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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="asio.reference.require_concept"></a><a class="link" href="require_concept.html" title="require_concept">require_concept</a>
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</h3></div></div></div>
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<p>
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<a class="indexterm" name="asio.indexterm.require_concept"></a>
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</p>
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<p>
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A customisation point that applies a concept-enforcing property to an object.
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</p>
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<pre class="programlisting"><span class="keyword">constexpr</span> <span class="identifier">unspecified</span> <span class="identifier">require_concept</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span>
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</pre>
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<p>
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The name <code class="computeroutput"><span class="identifier">require_concept</span></code>
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denotes a customization point object. The expression <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span>
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<span class="identifier">P</span><span class="special">)</span></code>
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for some subexpressions <code class="computeroutput"><span class="identifier">E</span></code>
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and <code class="computeroutput"><span class="identifier">P</span></code> (with types <code class="computeroutput"><span class="identifier">T</span> <span class="special">=</span> <span class="identifier">decay_t</span><span class="special"><</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">E</span><span class="special">)></span></code>
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and <code class="computeroutput"><span class="identifier">Prop</span> <span class="special">=</span>
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<span class="identifier">decay_t</span><span class="special"><</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">P</span><span class="special">)></span></code>)
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is expression-equivalent to:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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If <code class="computeroutput"><span class="identifier">is_applicable_property_v</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Prop</span><span class="special">></span> <span class="special">&&</span>
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<span class="identifier">Prop</span><span class="special">::</span><span class="identifier">is_requirable_concept</span></code> is not a well-formed
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constant expression with value <code class="computeroutput"><span class="keyword">true</span></code>,
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<code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code>
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is ill-formed.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput"><span class="identifier">E</span></code> if the expression
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<code class="computeroutput"><span class="identifier">Prop</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">static_query_v</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">==</span> <span class="identifier">Prop</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code>
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is a well-formed constant expression with value <code class="computeroutput"><span class="keyword">true</span></code>.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">P</span><span class="special">)</span></code> if the expression <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">P</span><span class="special">)</span></code>
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is well-formed.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput"><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code> if the expression <code class="computeroutput"><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code> is a valid expression with overload
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resolution performed in a context that does not include the declaration
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of the <code class="computeroutput"><span class="identifier">require_concept</span></code>
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customization point object.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require_concept</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P</span><span class="special">)</span></code>
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is ill-formed.
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</li>
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</ul></div>
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<h5>
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<a name="asio.reference.require_concept.h0"></a>
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<span><a name="asio.reference.require_concept.requirements"></a></span><a class="link" href="require_concept.html#asio.reference.require_concept.requirements">Requirements</a>
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</h5>
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<p>
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<span class="emphasis"><em>Header: </em></span><code class="literal">asio/require_concept.hpp</code>
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</p>
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<p>
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<span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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