html" title="Struct _self">_self</a> is special in that it is used as the default wherever possible.</p>
<p>What exactly is a placeholder? Placeholders act as a substitute for template parameters in concepts. The library automatically replaces all the placeholders used in a concept with the actual types involved when it stores an object in an <a class="link" href="any.html" title="Class template any">any</a>.</p>
<p>For example, in the following,</p>
<pre class="programlisting"><span class="identifier">any</span><span class="special"><</span><span class="identifier">copy_constructible</span><span class="special"><</span><span class="identifier">_a</span><span class="special">></span><span class="special">,</span> <span class="identifier">_a</span><span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">1</span><span class="special">)</span><span class="special">;</span>
</pre>
<p>The library sees that we're constructing an <a class="link" href="any.html" title="Class template any">any</a> that uses the <a class="link" href="_a.html" title="Struct _a">_a</a> placeholder with an <code class="computeroutput">int</code>. Thus it binds <a class="link" href="_a.html" title="Struct _a">_a</a> to int and instantiates <a class="link" href="copy_constructible.html" title="Struct template copy_constructible">copy_constructible<int></a>.</p>
<p>When there are multiple placeholders involved, you will have to use <a class="link" href="tuple.html" title="Class template tuple">tuple</a>, or pass the bindings explicitly, but the substitution still works the same way. </p>
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<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Steven Watanabe<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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