third_party/boost/type_traits/doc/detected.qbk

[/
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 detected]

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

__alias `Op<Args...>` if it is a valid template-id, otherwise
`boost::nonesuch`.

__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.hpp>`

__examples

Suppose you wish to determine whether a type has a `size()` const-member function, then given the meta-functions:

   template <class T>
   using size_member_tester = decltype(std::declval<const T&>().size());

   template <class T>
   using size_member_t = boost::detected_t<size_member_tester, T >;


Then the type `size_member_t<T>` is an alias for `size_member_tester<T>` if the operation is valid, and an alias for
`boost::nonesuch` otherwise.

See also: __is_detected, __is_detected_convertible, __is_detected_exact.

[endsect]