you are not interesting with your behaviour! this code you
insulting is generated by the qt designer. it isn't my code!!
i'm beginner you know and i'm understanding examples. this code
is working perfectly. i forgot to send the "firstdialog.ui" and
the "main()" function. when i click to the pushbutton, the
firstdialog window is appearing. what i wanted is that it apears
in the place of frame_3 of the mainwindow. but you are giving
responses like you want in this good forum(i think). this is my
last response for you,i wasted my time with somebody like you.
here are these 2 functions i forgot.

firstdialog.ui

#include <QApplication>
#include "mainwindow.h"

int main(int argc, char* argv[])
{
QApplication app(argc,argv);
MainWindow w;
w.setFixedSize(800,700);
w.show();

return app.exec();
}