CODEWITHSUNDEEP
c++qt
helmeligi
helmeligi

Reputation: 11

Can I use QCoreApplication::instance() for Orphan QObjects?

For QObjects created on the heap with no parent, I found that destructor is not called. So, I starting using the core application instance as their parent to ensure that they are cleaned by garbage collector. Is this the right action, or what should I exactly to safely de-allocated these objects?

Here is an example of what I am doing:

// Use application instance as parent to avoid memory leak if object is not deleted
m_qObject = new DataHandler(QCoreApplication::instance());

Upvotes: 1

Views: 466

Answers (1)

cbuchart
cbuchart

Reputation: 11555

Qt doesn't have an independent memory management, such as a garbage collector. The explicit parent-child relationship used is a classic ownership model that helps in the automatization of object life-cycles, it is deterministic (it will delete object on parent's destruction), and it is very natural with user interfaces.

If an object is created (in the heap) without a parent, it will generate a memory leak when the last reference to it is lost. Objects in the stack should not have a parent to prevent double-delete errors (check Qt documentation for further information)

If you set QApplication::instance as the parent of an object, you are basically telling it not to be destroyed until the application quits. If that's the case, then there is no such a great difference with other standard memory leaks, escept the object's destructor does something else than just releasing memory (such as closing OS-wide handles, saving files, etc).

There is a very interesting case I was curious about: what happened when the original parent (Q*Application::instance) was replaced at any given moment, as in this question?

The case is that the ownership of the object is transferred from the former Q*Application instance to the new one. It may become a problem if you rely on the existence of the object through the whole application life-span but several instances of Q*Application are allowed to be created and destroyed at any moment. One scenario would be a non-Qt application loading plug-ins or components built on Qt, in where you cannot control the creation/destruction order.

If that's the case, I'd suggest to find another way to destruct your object, such as having your own singleton inheriting from QObject that you can use as parent.

Upvotes: 1

Related Questions