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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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< span class = "firstname" > Beman< / span > < span class = "surname" > Dawes< / 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 >
< div > < p class = "copyright" > Copyright © 2008 Beman Dawes< / p > < / div >
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< div > < p class = "copyright" > Copyright © 2009, 2010 Vicente J. Botet Escriba< / p > < / div >
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< a name = "id3354293" > < / 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 >
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< 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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< a name = "declval.overview" > < / a > < a class = "link" href = "declval.html#declval.overview" title = "Overview" > Overview< / a >
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< 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 = "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 >
< 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 = "comment" > //noexcept; // as unevaluated operand
< / 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.
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< td align = "left" > < p > < small > Last revised: April 06, 2011 at 20:06:10 GMT< / small > < / p > < / td >
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