Overload.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include <type_traits>
 
#include "Templates/Projection.h"
 
namespace UE::Core::Private
{
    template <typename... InvocableTypes>
    struct TOverload : InvocableTypes...
    {
        using InvocableTypes::operator()...;
    };
 
    template <typename RetType, typename... ArgTypes>
    struct TOverloadWrapper
    {
        RetType (*Callable)(ArgTypes...);
 
        UE_REWRITE constexpr RetType operator()(ArgTypes... Args) const
        {
            return Callable((ArgTypes&&)Args...);
        }
    };
 
    // This wraps function pointers in an object so they can be inherited by TOverload
    template <typename RetType, typename... ArgTypes>
    [[nodiscard]] UE_REWRITE constexpr TOverloadWrapper<RetType, ArgTypes...> MakeCallableObject(RetType(*Callable)(ArgTypes...))
    {
        return { Callable };
    }
 
    template <typename CallableType>
        requires (!std::is_pointer_v<std::decay_t<CallableType>>)
    [[nodiscard]] UE_REWRITE constexpr CallableType&& MakeCallableObject(CallableType&& Callable)
    {
        return (CallableType&&)Callable;
    }
}
 
namespace UE
{
    // Combines a set of invocable objects into one and overloads them.
    template <typename... InvocableTypes>
    [[nodiscard]] UE_REWRITE constexpr auto Overload(InvocableTypes&&... Invocables)
    {
        return UE::Core::Private::TOverload{ UE::Core::Private::MakeCallableObject(Projection((InvocableTypes&&)Invocables))... };
    }
}