RelativePtr.h

Go to the documentation of this file.

// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Misc/AssertionMacros.h"
#include <limits>
#include <type_traits>
 
template<typename T, typename OffsetType = int16>
struct TRelativePtr
{
    static constexpr OffsetType NullValue = std::numeric_limits<OffsetType>::max();
 
    static_assert(std::is_integral_v<OffsetType>, "Offset type must be a signed integer");
 
    explicit TRelativePtr()
        : Offset(NullValue)
    {}
 
    explicit TRelativePtr(TYPE_OF_NULLPTR)
        : Offset(NullValue)
    {}
 
    explicit TRelativePtr(const void* BasePtr, const void* ThisPtr)
    {
        Reset(BasePtr, ThisPtr);
    }
 
    explicit operator bool() const
    {
        return Offset != NullValue;
    }
 
    T* Resolve(const void* BasePtr) const
    {
        return Offset == NullValue ? nullptr : reinterpret_cast<T*>(static_cast<uint8*>(const_cast<void*>(BasePtr)) + Offset);
    }
 
    template<typename U>
    U* Resolve(const void* BasePtr) const
    {
        return static_cast<U*>(Resolve(BasePtr));
    }
 
    void Reset(TYPE_OF_NULLPTR)
    {
        Offset = NullValue;
    }
 
    void Reset(const void* BasePtr, const void* ThisPtr)
    {
        const uintptr_t Base = reinterpret_cast<uintptr_t>(BasePtr);
        const uintptr_t This = reinterpret_cast<uintptr_t>(ThisPtr);
        if (This >= Base)
        {
            const uintptr_t OffsetPtr = This - Base;
            checkf(OffsetPtr >= 0 && OffsetPtr < std::numeric_limits<OffsetType>::max(),
                TEXT("Attempting to create a relative pointer outside the bounds of its capacity."));
            Offset = static_cast<OffsetType>(OffsetPtr);
        }
        else if constexpr (std::is_signed_v<OffsetType>)
        {
            const uintptr_t OffsetPtr = Base - This;
            checkf(OffsetPtr >= 0 && OffsetPtr < std::numeric_limits<OffsetType>::max(),
                TEXT("Attempting to create a relative pointer outside the bounds of its capacity."));
            Offset = -static_cast<OffsetType>(OffsetPtr);
        }
        else
        {
            checkf(false, TEXT("Attempting to assign a negative offset to an unsigned relative pointer!!"));
        }
    }
private:
 
    OffsetType Offset;
};