Question/Help with const init

Development Programming & Scripting C++
1

Hello, Is it possible to initialize a constant variable from a derrived class So I can change the value for each derrived class instances.

Ive tryed this, but it seems not to work, And I dont understand why.


Class AParent : public AActor{
Public:
AParent();
AParent(float Val) : Value(Val) {}

Protected:

const float Value;

};

class AChild : public AParent{
public:
AChild() : AParent(1.f) { // print out Value }
};

I would like to get an explanation/hint of what im doing wrong or a example how I can manage this to work.

2

In theory, it should. However, constructors in UE4 are different animals than in normal C++. Likely the class is getting re-initialized somewhere and losing that value. I’d write this using simple encapsulation and use PostLoad to set the value (as you are guaranteed that values set in PostLoad won’t get stomped during actor initialization).




Class AParent : public AActor{
Public:
AParent();

float GetValue() const { return Value; } 

Protected:

void SetValue(float MyValue) { Value = MyValue; }

float Value;

};

class AChild : public AParent{
public:
AChild();

virtual void PostLoad() override { SetValue(1.0f); }
};
3

Forgive me if im making stupid questions (Im still beginner with c++), But why using PostLoad? With your code the Value variable isnt const, Therefore I could assign a value directly from a derrived class since its protected.
I want to have a constant variable inside the parent class and initalize it inside a child class with a certain value thats not going to change anymore. Is that possible or should I give the child classes there own private const variable?

4

In regular C++ it’s possible. The class is constructed, and nothing else occurs unless you want it to.

In UE4, UStructs/UClasses follow a different paradigm:

https://docs.unrealengine.com/latest/INT/Programming/UnrealArchitecture/Actors/ActorLifecycle/

So let’s say you set the value in the constructor; if this actor was blueprinted and the blueprint was recompiled, or the level was reloaded, or someone duplicated the object - the value would get reset. PostLoad is called after all those various cases, so it’s safe.

In normal C++ you can leverage constructors quite a bit, in UE4 you have to leverage encapsulation a bit more and just realize that code is going to be slightly different than it’s pure C++ counterpart.

5

Allright, That explains a bit more. I will have a look at the link you provided.
Thanks for your time and explanation.