FGMPStructUnion

FGMPStructUnion

A value whose UScriptStruct type is decided at runtime, that still behaves like a first-class UE value.

Declared in Plugins/GMP/Source/GMP/Classes/GMPUnion.h:144

USTRUCT(BlueprintType, BlueprintInternalUseOnly,
        meta = (HasNativeMake = "/Script/GMP.GMPStructLib:MakeStructUnion"))
struct FGMPStructUnion
{
    UScriptStruct* GetScriptStruct() const;
    FName          GetTypeName() const;
    bool           IsValid(const UScriptStruct* InType = nullptr, uint32 ArrayIdx = 0) const;

    template<typename T>
    bool GetDynamicStruct(T& Data, uint32 Index = 0) const;

    GMP_API bool Serialize(FArchive& Ar);
    GMP_API bool Serialize(FStructuredArchive::FRecord Record);
    GMP_API bool NetSerialize(FArchive& Ar, UPackageMap* Map, bool& bOutSuccess);
    GMP_API bool Identical(const FGMPStructUnion* Other, uint32 PortFlags = 0) const;
    GMP_API void AddStructReferencedObjects(FReferenceCollector& Collector);
};

What makes it more than a blob

The last four methods, via TStructOpsTypeTraits:

Method	Consequence
Serialize	works in save games and asset serialisation
NetSerialize	replicates, with UPackageMap handling object references
Identical	property comparison and delta detection behave correctly
AddStructReferencedObjects	objects inside the held struct are visible to the garbage collector

Without the last one a stored payload holding an object reference would be collected out from under you. This is why StoreObjectMessage can safely keep a message alive indefinitely.

Array form

It holds either one value or N values of the same runtime type — GetDynamicStruct and IsValid both take an index, and GetArrayNum() bounds it. That is what lets a stored or serialised message carry a list without a second container type.

Where it is used internally

struct FGMPStoreSourceMsgs : public TMap<FSigSource, FGMPStructUnion>   // GMPUnion.h:526

This is the container behind StoreObjectMessage: keyed by source, holding the copied payload.

Reading a value back

GetDynamicStruct<T>(Data) is checked — it returns false rather than reinterpreting if the held type is not T. Check the result; do not assume.

IsValid(InType) tests the held type without extracting.

Blueprint

Access goes through the StructUnion node family — Set/Get StructUnion, StructTuple, DynStructOnScope. See UK2Neuron.

See also

StoreObjectMessage · GMPArchive · FRpcMessageUtils