When does a deep copy happen to a QList?

Question

In a class I\'m working on, I am juggling several QLists. I have heard that Qt tries not to make deep copies of lists whenever possible. From what I understan

Answer 1

When you pass a QList as a argument of a function or when you create copies without modifying it Qt just pass a "wrapper", the real data is never copied along. Implicit sharing is the Qt implementation of the Flyweight pattern, see here.

Answer 2

QLists are implemented using implicit sharing.

Object assignment (with operator=()) for implicitly shared objects is implemented using shallow copies.

This means that assignment alone will never cause the contained data to be copied. However, writing to a shared instance will cause the source object to be copied. This pattern is commonly known as copy-on-write.

So, to answer your question, if you never write to shared instances then they will never be copied. If you want to completely prevent copies being made then derive from QList and override and hide the copy constructor and assignment operator.

Answer 3

Just make sure to use constBegin(), constEnd(), pass by const reference, don't modify or copy.