0
class B
{
    public:
        A* GetA() const { return obj; }
    private:
        A* obj;
}

... 

B b;
b.GetA()->AInterfaceMethod(params);

So my questions are:

  1. What would be different had the function not been const?
  2. Are there any restrictions to what I can do with the pointer obtained via GetA()? Is it const? Does it point to a const A?
  3. When is this useful?

I encountered this in an Unreal tutorial (A is forward declared in the tutorial).

EDIT: I probably messed up, but the call does work. I've included the actual Unreal code below:

class ABatteryPickup : public APickup
{
    ABatteryPickup()
    {
        GetMesh()->SetSimulatePhysics(true);
    }
}

class Pickup
{
    public:
        class UStaticMeshComponent* GetMesh() const { return PickupMesh; }

    private:
        class UStaticMeshComponent* PickupMesh;
}

UStaticMeshComponent::SetSimulatePhysics() is not const.

Also, just tested this on a new, clean C++ project, and it works. screenshot proof

karnage
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  • Possible duplicate of [const function pointers](http://stackoverflow.com/questions/15506094/const-function-pointers) – Jonathan Mee Jun 22 '16 at 15:41
  • Constructor of `B` is binding pointer `Abc` to a temporary. Just lucky that `Print` is not accessing any data-member otherwise it would be invalid read. – Ajay Jun 22 '16 at 16:17
  • @Ajay Okay, that makes sense to me, but then how does GetMesh()->SetSimulatePhysics(true) work in Unreal? – karnage Jun 22 '16 at 16:20
  • Well, because the return type is non-const. The object being returned is not a const object. Change it to `const class UStaticMeshComponent* GetMesh() const` – Ajay Jun 22 '16 at 16:22
  • Hmm, let me try to rephrase: what is the point of `const` in `GetMesh() const` above? – karnage Jun 22 '16 at 16:25

2 Answers2

2
  1. What would be different had the function not been const?

If the function is a non-const member function, you can't call it on a const object.

const B b;
b.GetA();  // Fail, can't call a non-const member function on a const object
  1. Are there any restrictions to what I can do with the pointer obtained via GetA()? Is it const? Does it point to a const A?

No, nothing is different here. The returned value is not const itself, and not a pointer to const, it's just A* as it declared. If the object is const, the member variable obj will be const too, i.e. A* const, note it's still not a pointer to const, i.e. const A*. Anyway you return it by value with type A*, so nothing is different here.

  1. When is this useful?

Const member function and non-const member function could be overloaded. You might use them for different purpose. e.g.

class B
{
    public:
        A* GetA() { return obj; }             // returns A*
        const A* GetA() const { return obj; } // returns const A*
    private:
        A* obj;
}

... 

B b;
b.GetA();  // get A*
const B cb;
cb.GetA(); // get const A*
songyuanyao
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  • Thanks for the response, but b.GetA() seems to work even if GetA() is not const (I tested this on VS 2015 right now). I've added more details in the question. – karnage Jun 22 '16 at 16:08
  • @sloth If you mean my answer to 1st question, I meant make `b` const and `GetA()` non-const. But according to your screenshot, `b` is not const and `GetA()` is const. – songyuanyao Jun 22 '16 at 16:17
  • So if GetA() is const, but neither the returned pointer nor the object is const (meaning the caller can access Print()), then what purpose does GetA() const serve? – karnage Jun 22 '16 at 16:23
  • I had misunderstood your first bullet point! I finally got it. Thanks for that! But I'm still confused about what the point of having a `const` in `A* GetA() const` is? – karnage Jun 22 '16 at 16:30
  • @sloth I tried add some explanations, see my edited answer. – songyuanyao Jun 22 '16 at 16:58
  • Thanks a lot! Much appreciated. – karnage Jun 22 '16 at 23:01
1

What would be different had the function not been const?

const method has a contract - it does not change internal state of object B directly or indirectly, for example it would not change pointer obj to point somewhere else. So this code:

A* GetA() const { obj = new A; return obj; }

would fail to compile.

Are there any restrictions to what I can do with the pointer obtained via GetA()? Is it const? Does it point to a const A?

No, object of type A is unrelated and to make this method to return pointer of const A you need to change type of the returned pointer:

const A *GetA() const { return obj; } // now you cannot change A through pointer you get, unless you const_cast it.

When is this useful?

You can control separately what you can change inside method and what can be done with object, pointer to which you return. You just have choice.

Slava
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