115 lines
3.8 KiB
Plaintext
115 lines
3.8 KiB
Plaintext
[/
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/ Copyright (c) 2008 Howard Hinnant
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/ Copyright (c) 2009-20012 Vicente J. Botet Escriba
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/
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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/]
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[article Declval
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[quickbook 1.5]
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[authors [Hinnant, Howard]]
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[authors [Botet Escriba, Vicente J.]]
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[copyright 2008 Howard Hinnant]
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[copyright 2009-2012 Vicente J. Botet Escriba]
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[license
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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[@http://www.boost.org/LICENSE_1_0.txt])
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]
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]
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[/===============]
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[section Overview]
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[/===============]
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The motivation for `declval` was introduced in [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value N2958:
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Moving Swap Forward]. Here follows a rewording of this chapter.
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With the provision of decltype, late-specified return types, and default template-arguments for function templates a
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new generation of SFINAE patterns will emerge to at least partially compensate the lack of concepts on the C++0x timescale.
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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
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template<class T>
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T&& declval(); // not used
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as part of the function template declaration
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template<class To, class From>
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decltype(static_cast<To>(declval<From>())) convert(From&&);
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or as part of a class template definition
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template<class> class result_of;
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template<class Fn, class... ArgTypes>
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struct result_of<Fn(ArgTypes...)>
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{
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typedef decltype(declval<Fn>()(declval<ArgTypes>()...)) type;
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};
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The role of the function template declval() is a transformation of a type T into a value without using or evaluating this function.
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The name is supposed to direct the reader's attention to the fact that the expression `declval<T>()` is an lvalue if and only if
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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
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template<class T>
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typename std::add_rvalue_reference<T>::type declval(); // not used
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which ensures that we can also use cv void as template parameter. The careful reader might have noticed that `declval()`
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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.
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The provision of a new library component that allows the production of values in unevaluated expressions is considered
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important to realize constrained templates in C++0x where concepts are not available.
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This extremely light-weight function is expected to be part of the daily tool-box of the C++0x programmer.
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[endsect]
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[/=================]
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[section:reference Reference ]
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[/=================]
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`#include <boost/utility/declval.hpp>`
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namespace boost {
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template <typename T>
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typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand
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} // namespace boost
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The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.
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template <typename T>
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typename add_rvalue_reference<T>::type declval();
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[*Remarks:] If this function is used, the program is ill-formed.
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[*Remarks:] The template parameter T of declval may be an incomplete type.
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[*Example:]
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template <class To, class From>
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decltype(static_cast<To>(declval<From>())) convert(From&&);
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Declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To.
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[endsect]
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[/===============]
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[section History]
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[/===============]
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[heading boost 1.50]
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Fixes:
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* [@http://svn.boost.org/trac/boost/ticket/6570 #6570] Adding noexcept to boost::declval.
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[endsect]
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