Projection.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "AutoRTFM.h"
#include <type_traits>
 
namespace UE::Core::Private
{
    template <typename InvocableType>
    struct AUTORTFM_INFER TProjectionMemberFunction;
 
    template <typename ClassType, typename FunctionType>
    struct AUTORTFM_INFER TProjectionMemberFunction<FunctionType ClassType::*>
    {
        FunctionType ClassType::* MemberFunctionPtr;
 
        template <typename Arg0Type, typename... ArgTypes>
        constexpr decltype(auto) operator()(Arg0Type&& Arg0, ArgTypes&&... Args) const
        {
            using DecayedArg0Type = std::decay_t<Arg0Type>;
            if constexpr (std::is_base_of_v<ClassType, DecayedArg0Type>)
            {
                return (((Arg0Type&&)Arg0).*this->MemberFunctionPtr)((ArgTypes&&)Args...);
            }
            else
            {
                return ((*(Arg0Type&&)Arg0).*this->MemberFunctionPtr)((ArgTypes&&)Args...);
            }
        }
    };
 
    template <typename InvocableType>
    struct TProjectionMemberData;
 
    template <typename ClassType, typename MemberType>
    struct TProjectionMemberData<MemberType ClassType::*>
    {
        MemberType ClassType::* DataMemberPtr;
 
        template <typename Arg0Type>
        constexpr decltype(auto) operator()(Arg0Type&& Arg0) const
        {
            using DecayedArg0Type = std::decay_t<Arg0Type>;
            if constexpr (std::is_base_of_v<ClassType, DecayedArg0Type>)
            {
                return ((Arg0Type&&)Arg0).*this->DataMemberPtr;
            }
            else
            {
                return (*(Arg0Type&&)Arg0).*this->DataMemberPtr;
            }
        }
    };
 
    template <typename Class, typename MemberType>
    inline constexpr bool TIsMemberPointerToFunction(MemberType Class::*)
    {
        return std::is_function_v<MemberType>;
    }
}
 
template <typename Invocable0Type, typename... InvocableTypes>
AUTORTFM_INFER [[nodiscard]] constexpr auto Projection(Invocable0Type&& Invocable0, InvocableTypes&&... Invocables)
{
    if constexpr (sizeof...(InvocableTypes) == 0)
    {
        using DecayedInvocable0Type = std::decay_t<Invocable0Type>;
        if constexpr (!std::is_member_pointer_v<DecayedInvocable0Type>)
        {
            return (Invocable0Type&&)Invocable0;
        }
        else if constexpr (UE::Core::Private::TIsMemberPointerToFunction(DecayedInvocable0Type{}))
        {
            return UE::Core::Private::TProjectionMemberFunction<DecayedInvocable0Type>{ (Invocable0Type&&)Invocable0 };
        }
        else
        {
            return UE::Core::Private::TProjectionMemberData<DecayedInvocable0Type>{ (Invocable0Type&&)Invocable0 };
        }
    }
    else
    {
        return [Callable0 = Projection((Invocable0Type&&)Invocable0), CallableRest = Projection((InvocableTypes&&)Invocables...)](auto&&... Args) -> decltype(auto)
        {
            return CallableRest(Callable0((decltype(Args)&&)Args...));
        };
    }
}