CPP ctor and dtor in UnrealEngine

overview

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UCLASS(config=Game)
class AProjectSandboxCharacter : public ACharacter
{
    ...

public:
    AProjectSandboxCharacter();
    virtual ~AProjectSandboxCharacter();

    ...
}

somebody may have the experience your breakpoint stops in cpp constructor even the object is not placed in the world. you might say,
"Why this happens ? I do not get it why the constructor is called even while opening editor !"

vice versa, destructor is called when editor is closed. in your intuition, the cpp constructor or destructor seems to be called when the actor is created or deleted. but it does not as you saw. then, let us find what do cpp ctor and dtor do in unreal engine. we would get a clue to truth through it.

class default object

unreal engine provides their own macro system, which has several features below:

  • garbage collection
  • reference update
  • reflection
  • serialization
  • etc.

to support these features, unreal engine constructed massive macro magics and hacks. so, cpp in unreal engine acts unlike cpp as we know. this ctor/dtor issue is one of them(different behaviors). from reference #2, we can find the reference of Class Default Object, CDO.

cpp constructor makes the Class Default Object and it is copied whenever you create instance of the UObject. what UObject means in this post, is the object created by NewObject API. from reference #1, we can find the reference of NewObject API.

in summary…

  • if your class(or somewhat) should be utilized with APIs of unreal engine, this must inherit UObject and follow some conventions
  • if you do that, your class gonna have CDO, which is used for cloning object instance
  • in this condition, cpp ctor/dtor only works for CDO

※ for more information of CDO, visit reference #1 and reference #2
※ for more information of Unreal Reflection, visit reference #3

real ctor/dtor for unreal

that is why we could see that the breakpoint stops at cpp ctor/dtor before unreal editor opens. CDO is needed to display the asset editor of the class. try some tests for yourself.


the pictures above says, changes in cpp ctor will be shown in asset editor (if the asset inherits the class). not only the simple float variables, but it affects the various component or material things.

so, let us suppose real ctor/dtor for unreal should do its work when the instance of class is created in “game”, such as spawning bullets when player shoots the gun. there are several APIs for this purpose, but every child of UObject does not have common API.

here is a table for major classes. almost of gameplay framework classes inherit them.

child function
UUserWidget UUserWidget::NativeConstruct
AActor AActor::OnConstruction
UActorComponent UActorComponent::InitializeComponent

summary

  • due to supporting several features, cpp in unreal engine acts unlike cpp as we know.
  • cpp ctor/dtor is for CDO, not for the cloned instances in “game”. there are seperate APIs for them.
    • and it differs upon a class. there is no common unreal ctor/dtor.
  • if you plan to make custom class inherits UObject, you should consider how to make unreal ctor/dtor for it.
    • or just let the class inherits a class already implemented unreal ctor/dtor, such as UUserWidget