<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>
<div class="refsect1">
-<a name="idp655739008"></a><h2>Description</h2>
+<a name="idp568075536"></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>
projects / platform / upstream / boost.git / blobdiff