failure to initialize multiple windows using qt - qt

Question

I've been struggling with the problem of initializing of multiple forms in Qt under Windows (the version is 5.0.1 but I don't reckon this to matter much). Long story short, my application contains of 2 forms the one of which is supposed to be called on clicking the button on the another one.
The code runs as follows:
1) main.cpp:
#include "main.h"
#include <QApplication>
FmNewWord fmNewWord;
MainWindow w;
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
w.show();
return a.exec();
}
2) main.h:
#ifndef MAIN_H
#define MAIN_H
#include "mainwindow.h"
#include "fmnewword.h"
extern FmNewWord fmNewWord;
extern MainWindow w;
#endif // MAIN_H
3) mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include "ui_mainwindow.h"
class MainWindow : public QMainWindow, private Ui::MainWindow
{
Q_OBJECT
signals:
void sgnl_new_word();
public:
explicit MainWindow(QWidget *parent = 0);
protected:
void changeEvent(QEvent *e);
private slots:
void on_commandLinkButton_clicked();
void on_miExit_triggered();
void on_miNewWord_triggered();
};
#endif // MAINWINDOW_H
4) mainwindow.cpp:
#include "main.h"
#include <Windows.h>
extern FmNewWord fmNewWord;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
setupUi(this);
}
void MainWindow::changeEvent(QEvent *e)
{
QMainWindow::changeEvent(e);
switch (e->type()) {
case QEvent::LanguageChange:
retranslateUi(this);
break;
default:
break;
}
}
void MainWindow::on_commandLinkButton_clicked()
{
MessageBox(0, 0, 0, 0);
}
void MainWindow::on_miExit_triggered()
{
this->close();
}
void MainWindow::on_miNewWord_triggered()
{
fmNewWord.show();
}
5) fmnewword.cpp:
#include "fmnewword.h"
FmNewWord::FmNewWord(QWidget *parent) :
QMainWindow(parent)
{
setupUi(this); /// here the app crashes with parent == NULL
}
void FmNewWord::changeEvent(QEvent *e)
{
QMainWindow::changeEvent(e);
switch (e->type()) {
case QEvent::LanguageChange:
retranslateUi(this);
break;
default:
break;
}
}
6) fmnewword.h:
#ifndef FMNEWWORD_H
#define FMNEWWORD_H
#include "ui_fmnewword.h"
class FmNewWord : public QMainWindow, private Ui::FmNewWord
{
Q_OBJECT
public:
explicit FmNewWord(QWidget *parent = 0);
protected:
void changeEvent(QEvent *e);
};
#endif // FMNEWWORD_H
The code keeps crashing during inititialization in fmnewword.cpp on the line shown. Could anybody point out the reason? Thanks.

Answer 1

First of all, QApplication must be created before any widgets.
Your code should look llike this:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
And that's how the second form must be created:
class FmNewWord;
class MainWindow : public QMainWindow, private Ui::MainWindow
{
Q_OBJECT
...
private slots:
void on_miNewWord_triggered();
private:
FmNewWord *fmNewWord;
};
void MainWindow::on_miNewWord_triggered()
{
fmNewWord = new FmNewWord;
fmNewWord->show();
}