Parent-child relationship set to member variables in Qt

Question

I was reading Qt Documentation here. I found the following sentence under the title 'Thread Affinity'.

Note: A QObject's member variables do not automatically become its children. The parent-child relationship must be set by either passing a pointer to the child's constructor, or by calling setParent().

I don't understand what it mentioned as set parent-child relationship between object and it's member variables. I only know the parent-child relationship between super class and sub class.

Can anyone explain this sentence to me? If you can provide a example it is more helpful.

Thanks for reading.

Answer 1

Member variables will NOT become child objects without explicitly setting parent property. A Object subclass normally takes another QObject as it's parent in the constructor.

class Test : public QObject
{
  Q_OBJECT
  public:
    Test(QObject* prnt) 
      : QObject(prnt),
        timerNoPrnt(), // Test object is NOT the parent. This won't be deleted when Test object gets deleted.
        timer(this)    // Test object is the parent here. This will be deleted when Test object gets deleted. 
    {
      timerNoPrnt->setParent(this); // now parent assigned.
    }
  private:
    QTimer*           timerNoPrnt;   // member variable
    QTimer*           timer;
}