89 lines
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89 lines
7.5 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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<link rel="stylesheet" href="../../boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.75.2">
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<link rel="home" href="../../index.html" title="Asio">
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<link rel="up" href="../reference.html" title="Reference">
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<link rel="prev" href="prepend_t/values_.html" title="prepend_t::values_">
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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="prepend_t/values_.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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</p>
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<p>
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A customisation point that queries the value of a property.
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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">query</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">query</span></code> denotes a
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customization point object. The expression <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">query</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></code> is not a well-formed constant expression
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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">query</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">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></code> if the expression <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></code>
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is a well-formed constant expression.
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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">query</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">query</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">query</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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if the expression <code class="computeroutput"><span class="identifier">query</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">query</span></code> customization
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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">query</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.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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<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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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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