Qt Code: 
  1. #include "example.h"
  2. #include "ui_example.h"
  3. #include "hi.h"   //include the header file
  4.  
  5. example::example(QWidget *parent) :
  6.     QMainWindow(parent),
  7.     ui(new Ui::example)
  8. {
  9.     ui->setupUi(this);
  10.  
  11. // note that the declaration of pointer can be done only once
  12.  
  13.     connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(on_pushButton_clicked()));  //connect... the signal with slot ... 
  14.  
  15. }
  16.  
  17. example::~example()
  18. {
  19.     delete ui;
  20. }
  21.  
  22. void example::on_pushButton_clicked()
  23. {
  24.    //use the other class to instantiate the QDialog or the QDialog derived class (with or without ui) 
  25.  
  26.    hi *gamatos = new hi(this);  //this is valid if the class name is "hi"... replace that with your class name  //you need to pass 'this' as parent so that you don't get a memory leak
  27.  
  28.    gamatos->show(); //you show the dialog
  29. }
  30.  
  31. void example::changeEvent(QEvent *e)
  32. {
  33.     QMainWindow::changeEvent(e);
  34.     switch (e->type()) {
  35.     case QEvent::LanguageChange:
  36.         ui->retranslateUi(this);
  37.         break;
  38.     default:
  39.         break;
  40.     }
  41. }
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NOTE: the code changed a bit... i corrected in the previous post, but you already copied... you can't declare the pointer twice (you don't declare it in the constructor, only in the slot)