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### detected_or

    template<class Default, template<class...> class Op, class... Args>
    using detected_or = see-below;

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

Aliases: 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`

C++ Standard Paper: N4502

Compiler Compatibility: 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.