[/ 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]