detail/meta/detected.hpp

0001 //     __ _____ _____ _____
0002 //  __|  |   __|     |   | |  JSON for Modern C++
0003 // |  |  |__   |  |  | | | |  version 3.11.3
0004 // |_____|_____|_____|_|___|  https://github.com/nlohmann/json
0005 //
0006 // SPDX-FileCopyrightText: 2013-2023 Niels Lohmann <https://nlohmann.me>
0007 // SPDX-License-Identifier: MIT
0008
0009 #pragma once
0010
0011 #include <type_traits>
0012
0013 #include <nlohmann/detail/meta/void_t.hpp>
0014
0015 NLOHMANN_JSON_NAMESPACE_BEGIN
0016 namespace detail
0017 {
0018
0019 // https://en.cppreference.com/w/cpp/experimental/is_detected
0020 struct nonesuch
0021 {
0022     nonesuch() = delete;
0023     ~nonesuch() = delete;
0024     nonesuch(nonesuch const&) = delete;
0025     nonesuch(nonesuch const&&) = delete;
0026     void operator=(nonesuch const&) = delete;
0027     void operator=(nonesuch&&) = delete;
0028 };
0029
0030 template<class Default,
0031          class AlwaysVoid,
0032          template<class...> class Op,
0033          class... Args>
0034 struct detector
0035 {
0036     using value_t = std::false_type;
0037     using type = Default;
0038 };
0039
0040 template<class Default, template<class...> class Op, class... Args>
0041 struct detector<Default, void_t<Op<Args...>>, Op, Args...>
0042 {
0043     using value_t = std::true_type;
0044     using type = Op<Args...>;
0045 };
0046
0047 template<template<class...> class Op, class... Args>
0048 using is_detected = typename detector<nonesuch, void, Op, Args...>::value_t;
0049
0050 template<template<class...> class Op, class... Args>
0051 struct is_detected_lazy : is_detected<Op, Args...> { };
0052
0053 template<template<class...> class Op, class... Args>
0054 using detected_t = typename detector<nonesuch, void, Op, Args...>::type;
0055
0056 template<class Default, template<class...> class Op, class... Args>
0057 using detected_or = detector<Default, void, Op, Args...>;
0058
0059 template<class Default, template<class...> class Op, class... Args>
0060 using detected_or_t = typename detected_or<Default, Op, Args...>::type;
0061
0062 template<class Expected, template<class...> class Op, class... Args>
0063 using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>;
0064
0065 template<class To, template<class...> class Op, class... Args>
0066 using is_detected_convertible =
0067     std::is_convertible<detected_t<Op, Args...>, To>;
0068
0069 }  // namespace detail
0070 NLOHMANN_JSON_NAMESPACE_END