Executing Generic TBaseDynamicDelegate<> ExecuteIfBound is no member.

Development Programming & Scripting C++
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1

I’m trying to create a Backend Service usable by other cpp classes in my project.
I want the service to already deserialize the incoming data and parse it into a struct.

I’m currently not getting this work generically:

Header File:

    // SendMessageStruct works correctly
	template<typename InStructType>
	void SendMessageStruct(const FString& destination, const InStructType& InStruct)
	{
		SendMessageStruct(destination, InStructType::StaticStruct(), &InStruct);
	}
	void SendMessageStruct(const FString& destination, const UStruct* StructDefinition, const void* Struct);

// SubscribeStruct does not compile.
	template<typename OutStructType>
	void SubscribeStruct(const FString& Destination, TBaseDynamicDelegate<FWeakObjectPtr, void, OutStructType> Delegate)
	{
		const auto Callback = FStompSubscriptionEvent::CreateLambda([Delegate](const IStompMessage& Message)
		{
			OutStructType ResponseData;
			FJsonObjectConverter::JsonObjectStringToUStruct<OutStructType>(Message.GetBodyAsString(), &ResponseData, 0, 0, 0);
			Delegate.ExecuteIfBound(ResponseData);
		});
		Subscribe(Destination, Callback);
	}

	void Subscribe(const FString& Destination, const FStompSubscriptionEvent& Callback);

I’m calling it in the following way, with FNpcServerManagedData being a USTRUCT.

DECLARE_DYNAMIC_DELEGATE_OneParam(FOnNpcServerManagedDataUpdate, FNpcServerManagedData, Data);

...

	BackendCommunication->SubscribeStruct("/topic/npc/" + NpcId + "/flushManagedData", OnNpcServerManagedDataUpdate);

But when I’m trying to compile I get the following error:

0>C:\Projekte\Dukes of Frisia\Game\DukesOfFrisia\Source\Communication\Public\BackendCommunication.h(37): Error C2039 : "ExecuteIfBound" ist kein Member von "TBaseDynamicDelegate<FWeakObjectPtr,void,FNpcServerManagedData>".
0>C:\Projekte\Dukes of Frisia\Game\DukesOfFrisia\Source\Communication\Public\NpcCommunication.h(28): Reference  : Siehe Deklaration von "TBaseDynamicDelegate<FWeakObjectPtr,void,FNpcServerManagedData>"
0>C:\Projekte\Dukes of Frisia\Game\DukesOfFrisia\Source\Communication\Private\NpcCommunication.cpp(50): Reference  : Siehe Verweis auf die gerade kompilierte Instanziierung "void UBackendCommunication::SubscribeStruct<FNpcServerManagedData>(const FString &,TBaseDynamicDelegate<FWeakObjectPtr,void,FNpcServerManagedData>)" der Funktions-Vorlage.
0>C:\Projekte\Dukes of Frisia\Game\DukesOfFrisia\Source\Communication\Public\BackendCommunication.h(37): Error C2039 : "__this" ist kein Member von "TBaseDynamicDelegate<FWeakObjectPtr,void,FNpcServerManagedData>".
0>C:\Projekte\Dukes of Frisia\Game\DukesOfFrisia\Source\Communication\Public\NpcCommunication.h(28): Reference  : Siehe Deklaration von "TBaseDynamicDelegate<FWeakObjectPtr,void,FNpcServerManagedData>"

What am I doing wrong? Is it not possible to call ExecuteIfBound on a generic type?

2

Nope, Execute / ExecuteIfBound / Broadcast are declared in the subclasses.

Why not templatize the whole delegate type ?

template<typename OutStructType, typename DelegateType>
void SubscribeStruct(const FString& Destination, DelegateType Delegate)
{
    Delegate.ExecuteIfBound(ResponseData);
}

Alternatively, you can reproduce the auto-generated ExecuteIfBound call…
Should look like this :

if (Delegate.IsBound())
{
    struct FParms { OutStructType Data; } Parms = { ResponseData };
    Delegate.ProcessDelegate<UObject>(&Parms);
}

It’s not quite ideal though as you might not get any compile warning if you change the delegate signature, but probably crash at runtime.