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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">
<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.require"></a><a class="link" href="require.html" title="require">require</a>
</h3></div></div></div>
<p>
<a class="indexterm" name="asio.indexterm.require"></a>
</p>
<p>
A customisation point that applies a concept-preserving property to an object.
</p>
<pre class="programlisting"><span class="keyword">constexpr</span> <span class="identifier">unspecified</span> <span class="identifier">require</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span>
</pre>
<p>
The name <code class="computeroutput"><span class="identifier">require</span></code> denotes
a customisation point object. The expression <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span>
<span class="identifier">P0</span><span class="special">,</span> <span class="identifier">Pn</span><span class="special">...)</span></code>
for some subexpressions <code class="computeroutput"><span class="identifier">E</span></code>
and <code class="computeroutput"><span class="identifier">P0</span></code>, and where <code class="computeroutput"><span class="identifier">Pn</span><span class="special">...</span></code> represents
<code class="computeroutput"><span class="identifier">N</span></code> subexpressions (where
<code class="computeroutput"><span class="identifier">N</span></code> is 0 or more, and with
types <code class="computeroutput"><span class="identifier">T</span> <span class="special">=</span>
<span class="identifier">decay_t</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">E</span><span class="special">)&gt;</span></code>
and <code class="computeroutput"><span class="identifier">Prop0</span> <span class="special">=</span>
<span class="identifier">decay_t</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">P0</span><span class="special">)&gt;</span></code>)
is expression-equivalent to:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If <code class="computeroutput"><span class="identifier">is_applicable_property_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Prop0</span><span class="special">&gt;</span> <span class="special">&amp;&amp;</span>
<span class="identifier">Prop0</span><span class="special">::</span><span class="identifier">is_requirable</span></code> is not a well-formed
constant expression with value <code class="computeroutput"><span class="keyword">true</span></code>,
<code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">,</span> <span class="identifier">Pn</span><span class="special">...)</span></code>
is ill-formed.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput"><span class="identifier">E</span></code> if <code class="computeroutput"><span class="identifier">N</span> <span class="special">==</span> <span class="number">0</span></code> and the expression <code class="computeroutput"><span class="identifier">Prop0</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">static_query_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">==</span> <span class="identifier">Prop0</span><span class="special">::</span><span class="identifier">value</span><span class="special">()</span></code> is a well-formed constant expression
with value <code class="computeroutput"><span class="keyword">true</span></code>.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">P0</span><span class="special">)</span></code> if <code class="computeroutput"><span class="identifier">N</span>
<span class="special">==</span> <span class="number">0</span></code>
and the expression <code class="computeroutput"><span class="special">(</span><span class="identifier">E</span><span class="special">).</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">P0</span><span class="special">)</span></code> is a valid expression.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput"><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">)</span></code>
if <code class="computeroutput"><span class="identifier">N</span> <span class="special">==</span>
<span class="number">0</span></code> and the expression <code class="computeroutput"><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">)</span></code>
is a valid expression with overload resolution performed in a context
that does not include the declaration of the <code class="computeroutput"><span class="identifier">require</span></code>
customization point object.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">),</span> <span class="identifier">Pn</span><span class="special">...)</span></code>
if <code class="computeroutput"><span class="identifier">N</span> <span class="special">&gt;</span>
<span class="number">0</span></code> and the expression <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">),</span> <span class="identifier">Pn</span><span class="special">...)</span></code>
is a valid expression.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">require</span><span class="special">(</span><span class="identifier">E</span><span class="special">,</span> <span class="identifier">P0</span><span class="special">,</span> <span class="identifier">Pn</span><span class="special">...)</span></code>
is ill-formed.
</li>
</ul></div>
<h5>
<a name="asio.reference.require.h0"></a>
<span><a name="asio.reference.require.requirements"></a></span><a class="link" href="require.html#asio.reference.require.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">asio/require.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
</p>
</div>
<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<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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