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  __false_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]