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<title>Boost.Flyweight Documentation - Key-value flyweights reference</title>
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<h1><img src="../../../../boost.png" alt="Boost logo" align=
"middle" width="277" height="86">Key-value flyweights reference</h1>

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<h2>Contents</h2>

<ul>
  <li><a href="#key_extractor">Key extractors</a></li>
  <li><a href="#key_value_fwd_synopsis">Header
    <code>"boost/flyweight/key_value_fwd.hpp"</code> synopsis</a>
  </li>
  <li><a href="#synopsis">Header
    <code>"boost/flyweight/key_value.hpp"</code> synopsis</a>
    <ul>
      <li><a href="#key_value_construct">Class template <code>key_value</code></a></li>
    </ul>    
  </li>
</ul>

<h2><a name="key_extractor">Key extractors</a></h2>

<p>
Let <code>Key</code> be a type with some implicit equivalence relationship
and <code>Value</code> a type constructible from <code>Key</code>.
A <a href="https://boost.org/sgi/stl/DefaultConstructible.html"><code>Default
Constructible</code></a> type <code>KeyFromValue</code> is said
to be a key extractor from <code>Value</code> to <code>Key</code> if
<ol>
  <li><code>kfv(cv)</code> is defined and have type <code>const Key&amp;</code>,</li>
  <li><code>kfv(cv)</code> is equivalent to <code>kfv(Value(cv))</code>,</li>
  <li><code>kfv(Value(k))</code> is equivalent to <code>k</code>,</li>
</ol>
for every <code>kfv</code> of type <code>const KeyFromValue</code>,
<code>cv</code> of type <code>const Value</code> and
<code>k</code> of type <code>Key</code>.
</p>

<h2><a name="key_value_fwd_synopsis">Header
<a href="../../../../boost/flyweight/key_value_fwd.hpp"><code>"boost/flyweight/key_value_fwd.hpp"</code></a> synopsis</a></h2>

<blockquote><pre>
<span class=keyword>namespace</span> <span class=identifier>boost</span><span class=special>{</span>

<span class=keyword>namespace</span> <span class=identifier>flyweights</span><span class=special>{</span>

<span class=keyword>struct</span> <span class=identifier>no_key_from_value</span><span class=special>;</span>

<span class=keyword>template</span><span class=special>&lt;</span>
  <span class=keyword>typename</span> <span class=identifier>Key</span><span class=special>,</span><span class=keyword>typename</span> <span class=identifier>Value</span><span class=special>,</span>
  <span class=keyword>typename</span> <span class=identifier>KeyFromValue</span><span class=special>=</span><span class=identifier>no_key_from_value</span>
<span class=special>&gt;</span>
<span class=keyword>struct</span> <span class=identifier>key_value</span><span class=special>;</span>

<span class=special>}</span> <span class=comment>// namespace boost::flyweights</span>

<span class=special>}</span> <span class=comment>// namespace boost</span>
</pre></blockquote>

<h2><a name="synopsis">Header
<a href="../../../../boost/flyweight/key_value.hpp"><code>"boost/flyweight/key_value.hpp"</code></a> synopsis</a></h2>

<h3><a name="key_value_construct">Class template <code>key_value</code></a></h3>

<p>
In <a href="flyweight.html#flyweight"><code>flyweight</code></a> instantiations
of the form <code>flyweight&lt;T,...&gt;</code>, the associated
<code>key_type</code> and <code>value_type</code> are both equal to <code>T</code>.
Instantiations of the form <code>flyweight&lt;key_value&lt;Key,Value[,KeyFromValue]&gt;,...&gt;</code>
allow to specify these types separately. <code>Key</code> and <code>Value</code>
must be different types. When provided, <code>KeyFromValue</code>
must be a <a href="#key_extractor"><code>Key Extractor</code></a> from
<code>Value</code> to <code>Key</code>.
</p>

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<p>Revised April 24th 2019</p>

<p>&copy; Copyright 2006-2019 Joaqu&iacute;n M L&oacute;pez Mu&ntilde;oz.
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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