VersionedArchive.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Serialization/CustomVersion.h"
 
// Use TVersionedReader/TVersionedWriter to create an FArchiveProxy derived class using the provided 
// Reader/Writer class that'll automatically serialize the CustomVersions that were used during serialization.
 
template<class T> 
struct TUnderlyingArchiveContainer
{
    T Ar;
 
    template <typename... ArgsType>
    explicit TUnderlyingArchiveContainer(ArgsType&&... Args)
        : Ar(Forward<ArgsType>(Args)...)
    {
    }
};
 
template<class T> 
class TVersionedReader : public TUnderlyingArchiveContainer<T>, public FArchiveProxy
{   
public:
    
    template <typename... ArgsType>
    explicit TVersionedReader(ArgsType&&... Args)
        : TUnderlyingArchiveContainer<T>(Forward<ArgsType>(Args)...)
        , FArchiveProxy(this->Ar)
        
    {       
        FCustomVersionContainer CustomVersions;
 
        // Serialize the offset to the CustomVersions
        int64 VersionOffset = 0;
        *this << VersionOffset;
 
        // Preserve where we are so we can return and continue serializing after reading the CustomVersions
        int64 ReturnOffset = this->Tell();
        checkf(ReturnOffset != INDEX_NONE, TEXT("Underlying FArchive must support Seek/Tell to use TVersionnedReader"));
 
        // Go to CustomVersions, serialize them and set them in the underlying archive
        this->Seek(VersionOffset);
        CustomVersions.Serialize(*this);
        this->SetCustomVersions(CustomVersions);
 
        // Return to where we seeked from to be able to continue serialization
        this->Seek(ReturnOffset);
    }
};
 
template<class T>
class TVersionedWriter : public TUnderlyingArchiveContainer<T>, public FArchiveProxy
{   
    int64 VersionOffset = 0;
 
public:
        
        template <typename... ArgsType>
        explicit TVersionedWriter(ArgsType&&... Args)
            : TUnderlyingArchiveContainer<T>(Forward<ArgsType>(Args)...)
            , FArchiveProxy(this->Ar)
        {
            // Reserve space for version offset and remember where to write it
            VersionOffset = this->Tell();
            checkf(VersionOffset != INDEX_NONE, TEXT("Underlying FArchive must support Seek/Tell to use TVersionnedWriter"));
            *this << VersionOffset;
        }
    
        ~TVersionedWriter()
        {
            // Acquire the current set of CustomVersions
            FCustomVersionContainer CustomVersions;
            CustomVersions = this->GetCustomVersions();
 
            // Capture the offset where it'll be serialized, and then serialize the CustomVersion
            int64 CurrentOffset = this->Tell();
            CustomVersions.Serialize(*this);
            
            // Go back to the initial offset and serialize the CustomVersions offset
            this->Seek(VersionOffset);
            *this << CurrentOffset;
        }
};