how to detect if a widget is destroyed?

[Atr0p0s]

Hello everyone again!

I faced a such situation:
I have widget A that contains a pointer to child widget B and I want widget A to hide when X is clicked (it's by default), and widget B to destroy when its X is clicked (used B->setAttribute(Qt::WA_DeleteOnClose);). But I also want widget B to destroy when A is closed.
I can't use the pointer to B, because it can be already destroyed if widget B is closed(destroyed) by the user:

void A::closeEvent(QCloseEvent* event)
{
    if (p_B) {
       delete(p_B);
    }
    p_B = nullptr;
}

Can I get out of this situation preferably without changing widget B?

[Chris Kawa]

@Atr0p0s You can simply make B a child of A, i.e. pass A as a parent to B constructor or use the setParent method. A will delete its child when it is deleted itself.
If you prefer to do it manually for some reason use QPointer instead of a raw pointer. It is automatically nulled when the object it points to is deleted.

[JonB]

@Atr0p0s
void QObject::destroyed(QObject *obj = nullptr).

Use deleteLater() rather than delete.

Don't do any of this stuff, just use parentage (assuming you can here): child widget B should have A as its parent.

You shouldn't be in a complex situation like this, nobody else is.

[Chris Kawa]

@Atr0p0s You can simply make B a child of A, i.e. pass A as a parent to B constructor or use the setParent method. A will delete its child when it is deleted itself.
If you prefer to do it manually for some reason use QPointer instead of a raw pointer. It is automatically nulled when the object it points to is deleted.

[jeremy_k]

@Atr0p0s said in how to detect if a widget is destroyed?:

    if (p_B) {
       delete(p_B);

On the pedantic side, testing for a nullptr prior to deleting isn't generally necessary.
https://en.cppreference.com/w/cpp/language/delete:

If expression evaluates to a null pointer value, no destructors are called, and the deallocation function may or may not be called (it's unspecified), but the default deallocation functions are guaranteed to do nothing when passed a null pointer.

[Atr0p0s]

@JonB parentage isn't enough because I have to delete widget B when A is hidden (closed).
And thank you again! I didn't know about deleteLater() :)

[Atr0p0s]

@Chris-Kawa Thank you! QPointer does exactly what I want.

[Atr0p0s]

@jeremy_k Thanks, you are right! I was just trying somehow to get out of the situation :)

[JonB]

@Atr0p0s said in how to detect if a widget is destroyed?:

@JonB parentage isn't enough because I have to delete widget B when A is hidden (closed).

So you could use the void QObject::destroyed(QObject *obj = nullptr) I referenced you to for detecting if B gets destroyed and then set A.p_B to nullptr. Which I suspect is just what QPointer does.

[Atr0p0s]

@JonB I got it. Much appreciated, sir!