One post tagged with "cpp"

Generic Param Diffing in Unreal with Templated Structs​

In Unreal Engine, it's common to work with USTRUCT-based parameter types like FACCIntParam, FACCFloatParam, etc. These often inherit from a common base like FACCParam. But what happens when you want to write generic diffing logic, such as detecting added/removed/changed values in a templated function?

You might first think of polymorphism:

struct FACCParam
{
    virtual bool IsDiff(const FACCParam& Other) const;
};

Wrong approach — USTRUCTs do not support virtual methods reliably, and Unreal's reflection system doesn’t play well with polymorphic inheritance in structs.

Correct Approach: Use Template Specialization​

Instead of relying on virtual methods, use C++ templates and monomorphization. Here's why this works:

• Each FACCIntParam, FACCFloatParam, etc. implements its own IsDiff(const TParam&).
• When using TParam in a template, the compiler generates a concrete version at compile time.
• You don’t need any virtual functions, nor any base IsDiff.

Example: Generic Diff Template​

template<typename TParam>
void DetectModifiedParams(const TArray<TParam>& NewParams, const TArray<TParam>& OldParams)
{
    // Map param by their unique composite key (e.g., Key + CustomInfo)
    TMap<FName, TParam> NewMap;
    for (const TParam& Param : NewParams)
    {
        NewMap.Add(Param.GetUniqueKey(), Param);
    }

    TMap<FName, TParam> OldMap;
    for (const TParam& Param : OldParams)
    {
        OldMap.Add(Param.GetUniqueKey(), Param);
    }

    // Compare
    for (const auto& Elem : NewMap)
    {
        const FName& UniqueKey = Elem.Key;
        const TParam& NewValue = Elem.Value;

        if (const TParam* OldValue = OldMap.Find(UniqueKey))
        {
            if (NewValue.IsDiff(*OldValue))
            {
                // modified
            }
        }
        else
        {
            // added
        }
    }

    for (const auto& Elem : OldMap)
    {
        if (!NewMap.Contains(Elem.Key))
        {
            // removed
        }
    }
}

IsDiff stays local to each child​

In FACCIntParam:

bool IsDiff(const FACCIntParam& Other) const
{
    return !Equals(Other) || Value != Other.Value;
}

No need to declare anything in the base FACCParam.

Summary​

• ❌ Do not use virtual methods in USTRUCTs.
• ✅ Templates let you write reusable diff logic with zero runtime cost.
• ✅ Keep IsDiff() defined per param type.
• ✅ Use TParam::IsDiff() safely inside template functions.

Clean. Performant. Unreal-friendly.