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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).
- ]
- [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.
- [endsect]
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