[ceph.git] / ceph / src / boost / libs / type_traits / doc / is_assignable.qbk

[/ 
  Copyright 2015 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:is_assignable is_assignable]

   template <class T, class U>
   struct is_assignable : public __tof {};
    
__inherit If `std::declval<T>() = std::declval<U>()` then inherits from __true_type, 
otherwise from  __flase_type.  Type `T` must be a complete type.

Note that this trait is somewhat tricky to use correctly: for example:

   is_assignable<int, int>::value

is `false` since `std::declval<int>()` is an ['xvalue] which can not be assigned to!

If you're intention is to check for copy-assignment from some type U then use:

   is_assignable<T&, const U&>::value

If you're intention is to check for move-assignment then use:

   is_assignable<T&, U&&>::value

or simply:

   is_assignable<T&, U>::value


__compat Requires the C++11 features `decltype` and SFINAE-expressions for full support.

__header ` #include <boost/type_traits/is_assignable.hpp>` or ` #include <boost/type_traits.hpp>`

[endsect]
Commit	Line	Data
7c673cae FG	1	[/
	2	Copyright 2015 John Maddock.
	3	Distributed under the Boost Software License, Version 1.0.
	4	(See accompanying file LICENSE_1_0.txt or copy at
	5	http://www.boost.org/LICENSE_1_0.txt).
	6	]
	7
	8	[section:is_assignable is_assignable]
	9
	10	template <class T, class U>
	11	struct is_assignable : public __tof {};
	12
	13	__inherit If `std::declval<T>() = std::declval<U>()` then inherits from __true_type,
	14	otherwise from __flase_type. Type `T` must be a complete type.
	15
	16	Note that this trait is somewhat tricky to use correctly: for example:
	17
	18	is_assignable<int, int>::value
	19
	20	is `false` since `std::declval<int>()` is an ['xvalue] which can not be assigned to!
	21
	22	If you're intention is to check for copy-assignment from some type U then use:
	23
	24	is_assignable<T&, const U&>::value
	25
	26	If you're intention is to check for move-assignment then use:
	27
	28	is_assignable<T&, U&&>::value
	29
	30	or simply:
	31
	32	is_assignable<T&, U>::value
	33
	34
	35	__compat Requires the C++11 features `decltype` and SFINAE-expressions for full support.
	36
	37	__header ` #include <boost/type_traits/is_assignable.hpp>` or ` #include <boost/type_traits.hpp>`
	38
	39	[endsect]
	40