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<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Declval</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="declval.html" title="Declval"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> <td align="center"><a href="../../../../index.html">Home</a></td> <td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"></div> <div class="article"> <div class="titlepage"> <div> <div><h2 class="title"> <a name="declval"></a>Declval</h2></div> <div><div class="authorgroup"> <div class="author"><h3 class="author"> Howard Hinnant </h3></div> <div class="author"><h3 class="author"> Vicente J. Botet Escriba </h3></div> </div></div> <div>Copyright © 2008 Howard Hinnant</div> <div>Copyright © 2009-2012 Vicente J. Botet Escriba</div> <div><div class="legalnotice"> <a name="declval.legal"></a> Distributed under the Boost Software License, Version 1.0. (See accompanying 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>) </div></div> </div> <hr> </div> <div class="toc"> Table of Contents <dl class="toc"> <dt><a href="declval.html#declval.overview">Overview</a></dt> <dt><a href="declval.html#declval.reference">Reference </a></dt> <dt><a href="declval.html#declval.history">History</a></dt> </dl> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="declval.overview"></a><a class="link" href="declval.html#declval.overview" title="Overview">Overview</a> </h2></div></div></div> The motivation for <code class="computeroutput">declval</code> was introduced in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value" target="_top">N2958: Moving Swap Forward</a>. Here follows a rewording of this chapter. With the provision of decltype, late-specified return types, and default template-arguments for function templates a new generation of SFINAE patterns will emerge to at least partially compensate the lack of concepts on the C++0x timescale. Using this technique, it is sometimes necessary to obtain an object of a known type in a non-using context, e.g. given the declaration <pre class="programlisting">template<class T> T&& declval(); // not used </pre> as part of the function template declaration <pre class="programlisting">template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&); </pre> or as part of a class template definition <pre class="programlisting">template<class> class result_of; template<class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)> { typedef decltype(declval<Fn>()(declval<ArgTypes>()...)) type; }; </pre> The role of the function template declval() is a transformation of a type T into a value without using or evaluating this function. The name is supposed to direct the reader's attention to the fact that the expression <code class="computeroutput">declval<T>()</code> is an lvalue if and only if T is an lvalue-reference, otherwise an rvalue. To extend the domain of this function we can do a bit better by changing its declaration to <pre class="programlisting">template<class T> typename std::add_rvalue_reference<T>::type declval(); // not used </pre> which ensures that we can also use cv void as template parameter. The careful reader might have noticed that <code class="computeroutput">declval()</code> already exists under the name create() as part of the definition of the semantics of the type trait is_convertible in the C++0x standard. The provision of a new library component that allows the production of values in unevaluated expressions is considered important to realize constrained templates in C++0x where concepts are not available. This extremely light-weight function is expected to be part of the daily tool-box of the C++0x programmer. </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="declval.reference"></a><a class="link" href="declval.html#declval.reference" title="Reference">Reference </a> </h2></div></div></div> <code class="computeroutput">#include <boost/utility/declval.hpp></code> <pre class="programlisting">namespace boost { template <typename T> typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand } // namespace boost </pre> The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands. <pre class="programlisting">template <typename T> typename add_rvalue_reference<T>::type declval(); </pre> Remarks: If this function is used, the program is ill-formed. Remarks: The template parameter T of declval may be an incomplete type. Example: <pre class="programlisting">template <class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&); </pre> Declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To. </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="declval.history"></a><a class="link" href="declval.html#declval.history" title="History">History</a> </h2></div></div></div> <h4> <a name="declval.history.h0"></a> <a name="declval.history.boost_1_50"></a><a class="link" href="declval.html#declval.history.boost_1_50">boost 1.50</a> </h4> Fixes: <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> <a href="http://svn.boost.org/trac/boost/ticket/6570" target="_top">#6570</a> Adding noexcept to boost::declval. </li></ul></div> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left">Last revised: October 30, 2014 at 10:17:15 GMT</td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"></div> </body> </html>