How do I check for a valid cast?

Development Programming & Scripting
1

Hello community,

right now, I am trying to implement the IGenericTeamAgentInterface for the AI and I got it up and running. But I encounter a problem I cannot solve on my own, as all of my search keywords seem to be too generic to deliver an appropriate result.
I am not new to C++ but to Unreal and I kinda miss a lot.

I am overriding the GetGenericTeamId() function from the TeamAgentInterface inside a PlayerCharacter. But as most of the logic is stored inside the PlayerController, the function basically only calls this one instead. The Engine crashes by that procedure at the moment where I hit simulate instead preview. I assume that the PlayerCharacter is missing a valid PlayerController in that case as the VS Exception is “Access violation reading location 0x0000000000000000”. So far so good, but how do I check for a valid memory location in that case?

Here is the code:

FGenericTeamId ALAME_PlayerCharacter::GetGenericTeamId() const
{    	
	return Cast<IGenericTeamAgentInterface>(GetController())->GetGenericTeamId();
}
2

Something like this:

FGenericTeamId ALAME_PlayerCharacter::GetGenericTeamId() const
{
Controller* YouController = Cast<IGenericTeamAgentInterface>(GetController())->GetGenericTeamId();

if (YouController)
{
	return Cast<IGenericTeamAgentInterface>(GetController())->GetGenericTeamId();
}

return nullptr;

}

I’m not sure “Controller” is the right class, but I hope you get the idea.

Using the “if(bool)” you can check if the cast was valid. Storing the cast on a pointer is so you don’t have to cast twice; on the check and on the return.

pointers “filled” with nullptr don’t crash the engine.

3

Thank you, your answer was not directly applicable but it points me in the right direction.

“Cast(GetController())->GetGenericTeamId();” returns a FGenericTeamId Object (not a pointer). To tinker around with that knowledge, I split up the whole expression. Also, the return value of the whole function itself is an object, not a pointer.

The new function solves the problem. And I could be happy with it, but still, I ask my self if it is the most optimized and convenient solution to the problem. Does anyone have an opinion on that? And, do I ever have to save all my return values like that? Sounds like an immense overhead to me.

Here’s the new function:

FGenericTeamId ALAME_PlayerCharacter::GetGenericTeamId() const
{
	FGenericTeamId value = 255;

	AController* currentController = GetController();
	if (currentController)
	{
		IGenericTeamAgentInterface* currentInterface = Cast<IGenericTeamAgentInterface>(currentController);
		if (currentInterface)
			value = Cast<IGenericTeamAgentInterface>(GetController())->GetGenericTeamId();
	}
	return value;
}

The step to create the “currentInterface” is not needed, as it hadn’t failed until now.

4

The first check is unnecessary. You can immediately try to cast the result of GetController(), even if it is null. When you try to cast a nullptr, you get back a nullptr, no harm done.

There’s also no reason to cast twice. Save the result of the cast in a variable and use the variable.

1 Like
5

This. That bothered me so much that he did it like that.