QMainWindow not showing up - c++

Question

I have made a quick test program which doesn't seem to bring up anything when executed. Here is my code:
z_lock.h
#ifndef Z_LOCK_H
#define Z_LOCK_H
#include <QtGui>
class z_lock : public QMainWindow
{
Q_OBJECT
public:
z_lock();
private slots:
void password_check();
void quit();
private:
QStackedWidget *book;
QWidget *page1;
QWidget *page2;
QLineEdit *input;
};
#endif
z_lock.cpp
#include "z_lock.h"
z_lock::z_lock(){
book = new QStackedWidget;
//page1
page1 = new QWidget;
QLabel *label1 = new QLabel("Enter password.");
input = new QLineEdit;
QPushButton *goButton = new QPushButton("Go");
connect(goButton, SIGNAL(clicked()), this, SLOT(password_check()));
QHBoxLayout *layout1;
layout1 -> addWidget(label1);
layout1 -> addWidget(input);
layout1 -> addWidget(goButton);
page1 -> setLayout(layout1);
//page2
page2 = new QWidget;
QLabel *label2 = new QLabel("Welcome, you're in.");
QVBoxLayout *layout2;
layout2 -> addWidget(label2);
page2 -> setLayout(layout2);
book -> addWidget(page1);
book -> addWidget(page2);
setCentralWidget(book);
}
void z_lock::quit(){
close();
}
void z_lock::password_check(){
QString guess = input -> text();
if (guess == "apple"){
z_lock::book -> setCurrentIndex(1);
}
}
main.cpp
#include "z_lock.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
z_lock *that;
that -> show();
return app.exec();
}
Please, any help would do. I have carried out research about everything to do with QMainWindow and still nothing. This will probably be stupidly easy to solve and I'm sorry for such a basic question.

Answer 1

You have several issues in your code... Let us start with the most important:
You are allocating the object in your main.cpp on the heap which is a sort-of issue in itself, but even if you do that, you should use the new keyword for that in C++.
That being said, you really should not allocate it here on the heap: partially because it would leak the memory, but more importantly there is no need for it.
Therefore, replace these lines:
z_lock *that;
that -> show();
with simply this:
z_lock that;
that.show();
This is pretty much the same issue for your layouts here, too:
QHBoxLayout *layout1;
QVBoxLayout *layout2;
Please change them to:
QHBoxLayout *layout1 = new QHBoxLayout;
QVBoxLayout *layout2 = new QVBoxLayout;
Furthermore, you should not include the whole QtGui module like this:
#include <QtGui>
Just include the classes which you eventually need. Even though this is in some Qt 4 book, this is bad practice.
This method is also useless in your code since it does nothing except wrapping the existing slot:
void z_lock::quit(){
close();
}
Also, you may wish to use the tr() method for things like this to get translation for other languages:
QLabel *label1 = new QLabel("Enter password.");

Answer 2

z_lock *that;
that -> show();
The problem is that is an uninitialized pointer. It doesn't point to a valid z_lock instance for you to call methods on.
You need to do either allocate and construct a z_lock instance with new:
z_lock* that = new z_lock;
that->show();
Or, more simply, switch to stack allocation:
z_lock that;
that.show();
The same applies to the "layout" variables in the z_lock constructor.
This is an example of why it's a good idea to compile at a high warning level and fix warnings. The compiler can easily notice and point out the invalid use of uninitialized variables.