Home/ Questions/Q 7802081
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-02T01:09:09+00:00

Like the title,why the q_ptr pointer is assigned to this pointer of QObject? in

  • 0

Like the title,why the “q_ptr” pointer is assigned to “this” pointer of QObject? in source code.

QObject::QObject(QObjectPrivate &dd, QObject *parent)
: d_ptr(&dd)
{
>>Q_D(QObject);
>>d_ptr->q_ptr = this;/*question*/
.......

Then,when use Q_Q() macro in source code like blow:

Q_Q(QWidget)

It will return the q pointer handled by the function q_fun():

QWidget*q_func() {return static_cast<QWidget*>(q_ptr);}

As all we know,static_castis not safe when cast from parent to child.

I am very frustrated about /*question*/ ,can any guy tell me the secret?Thanks!

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team
    d_ptr->q_ptr = this;/*question*/

    This is where the private implementation object (PIMPL idiom) is told about the object it is working for/with (the non-private QObject). Here’s a good link for info about Qt and d pointers (d_ptr).

    Q_Q macro returns the pointer to the QObject, so you can emit signals from it (among other things). As for the static_cast bit, that is safe because the macro is defined differently for each class created by the Q_DECLARE_PRIVATE and Q_DECLARE_PUBLIC macros: the result being, static_cast is always casting to the correct type. Again, I recommend reading the link.

    • 0