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87 lines
4.6 KiB
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>query</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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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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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="prefer_result/type.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="query_result.html"><img src="../../next.png" alt="Next"></a>
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</div>
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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.query"></a><a class="link" href="query.html" title="query">query</a>
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</h3></div></div></div>
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<p>
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<a class="indexterm" name="asio.indexterm.query"></a>
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A customisation point that queries the value of
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a property.
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</p>
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<pre class="programlisting">constexpr unspecified query = unspecified;
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</pre>
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<p>
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The name <code class="computeroutput">query</code> denotes a customization point object. The expression
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<code class="computeroutput">asio::query(E, P)</code> for some subexpressions <code class="computeroutput">E</code> and
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<code class="computeroutput">P</code> (with types <code class="computeroutput">T = decay_t<decltype(E)></code> and
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<code class="computeroutput">Prop = decay_t<decltype(P)></code>) is expression-equivalent
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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">is_applicable_property_v<T, Prop></code> is not a well-formed
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constant expression with value <code class="computeroutput">true</code>, <code class="computeroutput">asio::query(E,
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P)</code> is ill-formed.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">Prop::template static_query_v<T></code> if the
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expression <code class="computeroutput">Prop::template static_query_v<T></code> is a well-formed
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constant expression.
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</li>
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<li class="listitem">
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Otherwise, <code class="computeroutput">(E).query(P)</code> if the expression <code class="computeroutput">(E).query(P)</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">query(E, P)</code> if the expression <code class="computeroutput">query(E,
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P)</code> is a valid expression with overload resolution performed in
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a context that does not include the declaration of the <code class="computeroutput">query</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">asio::query(E, P)</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="asio.reference.query.h0"></a>
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<span><a name="asio.reference.query.requirements"></a></span><a class="link" href="query.html#asio.reference.query.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/query.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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<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 © 2003-2020 Christopher M.
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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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</div></td>
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</tr></table>
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