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113 lines
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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="boost_asio.reference.prefer"></a><a class="link" href="prefer.html" title="prefer">prefer</a>
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</h3></div></div></div>
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<p>
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<a class="indexterm" name="boost_asio.indexterm.prefer"></a>
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A customisation point that attempts to apply
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a property to an object.
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</p>
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<pre class="programlisting">constexpr unspecified prefer = unspecified;
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</pre>
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<p>
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The name <code class="computeroutput">prefer</code> denotes a customisation point object. The expression
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<code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> for some subexpressions <code class="computeroutput">E</code>
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and <code class="computeroutput">P0</code>, and where <code class="computeroutput">Pn...</code> represents <code class="computeroutput">N</code>
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subexpressions (where <code class="computeroutput">N</code> is 0 or more, and with types <code class="computeroutput">T
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= decay_t<decltype(E)></code> and <code class="computeroutput">Prop0 = decay_t<decltype(P0)></code>)
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is expression-equivalent to:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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If <code class="computeroutput">is_applicable_property_v<T, Prop0> && Prop0::is_preferable</code>
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is not a well-formed constant expression with value <code class="computeroutput">true</code>,
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<code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> is ill-formed.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code> and the expression
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<code class="computeroutput">Prop0::template static_query_v<T> == Prop0::value()</code>
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is a well-formed constant expression with value <code class="computeroutput">true</code>.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">(E).require(P0)</code> if <code class="computeroutput">N == 0</code> and the
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expression <code class="computeroutput">(E).require(P0)</code> is a valid expression.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">require(E, P0)</code> if <code class="computeroutput">N == 0</code> and the
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expression <code class="computeroutput">require(E, P0)</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">require</code> customization point object.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">(E).prefer(P0)</code> if <code class="computeroutput">N == 0</code> and the
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expression <code class="computeroutput">(E).prefer(P0)</code> is a valid expression.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">prefer(E, P0)</code> if <code class="computeroutput">N == 0</code> and the
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expression <code class="computeroutput">prefer(E, P0)</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">prefer</code> customization point object.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code>.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">boost::asio::prefer(boost::asio::prefer(E, P0), Pn...)</code>
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if <code class="computeroutput">N > 0</code> and the expression <code class="computeroutput">boost::asio::prefer(boost::asio::prefer(E,
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P0), Pn...)</code> is a valid expression.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> 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="boost_asio.reference.prefer.h0"></a>
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<span class="phrase"><a name="boost_asio.reference.prefer.requirements"></a></span><a class="link" href="prefer.html#boost_asio.reference.prefer.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">boost/asio/prefer.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">boost/asio.hpp</code>
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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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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</p>
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