FunctionWithContext.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "HAL/Platform.h"
#include "Templates/Invoke.h"
#include <type_traits>
 
namespace UE
{
 
template <typename FunctionType>
class TFunctionWithContext;
 
template <typename ReturnType, typename... ArgTypes>
class TFunctionWithContext<ReturnType (ArgTypes...)>
{
public:
    using FunctionType = ReturnType (void*, ArgTypes...);
 
    template <typename LambdaType>
    inline TFunctionWithContext(LambdaType&& Lambda UE_LIFETIMEBOUND)
        requires (!std::is_same_v<std::decay_t<LambdaType>, TFunctionWithContext>) &&
            std::is_invocable_r_v<ReturnType, std::decay_t<LambdaType>, ArgTypes...>
        : Function(&Call<std::decay_t<LambdaType>>)
        , Context(&Lambda)
    {
    }
 
    template <typename LambdaType>
    inline TFunctionWithContext& operator=(LambdaType&& Lambda UE_LIFETIMEBOUND)
        requires (!std::is_same_v<std::decay_t<LambdaType>, TFunctionWithContext>) &&
            std::is_invocable_r_v<ReturnType, std::decay_t<LambdaType>, ArgTypes...>
    {
        Function = &Call<std::decay_t<LambdaType>>;
        Context = &Lambda;
        return *this;
    }
 
    inline explicit TFunctionWithContext(FunctionType* InFunction, void* InContext)
        : Function(InFunction)
        , Context(InContext)
    {
    }
 
    inline constexpr TFunctionWithContext(decltype(nullptr))
    {
    }
 
    inline explicit operator bool() const
    {
        return !!Function;
    }
 
    inline ReturnType operator()(ArgTypes... Args) const
    {
        return Function(Context, (ArgTypes&&)Args...);
    }
 
    inline FunctionType* GetFunction() const
    {
        return Function;
    }
 
    inline void* GetContext() const
    {
        return Context;
    }
 
private:
    template <typename LambdaType>
    static ReturnType Call(void* Lambda, ArgTypes... Args)
    {
        return Invoke(*(LambdaType*)Lambda, (ArgTypes&&)Args...);
    }
 
    FunctionType* Function = nullptr;
    void* Context = nullptr;
};
 
} // UE