1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>prefer</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../reference.html" title="Reference"> 9<link rel="prev" href="post/overload3.html" title="post (3 of 3 overloads)"> 10<link rel="next" href="prefer_result.html" title="prefer_result"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 15<td align="center"><a href="../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="post/overload3.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="prefer_result.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_asio.reference.prefer"></a><a class="link" href="prefer.html" title="prefer">prefer</a> 28</h3></div></div></div> 29<p> 30 <a class="indexterm" name="boost_asio.indexterm.prefer"></a> 31A customisation point that attempts to apply 32 a property to an object. 33 </p> 34<pre class="programlisting">constexpr unspecified prefer = unspecified; 35</pre> 36<p> 37 The name <code class="computeroutput">prefer</code> denotes a customisation point object. The expression 38 <code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> for some subexpressions <code class="computeroutput">E</code> 39 and <code class="computeroutput">P0</code>, and where <code class="computeroutput">Pn...</code> represents <code class="computeroutput">N</code> 40 subexpressions (where <code class="computeroutput">N</code> is 0 or more, and with types <code class="computeroutput">T 41 = decay_t<decltype(E)></code> and <code class="computeroutput">Prop0 = decay_t<decltype(P0)></code>) 42 is expression-equivalent to: 43 </p> 44<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 45<li class="listitem"> 46 If <code class="computeroutput">is_applicable_property_v<T, Prop0> && Prop0::is_preferable</code> 47 is not a well-formed constant expression with value <code class="computeroutput">true</code>, 48 <code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> is ill-formed. 49 </li> 50<li class="listitem"> 51 Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code> and the expression 52 <code class="computeroutput">Prop0::template static_query_v<T> == Prop0::value()</code> 53 is a well-formed constant expression with value <code class="computeroutput">true</code>. 54 </li> 55<li class="listitem"> 56 Otherwise, <code class="computeroutput">(E).require(P0)</code> if <code class="computeroutput">N == 0</code> and the 57 expression <code class="computeroutput">(E).require(P0)</code> is a valid expression. 58 </li> 59<li class="listitem"> 60 Otherwise, <code class="computeroutput">require(E, P0)</code> if <code class="computeroutput">N == 0</code> and the 61 expression <code class="computeroutput">require(E, P0)</code> is a valid expression with overload 62 resolution performed in a context that does not include the declaration 63 of the <code class="computeroutput">require</code> customization point object. 64 </li> 65<li class="listitem"> 66 Otherwise, <code class="computeroutput">(E).prefer(P0)</code> if <code class="computeroutput">N == 0</code> and the 67 expression <code class="computeroutput">(E).prefer(P0)</code> is a valid expression. 68 </li> 69<li class="listitem"> 70 Otherwise, <code class="computeroutput">prefer(E, P0)</code> if <code class="computeroutput">N == 0</code> and the 71 expression <code class="computeroutput">prefer(E, P0)</code> is a valid expression with overload 72 resolution performed in a context that does not include the declaration 73 of the <code class="computeroutput">prefer</code> customization point object. 74 </li> 75<li class="listitem"> 76 Otherwise, <code class="computeroutput">E</code> if <code class="computeroutput">N == 0</code>. 77 </li> 78<li class="listitem"> 79 Otherwise, <code class="computeroutput">boost::asio::prefer(boost::asio::prefer(E, P0), Pn...)</code> 80 if <code class="computeroutput">N > 0</code> and the expression <code class="computeroutput">boost::asio::prefer(boost::asio::prefer(E, 81 P0), Pn...)</code> is a valid expression. 82 </li> 83<li class="listitem"> 84 Otherwise, <code class="computeroutput">boost::asio::prefer(E, P0, Pn...)</code> is ill-formed. 85 </li> 86</ul></div> 87<h5> 88<a name="boost_asio.reference.prefer.h0"></a> 89 <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> 90 </h5> 91<p> 92 <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/prefer.hpp</code> 93 </p> 94<p> 95 <span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code> 96 </p> 97</div> 98<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 99<td align="left"></td> 100<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 101 Kohlhoff<p> 102 Distributed under the Boost Software License, Version 1.0. (See accompanying 103 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>) 104 </p> 105</div></td> 106</tr></table> 107<hr> 108<div class="spirit-nav"> 109<a accesskey="p" href="post/overload3.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="prefer_result.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 110</div> 111</body> 112</html> 113
