prefer.html
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.prefer"></a><a class="link" href="prefer.html" title="prefer">prefer</a>
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<p>
<a class="indexterm" name="asio.indexterm.prefer"></a>
A customisation point that attempts to apply a
property to an object.
</p>
<pre class="programlisting">constexpr unspecified prefer = unspecified;
</pre>
<p>
The name <code class="computeroutput">prefer</code> denotes a customisation point object. The expression
<code class="computeroutput">asio::prefer(E, P0, Pn...)</code> for some subexpressions <code class="computeroutput">E</code>
and <code class="computeroutput">P0</code>, and where <code class="computeroutput">Pn...</code> represents <code class="computeroutput">N</code>
subexpressions (where <code class="computeroutput">N</code> is 0 or more, and with types <code class="computeroutput">T
= decay_t<decltype(E)></code> and <code class="computeroutput">Prop0 = decay_t<decltype(P0)></code>)
is expression-equivalent to:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If <code class="computeroutput">is_applicable_property_v<T, Prop0> && Prop0::is_preferable</code>
is not a well-formed constant expression with value <code class="computeroutput">true</code>,
<code class="computeroutput">asio::prefer(E, P0, Pn...)</code> is ill-formed.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code> and the expression
<code class="computeroutput">Prop0::template static_query_v<T> == Prop0::value()</code>
is a well-formed constant expression with value <code class="computeroutput">true</code>.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">(E).require(P0)</code> if <code class="computeroutput">N == 0</code> and the
expression <code class="computeroutput">(E).require(P0)</code> is a valid expression.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">require(E, P0)</code> if <code class="computeroutput">N == 0</code> and the
expression <code class="computeroutput">require(E, P0)</code> is a valid expression with overload
resolution performed in a context that does not include the declaration
of the <code class="computeroutput">require</code> customization point object.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">(E).prefer(P0)</code> if <code class="computeroutput">N == 0</code> and the
expression <code class="computeroutput">(E).prefer(P0)</code> is a valid expression.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">prefer(E, P0)</code> if <code class="computeroutput">N == 0</code> and the
expression <code class="computeroutput">prefer(E, P0)</code> is a valid expression with overload
resolution performed in a context that does not include the declaration
of the <code class="computeroutput">prefer</code> customization point object.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code>.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">asio::prefer(asio::prefer(E, P0), Pn...)</code> if
<code class="computeroutput">N > 0</code> and the expression <code class="computeroutput">asio::prefer(asio::prefer(E,
P0), Pn...)</code> is a valid expression.
</li>
<li class="listitem">
Otherwise, <code class="computeroutput">asio::prefer(E, P0, Pn...)</code> is ill-formed.
</li>
</ul></div>
<h5>
<a name="asio.reference.prefer.h0"></a>
<span><a name="asio.reference.prefer.requirements"></a></span><a class="link" href="prefer.html#asio.reference.prefer.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">asio/prefer.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio.hpp</code>
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 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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