Trouble initializing UCLASS with class member in default constructor.

Development Programming & Scripting C++
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1

I have this error
error: constructor for 'UActionQueue' must explicitly initialize the member 'queue' which does not have a default constructor

Here is the code in question

// ActionQueue.h
UCLASS( ClassGroup=(Custom), meta=(BlueprintSpawnableComponent) )
class CPPTEST_API UActionQueue : public UActorComponent
{
	GENERATED_BODY()
	TFragmentQueue<FActionStruct> queue;

public:	
	// Sets default values for this components properties
	UActionQueue();

protected:
	// Called when the game starts
	virtual void BeginPlay() override;

public:	
	// Called every frame
	virtual void TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction) override;
};

// ActionQueue.cpp
UActionQueue::UActionQueue() 
: queue(1,1)
{
	PrimaryComponentTick.bCanEverTick = true;

	// ...
}

template<class T>
class CPPTEST_API TFragmentQueue
{
private
  // member variables ...
public:
    TFragmentQueue(int queuedSize, int previousSize)
    {
        // init code
    }
    
  // rest of class code ...
}

So as far as I know this is valid C++ but Unreal has some problem with it and I don’t get why. I’m trying to initialize queue in the default constructor of UActionQueue but I just get this error. Can I not have objects without default constructors? I’ve looked through the Gameplay Classes and Epic C++ Coding Standard articles and saw UObjects need default constructors but TFragmentQueue is not a UObject, its just a basic class.

2

I’ll mark this as the answer if nobody has anything to add but what worked is moving the assignment of queue into the declaration in the header.

UCLASS( ClassGroup=(Custom), meta=(BlueprintSpawnableComponent) )
class CPPTEST_API UActionQueue : public UActorComponent
{
	GENERATED_BODY()
	TFragmentQueue<FActionStruct> queue = TFragmentQueue<FActionStruct>(1,1);

public:	
	// Sets default values for this components properties
	UActionQueue();

};

I still don’t see why I cant use the constructor since that’s what it is for and documentation says to use it for that but whatever.

3

Where was the error reported?

While everything you’ve written is valid C++, you have to remember that Unreal generates code for your class as well. I suspect that the error is in a generated constructor that you don’t have any control over.

An alternative would be to have a default constructor and a way to set those separately. But your header based solution works too in this case.

4

Here is the full error
CompilerResultsLog: /home/user-name/Documents/Unreal Projects/CPPTest/Intermediate/Build/Linux/UnrealEditor/Inc/CPPTest/UHT/ActionQueue.gen.cpp:553:31: error: constructor for 'UActionQueue' must explicitly initialize the member 'queue' which does not have a default constructor

That “.gen.cpp” makes me think you’re right about the generated code. If that’s the case what options do I have?

5

The only solutions I can think of are the two I gave in my previous post. Either keep doing what you’re already doing or add a default constructor. Since you’re dealing with a template and it can’t be a UPROPERTY you have slightly more flexibility than you might otherwise.

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