Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the master branch, built from commit f5c5562f6c.
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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:

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.


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