[/
  (C) Copyright 2009-2011  Frederic Bron.
  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:has_left_shift has_left_shift]
   template <class Lhs, class Rhs=Lhs, class Ret=dont_care>
   struct has_left_shift : public __tof {};

__inherit
If (i) `lhs` of type `Lhs` and `rhs` of type `Rhs` can be used in expression `lhs<<rhs`,
and (ii) `Ret=dont_care` or the result of expression `lhs<<rhs` is convertible to `Ret`
then inherits from __true_type,
otherwise inherits from __false_type.

The default behaviour (`Ret=dont_care`) is to not check for the return value of binary `operator<<`.
If `Ret` is different from the default `dont_care` type, the return value is checked to be convertible to `Ret`.
Convertible to `Ret` means that the return value of the operator can be used as argument to a function expecting `Ret`:
``
void f(Ret);
Lhs lhs;
Rhs rhs;
f(lhs<<rhs); // is valid if has_left_shift<Lhs, Rhs, Ret>::value==true
``
If `Ret=void`, the return type is checked to be exactly `void`.

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

[has_binary_operator_compat]

__examples

[:`has_left_shift<Lhs, Rhs, Ret>::value_type` is the type `bool`.]
[:`has_left_shift<Lhs, Rhs, Ret>::value` is a `bool` integral constant expression.]
[:`has_left_shift<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
[:`has_left_shift<long>` inherits from `__true_type`.]

[:`has_left_shift<int, int, int>` inherits from `__true_type`.]
[:`has_left_shift<const int, int>` inherits from `__true_type`.]
[:`has_left_shift<std::ostream, int>` inherits from `__true_type`.]
[:`has_left_shift<std::ostream, char*, std::ostream>` inherits from `__true_type`.]
[:`has_left_shift<std::ostream, std::string>` inherits from `__true_type`.]

[:`has_left_shift<int, double, bool>` inherits from `__false_type`.]
[:`has_left_shift<int, int, std::string>` inherits from `__false_type`.]


[*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]

[binary_operator_known_issues has_left_shift..<<..A..contains<T>..const ]

[endsect]