1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Declval</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="declval.html" title="Declval"> 8</head> 9<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 10<table cellpadding="2" width="100%"><tr> 11<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 12<td align="center"><a href="../../../../index.html">Home</a></td> 13<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 14<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 15<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 16<td align="center"><a href="../../../../more/index.htm">More</a></td> 17</tr></table> 18<hr> 19<div class="spirit-nav"></div> 20<div class="article"> 21<div class="titlepage"> 22<div> 23<div><h2 class="title"> 24<a name="declval"></a>Declval</h2></div> 25<div><div class="authorgroup"> 26<div class="author"><h3 class="author"> 27<span class="firstname">Howard</span> <span class="surname">Hinnant</span> 28</h3></div> 29<div class="author"><h3 class="author"> 30<span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span> 31</h3></div> 32</div></div> 33<div><p class="copyright">Copyright © 2008 Howard Hinnant</p></div> 34<div><p class="copyright">Copyright © 2009-2012 Vicente 35 J. Botet Escriba</p></div> 36<div><div class="legalnotice"> 37<a name="declval.legal"></a><p> 38 Distributed under the Boost Software License, Version 1.0. (See accompanying 39 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>) 40 </p> 41</div></div> 42</div> 43<hr> 44</div> 45<div class="toc"> 46<p><b>Table of Contents</b></p> 47<dl class="toc"> 48<dt><span class="section"><a href="declval.html#declval.overview">Overview</a></span></dt> 49<dt><span class="section"><a href="declval.html#declval.reference">Reference </a></span></dt> 50<dt><span class="section"><a href="declval.html#declval.history">History</a></span></dt> 51</dl> 52</div> 53<div class="section"> 54<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 55<a name="declval.overview"></a><a class="link" href="declval.html#declval.overview" title="Overview">Overview</a> 56</h2></div></div></div> 57<p> 58 The motivation for <code class="computeroutput"><span class="identifier">declval</span></code> 59 was introduced in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value" target="_top">N2958: 60 Moving Swap Forward</a>. Here follows a rewording of this chapter. 61 </p> 62<p> 63 With the provision of decltype, late-specified return types, and default template-arguments 64 for function templates a new generation of SFINAE patterns will emerge to at 65 least partially compensate the lack of concepts on the C++0x timescale. Using 66 this technique, it is sometimes necessary to obtain an object of a known type 67 in a non-using context, e.g. given the declaration 68 </p> 69<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 70<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used</span> 71</pre> 72<p> 73 as part of the function template declaration 74 </p> 75<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span> 76<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span> 77</pre> 78<p> 79 or as part of a class template definition 80 </p> 81<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">result_of</span><span class="special">;</span> 82 83<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Fn</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">ArgTypes</span><span class="special">></span> 84<span class="keyword">struct</span> <span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">(</span><span class="identifier">ArgTypes</span><span class="special">...)></span> 85<span class="special">{</span> 86 <span class="keyword">typedef</span> <span class="keyword">decltype</span><span class="special">(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">>()(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">ArgTypes</span><span class="special">>()...))</span> <span class="identifier">type</span><span class="special">;</span> 87<span class="special">};</span> 88</pre> 89<p> 90 The role of the function template declval() is a transformation of a type T 91 into a value without using or evaluating this function. The name is supposed 92 to direct the reader's attention to the fact that the expression <code class="computeroutput"><span class="identifier">declval</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span></code> is 93 an lvalue if and only if T is an lvalue-reference, otherwise an rvalue. To 94 extend the domain of this function we can do a bit better by changing its declaration 95 to 96 </p> 97<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 98<span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used</span> 99</pre> 100<p> 101 which ensures that we can also use cv void as template parameter. The careful 102 reader might have noticed that <code class="computeroutput"><span class="identifier">declval</span><span class="special">()</span></code> already exists under the name create() as 103 part of the definition of the semantics of the type trait is_convertible in 104 the C++0x standard. 105 </p> 106<p> 107 The provision of a new library component that allows the production of values 108 in unevaluated expressions is considered important to realize constrained templates 109 in C++0x where concepts are not available. This extremely light-weight function 110 is expected to be part of the daily tool-box of the C++0x programmer. 111 </p> 112</div> 113<div class="section"> 114<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 115<a name="declval.reference"></a><a class="link" href="declval.html#declval.reference" title="Reference">Reference </a> 116</h2></div></div></div> 117<p> 118 <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">declval</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 119 </p> 120<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> 121 122 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 123 <span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">()</span> <span class="keyword">noexcept</span><span class="special">;</span> <span class="comment">// as unevaluated operand</span> 124 125<span class="special">}</span> <span class="comment">// namespace boost</span> 126</pre> 127<p> 128 The library provides the function template declval to simplify the definition 129 of expressions which occur as unevaluated operands. 130 </p> 131<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 132<span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> 133</pre> 134<p> 135 <span class="bold"><strong>Remarks:</strong></span> If this function is used, the program 136 is ill-formed. 137 </p> 138<p> 139 <span class="bold"><strong>Remarks:</strong></span> The template parameter T of declval 140 may be an incomplete type. 141 </p> 142<p> 143 <span class="bold"><strong>Example:</strong></span> 144 </p> 145<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span> 146<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span> 147</pre> 148<p> 149 Declares a function template convert which only participates in overloading 150 if the type From can be explicitly converted to type To. 151 </p> 152</div> 153<div class="section"> 154<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 155<a name="declval.history"></a><a class="link" href="declval.html#declval.history" title="History">History</a> 156</h2></div></div></div> 157<h4> 158<a name="declval.history.h0"></a> 159 <span class="phrase"><a name="declval.history.boost_1_50"></a></span><a class="link" href="declval.html#declval.history.boost_1_50">boost 160 1.50</a> 161 </h4> 162<p> 163 Fixes: 164 </p> 165<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 166 <a href="http://svn.boost.org/trac/boost/ticket/6570" target="_top">#6570</a> 167 Adding noexcept to boost::declval. 168 </li></ul></div> 169</div> 170</div> 171<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 172<td align="left"><p><small>Last revised: August 11, 2020 at 14:59:07 GMT</small></p></td> 173<td align="right"><div class="copyright-footer"></div></td> 174</tr></table> 175<hr> 176<div class="spirit-nav"></div> 177</body> 178</html> 179
