QSharedData and operator =

Question

I recently wanted to implement implicit sharing functions like Qt with its QSharedData and QSharedDataPointer , so I looked at their sources and instead of QSharedData found these three lines:

 private: // using the assignment operator would lead to corruption in the ref-counting QSharedData &operator=(const QSharedData &); 

However, I do not understand how operator= to break the reference count.

If I simply did not make it closed and left its implementation empty, would it serve the same purpose?

i.e. if I just wrote this:

  public: QSharedData &operator=(const QSharedData & ) { return *this; } 

Answer 1

The purpose of QSharedData is to maintain reference counting. If you assign each other, what should happen with the counting of links on each side? As you correctly identified: nothing. It just doesn’t make sense to assign one QSharedData another, and therefore a reasonable course of action is to prevent it at compile time.

Answer 2

No, it would be bad if he did the reference counting, he needed to keep accounting records, and just returning this means that there are copies of QSharedData unaccounted for. this C ++ faq example basically shows what accounting is necessary for operator = in a counted object.