ceph/src/boost/libs/type_traits/doc/is_assignable.qbk

   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