THUAI6/CAPI/cpp/grpc/include/absl/functional/internal/function_ref.h

// Copyright 2019 The Abseil Authors.
//
// 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
//
//      https://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.

#ifndef ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
#define ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_

#include <cassert>
#include <functional>
#include <type_traits>

#include "absl/base/internal/invoke.h"
#include "absl/meta/type_traits.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace functional_internal
    {

        // Like a void* that can handle function pointers as well. The standard does not
        // allow function pointers to round-trip through void*, but void(*)() is fine.
        //
        // Note: It's important that this class remains trivial and is the same size as
        // a pointer, since this allows the compiler to perform tail-call optimizations
        // when the underlying function is a callable object with a matching signature.
        union VoidPtr
        {
            const void* obj;
            void (*fun)();
        };

        // Chooses the best type for passing T as an argument.
        // Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
        // passed by value.
        template<typename T>
        constexpr bool PassByValue()
        {
            return !std::is_lvalue_reference<T>::value &&
                   absl::is_trivially_copy_constructible<T>::value &&
                   absl::is_trivially_copy_assignable<
                       typename std::remove_cv<T>::type>::value &&
                   std::is_trivially_destructible<T>::value &&
                   sizeof(T) <= 2 * sizeof(void*);
        }

        template<typename T>
        struct ForwardT : std::conditional<PassByValue<T>(), T, T&&>
        {
        };

        // An Invoker takes a pointer to the type-erased invokable object, followed by
        // the arguments that the invokable object expects.
        //
        // Note: The order of arguments here is an optimization, since member functions
        // have an implicit "this" pointer as their first argument, putting VoidPtr
        // first allows the compiler to perform tail-call optimization in many cases.
        template<typename R, typename... Args>
        using Invoker = R (*)(VoidPtr, typename ForwardT<Args>::type...);

        //
        // InvokeObject and InvokeFunction provide static "Invoke" functions that can be
        // used as Invokers for objects or functions respectively.
        //
        // static_cast<R> handles the case the return type is void.
        template<typename Obj, typename R, typename... Args>
        R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args)
        {
            auto o = static_cast<const Obj*>(ptr.obj);
            return static_cast<R>(
                absl::base_internal::invoke(*o, std::forward<Args>(args)...)
            );
        }

        template<typename Fun, typename R, typename... Args>
        R InvokeFunction(VoidPtr ptr, typename ForwardT<Args>::type... args)
        {
            auto f = reinterpret_cast<Fun>(ptr.fun);
            return static_cast<R>(
                absl::base_internal::invoke(f, std::forward<Args>(args)...)
            );
        }

        template<typename Sig>
        void AssertNonNull(const std::function<Sig>& f)
        {
            assert(f != nullptr);
            (void)f;
        }

        template<typename F>
        void AssertNonNull(const F&)
        {
        }

        template<typename F, typename C>
        void AssertNonNull(F C::*f)
        {
            assert(f != nullptr);
            (void)f;
        }

        template<bool C>
        using EnableIf = typename ::std::enable_if<C, int>::type;

    }  // namespace functional_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_