CODEWITHSUNDEEP
c++qtoop
parsecer
parsecer

Reputation: 5106

keypressevent and its overload

I have a following class which handles escape key press.

         class KeyPress : public QWidget
       {
           Q_OBJECT
          public:
            KeyPress(QWidget * parent=0);

          protected:
             void keyPressEvent(QKeyEvent * event);

       };

And the .cpp file:

          void KeyPress::keyPressEvent(QKeyEvent *event)
           {

                if (event->key() == Qt::Key_Escape) {
                    qApp->quit();
                           }
            }

It works fine, but if I change the method's name, to say, keyPressed(QKeyEvent * event), it doesn't work. The keyPressEvent(QKeyEvent * event) method according to docs is a protected method of QWidget. So the initial code just overloads that method, dosn't it? And the overload works. But why a completely new version of the method with another name but the same implementation wouldn't work?

Upvotes: 0

Views: 368

Answers (1)

fghj
fghj

Reputation: 9394

It is not overload, it is override of virtual function. Read about c++ virtual functions.

For example:

#include <iostream>

using std::cout;
struct Foo { /*virtual*/void f() { cout << "foo:f\n"; } };
struct Boo : public Foo { void f() { cout << "boo:f\n"; } };

int main()
{
    Boo boo;
    Foo *ptr = &boo;
    ptr->f();
}

such code print foo:f, because of for ptr->f() compiler generate something like this:

address_of_function = &Foo::f;
call address_of_function

but if you uncomment virtual in my example code will print boo:f, because of virtual cause compiler generate similar to such code:

 address_of_function = ptr->virtual_table_of_Foo_class[offset_of_f]
 call address_of_function

ptr point to Boo class virtual table, and address_of_function will be equal &Boo::f, this how virtual functions works.

So in your case Foo == QWidget and it have code like this inside of Qt:

this->keyPressEvent();

, which take address of keyPressEvent in virtual table, and call it. Obviously if you implement KeyPress::someOtherFunction, it will be not called, because of already compiled code of Qt have no call to it.

Upvotes: 2

Related Questions