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< a name = "declval" ></ a > Declval</ h2 ></ div >
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< span class = "firstname" > Howard</ span > < span class = "surname" > Hinnant</ span >
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< div class = "author" >< h3 class = "author" >
< span class = "firstname" > Vicente J.</ span > < span class = "surname" > Botet Escriba</ span >
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< div >< p class = "copyright" > Copyright © 2008 Howard Hinnant</ p ></ div >
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< div >< p class = "copyright" > Copyright © 2009 -2012 Vicente J. Botet Escriba</ p ></ div >
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< a name = "idp13456288" ></ a >< p >
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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 > )
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< p >< b > Table of Contents</ b ></ p >
< dl >
< dt >< span class = "section" >< a href = "declval.html#declval.overview" > Overview</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "declval.html#declval.reference" > Reference </ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "declval.html#declval.history" > History</ a ></ span ></ dt >
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< div class = "section declval_overview" >
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< 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 >
< p >
The motivation for < code class = "computeroutput" >< span class = "identifier" > declval</ span ></ 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.
</ p >
< p >
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
</ p >
< 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 >
< span class = "identifier" > T</ span >< span class = "special" > && </ span > < span class = "identifier" > declval</ span >< span class = "special" > ();</ span > < span class = "comment" > // not used
</ span ></ pre >
< p >
as part of the function template declaration
</ p >
< 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 >
< span class = "identifier" > 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 >
</ pre >
< p >
or as part of a class template definition
</ p >
< 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 >
< 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 >
< 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 >
< span class = "special" > {</ span >
< span class = "keyword" > typedef</ span > < span class = "identifier" > 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 >
< span class = "special" > };</ span >
</ pre >
< p >
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" >< span class = "identifier" > declval</ span >< span class = "special" > < </ span >< span class = "identifier" > T</ span >< span class = "special" > > ()</ span ></ 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
</ p >
< 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 >
< 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 ></ pre >
< p >
which ensures that we can also use cv void as template parameter. The careful
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
part of the definition of the semantics of the type trait is_convertible in
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the C++0x standard.
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</ p >
< p >
The provision of a new library component that allows the production of values
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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.
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</ p >
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< div class = "section declval_reference" >
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< div class = "titlepage" >< div >< div >< h2 class = "title" style = "clear: both" >
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< a name = "declval.reference" ></ a >< a class = "link" href = "declval.html#declval.reference" title = "Reference" > Reference </ a >
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</ h2 ></ div ></ div ></ div >
< p >
< 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 >
</ p >
< pre class = "programlisting" >< span class = "keyword" > namespace</ span > < span class = "identifier" > boost</ span > < span class = "special" > {</ span >
< span class = "keyword" > template</ span > < span class = "special" > < </ span >< span class = "keyword" > typename</ span > < span class = "identifier" > T</ span >< span class = "special" > > </ span >
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< 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 = "identifier" > noexcept</ span >< span class = "special" > ;</ span > < span class = "comment" > // as unevaluated operand
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</ span >
< span class = "special" > }</ span > < span class = "comment" > // namespace boost
</ span ></ pre >
< p >
The library provides the function template declval to simplify the definition
of expressions which occur as unevaluated operands.
</ p >
< 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 >
< 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 >
</ pre >
< p >
< span class = "bold" >< strong > Remarks:</ strong ></ span > If this function is used, the program
is ill-formed.
</ p >
< p >
< span class = "bold" >< strong > Remarks:</ strong ></ span > The template parameter T of declval
may be an incomplete type.
</ p >
< p >
< span class = "bold" >< strong > Example:</ strong ></ span >
</ p >
< 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 >
< span class = "identifier" > 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 >
</ pre >
< p >
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Declares a function template convert which only participates in overloading
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if the type From can be explicitly converted to type To.
</ p >
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< div class = "section declval_history" >
< 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 >
< a name = "declval.history.boost_1_50" ></ a >< h4 >
< a name = "idp13559952" ></ a >
< a class = "link" href = "declval.html#declval.history.boost_1_50" > boost 1.50</ a >
</ h4 >
< p >
New Features:
</ p >
< div class = "itemizedlist" >< ul class = "itemizedlist" 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 >
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< td align = "left" >< p >< small > Last revised: March 25, 2012 at 18:26:23 GMT</ small ></ p ></ td >
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