Is this the corrected way to use QPointer?

Question

How could I check whether a pointer's content is deleted? If I use QPointer like this:

myClass::myClass(myStruct* p){
 _p = p;//_p is a QPointer<myStruct>
}

myClass::function(){
if(_p) {_p->function();}
}

then I have

myStruct* p = new myStruct();
myClass A(p);
delete p;
A.function();

will the last A.function() cuase the _p->function() be called and therefore cause access violation? when I delete p, what will happen to _p?

With regular pointers there is no way to check, see `QSharedPointer` and `QWeakPointer` — dtech, Dec 11 '14 at 19:28
But the QPointer's doc says it will be cast to 0 once it is deleted? — Nyaruko, Dec 11 '14 at 19:48
@ddriver Do I misread you? `QObject` destructor will clear all `QPointer`s pointing to it. — hyde, Dec 11 '14 at 19:56
@hyde - yes you are right. My bad. But it only works for `QObject` derived. — dtech, Dec 11 '14 at 20:02

score 2 · Accepted Answer · answered Dec 11 '14 at 20:10

2

If myStruct is a QObject subclass, then your code should work, as much as can be said from shown snippets. The QObject destructor will clear every QPointer pointing to the destructed instance. If it isn't a QObject subclass, then you should get compile/link error.

Quote from the docs:

Note that class T must inherit QObject, or a compilation or link error will result.

answered Dec 11 '14 at 20:10

hyde

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But when is the QPointer set back to NULL? once the delete p is called, is it set back to NULL immediately(assume the delete p and the QPointer are in the same thread )? If there's another thread emiting signal to trigger myClass::function(), will the example in my question still be safe? – Nyaruko Dec 12 '14 at 21:14
1

In C++, with no garbage collection, `delete` will cause the object destructors to be called immediately, in reverse order, so `QObject::~QObject` is the last destructor to be called, but it will be called before `delete` "returns". If you use inter-thread signals, they should be queued, so deleting the object should not be a problem. – hyde Dec 13 '14 at 08:21
1

@Nyaruko If you call any method of the object being deleted from another thread while it is being deleted (or doing anything else which is not explicitly thread safe), be it direct method call, or via `Qt::DirectConnection` signal, it's basically a bug in your program. – hyde Dec 13 '14 at 08:23

Is this the corrected way to use QPointer?

1 Answers1