third_party/boost/type_traits/doc/detected_or.qbk

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Copyright 2018 Glen Joseph Fernandes
(glenjofe@gmail.com)

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).
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[section:detected_or detected_or]

    template<class Default, template<class...> class Op, class... Args>
    using detected_or = __below;

    template<class Default, template<class...> class Op, class... Args>
    using detected_or_t = typename detected_or<Default, Op, Args...>::type;

__alias An unspecified type with two public member type definitions:

* `value_t` is __true_type if `Op<Args...>` is a valid template-id, otherwise
  __false_type
* `type` is `Op<Args...>` if it is a valid template-id, otherwise `Default`

__std_paper [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf N4502]

__compat Requires C++11 variadic templates and C++11 template aliases.

__header `#include <boost/type_traits/detected_or.hpp>`

__examples

Suppose we wish to declare a type that represents the difference between two values of type T, it should be
T::difference_type if such a type exists, or std::ptrdiff_t otherwise:

    template<class T>
    using difference_t = typename T::difference_type;

    template<class T>
    using difference_type = boost::detected_or_t<std::ptrdiff_t, difference_t, T>;

Now the type `difference_type<T>` gives us what we need.

See also: __is_detected, __is_detected_convertible, __is_detected_exact.

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