Is using "this" in contructor's initialization list specificly dangerous with Qt?

Question

I need reliable information about "this" subject:

class MyClass, public QWidget
{
public:
    MyClass( QWidget * parent = NULL )
         :QWidget( parent ),
         mpAnotherWidget( new QWidget( this ) ){};
private:
    QWidget * mpAnotherWidget;
};

Of course, calling virtual functions in contructor OR initialization list is a bad idea. The Question is: can this code

mpAnotherWidget( new QWidget( this ) )

lead to undefined behavior?! And if so: why ?

Please quote your sources if you can! Thanks!

Answer 1

It depends on what QWidget does with the pointer it is given. It is perfectly fine to pass around a reference or pointer to a half-constructed object as long as the called code does not access the underlying object. You need to look into the documentation of QWidget to know whether it touches the object or just stores the pointer.

---

In the particular case of Qt, reading the documentation, it is calling the constructor of QWidget, the argument is of type QWidget*, and this is only used to cast to the base pointer. Obtaining the pointer to the base is guaranteed in 12.7/3 as the requirement for the conversion is that the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from B shall have started. The pointer is then passed to the QWidget constructor that may use it in any way it wants, since the constructor for the base QWidget has already completed before the constructor for the mpAnotherWidget starts.