Access UI elements from another class c++ - c++

Question

I have a problem with accessing ui elements from another class(with instance). I have a second QMainWindow in my application, I can access in secondWindow.cxx class all ui elements but not in read.cxx class. My code looks like following. Where is my mistake? Thank you for your help.
-------------------------------secondWindow.h------------------------------------
#ifndef __secondWindow_h
#define __secondWindow_h
#include "ui_secondwindow.h"
class secondWindow : public QMainWindow
{
friend class read;
igstkStandardClassBasicTraitsMacro(secondWindow, QMainWindow);
Q_OBJECT
public:
igstkStateMachineMacro();
secondWindow();
virtual ~secondWindow();
void createSignalAndSlots();
public slots:
void secondWindowTest();
protected:
private:
Ui::secondMainWindow m_secondWindowUI;
};
#endif
-------------------------------secondWindow.cxx------------------------------------
#include "secondWindow.moc"
#include "secondWindow.h"
#include "read.h"
secondWindow::secondWindow() :m_StateMachine(this)
{
m_secondWindowUI.setupUi(this);
createSignalAndSlots();
}
void secondWindow::createSignalAndSlots()
{
connect(m_secondWindowUI.pushButton1, SIGNAL(clicked()),this, SLOT(secondWindowTest()));
connect(m_secondWindowUI.pushButton2, SIGNAL(clicked()), read::instance(), SLOT(readTest()));
}
void secondWindow::secondWindowTest()
{
m_secondWindowUI.pushButton1->setEnabled(true); //OK
}
secondWindow::~secondWindow(){}
---------------------------------read.h--------------------------------------
#pragma once
#include "secondWindow.h"
class read : public QObject
{
Q_OBJECT
public:
static read *instance();
read();
virtual ~read() {}
public slots:
void readTest();
protected:
secondWindow *m_readUI;
static read *m_read;
private:
};
---------------------------------read.cxx--------------------------------------
#include <read.moc>
#include "secondWindow.h"
#include "read.h"
read *read::m_read= NULL;
read::read()
{
m_readUI = dynamic_cast<secondWindow*>( QApplication::instance() );
}
read *read::instance()
{
if(m_read == NULL)
m_read = new read();
return m_read;
}
void read::readTest()
{
m_readUI->m_secondWindowUI.qlabelTest->setText("test"); //segmentation fault
}

Answer 1

You are casting a QApplication::instance(), which is a QApplication * deriving from QCoreApplication * deriving from QObject *. That won't work, it's not a secondWindow *, not even a QMainWindow *, not even a QWidget *.
Apart from that, your coding style is rather strange -- in Qt, it's customary to use CamelCase for classes, not thisStuff which usually applies to functions and methods. Including <read.moc> is just wrong. Why is read::m_read static? Finally, the coupling between the two window classes is set up in a strange way (accessing global stuff like QApplication just to get a reference to another window smells ugly code). A much better and more obvious approach is to either wrap all of your windows in a parent object or setting up the dependencies explicitly, perhaps like this:
MainWindow *mainWindow = new MainWindow();
SecondWindow *second = new SecondWindow(mainWindow);
UtilityWindow *utilityWin = new UtilityWindow(second);