1 // 2 // uses_executor.hpp 3 // ~~~~~~~~~~~~~~~~~ 4 // 5 // Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com) 6 // 7 // Distributed under the Boost Software License, Version 1.0. (See accompanying 8 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 9 // 10 11 #ifndef BOOST_ASIO_USES_EXECUTOR_HPP 12 #define BOOST_ASIO_USES_EXECUTOR_HPP 13 14 #if defined(_MSC_VER) && (_MSC_VER >= 1200) 15 # pragma once 16 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200) 17 18 #include <boost/asio/detail/config.hpp> 19 #include <boost/asio/detail/type_traits.hpp> 20 21 #include <boost/asio/detail/push_options.hpp> 22 23 namespace boost { 24 namespace asio { 25 26 /// A special type, similar to std::nothrow_t, used to disambiguate 27 /// constructors that accept executor arguments. 28 /** 29 * The executor_arg_t struct is an empty structure type used as a unique type 30 * to disambiguate constructor and function overloading. Specifically, some 31 * types have constructors with executor_arg_t as the first argument, 32 * immediately followed by an argument of a type that satisfies the Executor 33 * type requirements. 34 */ 35 struct executor_arg_t 36 { 37 /// Constructor. executor_arg_tboost::asio::executor_arg_t38 BOOST_ASIO_CONSTEXPR executor_arg_t() BOOST_ASIO_NOEXCEPT 39 { 40 } 41 }; 42 43 /// A special value, similar to std::nothrow, used to disambiguate constructors 44 /// that accept executor arguments. 45 /** 46 * See boost::asio::executor_arg_t and boost::asio::uses_executor 47 * for more information. 48 */ 49 #if defined(BOOST_ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION) 50 constexpr executor_arg_t executor_arg; 51 #elif defined(BOOST_ASIO_MSVC) 52 __declspec(selectany) executor_arg_t executor_arg; 53 #endif 54 55 /// The uses_executor trait detects whether a type T has an associated executor 56 /// that is convertible from type Executor. 57 /** 58 * Meets the BinaryTypeTrait requirements. The Asio library provides a 59 * definition that is derived from false_type. A program may specialize this 60 * template to derive from true_type for a user-defined type T that can be 61 * constructed with an executor, where the first argument of a constructor has 62 * type executor_arg_t and the second argument is convertible from type 63 * Executor. 64 */ 65 template <typename T, typename Executor> 66 struct uses_executor : false_type {}; 67 68 } // namespace asio 69 } // namespace boost 70 71 #include <boost/asio/detail/pop_options.hpp> 72 73 #endif // BOOST_ASIO_USES_EXECUTOR_HPP 74