/* * Copyright (c) Meta Platforms, Inc. and affiliates. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ #pragma once #include <type_traits> namespace folly { namespace fibers { /** * For any functor F taking >= 1 argument, * FirstArgOf<F>::type is the type of F's first parameter. * * Rationale: we want to declare a function func(F), where F has the * signature `void(X)` and func should return T<X> (T and X are some types). * Solution: * * template <typename F> * T<typename FirstArgOf<F>::type> * func(F&& f); */ namespace detail { template <typename F> struct ExtractFirstArg; template <typename Ret, typename T, typename First, typename... Args> struct ExtractFirstArg<Ret (T::*)(First, Args...)> { typedef First type; }; template <typename Ret, typename T, typename First, typename... Args> struct ExtractFirstArg<Ret (T::*)(First, Args...) const> { typedef First type; }; template <typename Ret, typename First, typename... Args> struct ExtractFirstArg<Ret(First, Args...)> { typedef First type; }; } // namespace detail template <typename F, typename Enable = void> struct FirstArgOf; /** Specialization for non-function-object callables */ template <typename F> struct FirstArgOf<F, typename std::enable_if<!std::is_class<F>::value>::type> { typedef typename detail::ExtractFirstArg< typename std::remove_pointer<F>::type>::type type; }; /** Specialization for function objects */ template <typename F> struct FirstArgOf<F, typename std::enable_if<std::is_class<F>::value>::type> { typedef typename FirstArgOf<decltype(&F::operator())>::type type; }; } // namespace fibers } // namespace folly