How to return an array reference in C++

[calmattt]

Hi,

I would like to have a const getter method to access a private array in my class. By removing the const keyword the problem is gone.
Is it possible? Does it make sense?

I also notice that if I use the GetStarClusters() in a blueprint the performance is way way worse than accessing the property directly. Is it normal? It seems it’s creating copies from the array when I call the method.

NOTE: FStarCluster is a BlueprintType struct .

public: 
	UFUNCTION(BlueprintCallable, BlueprintPure, Category = "Stars System")
	FORCEINLINE TArray<FStarCluster>& GetStarClusters() const { return StarClusters; }

private: 
	UPROPERTY(BlueprintReadOnly, Category = "Stars System")
	TArray<FStarCluster> StarClusters;

Thanks in advance!
Mateus.

[Chatouille]

See this (first answer)

c++ const member function returning reference to class member

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Blueprint VM tends to do lots of copying around arguments regardless of whether you specify them as ref or not. You can take a look at ProcessEvent and the UHT generated code (files in .generated.h and .gen.cpp) to understand the way it works. Accessing the property directly is faster.

[calmattt]

Thanks for the quick reply!

If I understood correctly, the suggestion is to add const to the return type, right?

	UFUNCTION(BlueprintCallable, BlueprintPure, Category = "Stars System")
	FORCEINLINE **const** TArray<FStarCluster>& GetStarClusters() const { return StarClusters; }

this is causing this compilation error:

Inappropriate keyword 'const' on variable of type 'TArray'

This error makes no sense to me as I can use const for parameters of type TArray. I understand the contexts are different, but why?
I’m wondering if I’m doing something wrong but I’m blind.

[Chatouille]

That one is not a C++ error but is thrown by UHT.

It just tells you that it makes no sense to return a const ref here because blueprints will be able to manipulate it anyways, since it doesn’t support const-ness in C++ terms.

Using const& in a function argument is different. By default for reference arguments the BP system produces an output pin. const& tells the BP to produce an input pin, while still benefiting from optimization (no copies) on C++ side. It does still generate copies for the BP side though…

[calmattt]

Now everything makes sense

Thanks for the explanation, really appreciate that!