connect() problem

Hello you all!
I'm a newbie in Qt programming so my question will probably seem a stupid question but all guides I read didn't help me...
I'm developing a C/C++ program in Win Vista & Qt Creator IDE with the Qt framework. It is a Qt GUI Application (the one you can create by clicking New -> project)
I'd like to connect the event of clicking a PushButton with a function I wrote earlier. Here is the code of my mainwindow.cpp file:

Code:

---

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include<QMessageBox>
class act: public QObject
{
 public slots:
     void buttonClicked()
     {
         QMessageBox("Title","Msg",QMessageBox::NoIcon,QMessageBox::Ok,QMessageBox::NoButton,QMessageBox::NoButton);
     }
 };
  MainWindow::MainWindow(QWidget *parent) :
     QMainWindow(parent),
     ui(new Ui::MainWindow)
 {
QPushButton *PB= new QPushButton(this);
     PB->setText("Button text");
      act a;
     connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));
     PB->show();
     }
  MainWindow::~MainWindow()
 {
     delete ui;
 }

---

It is executed correctly but when I push the button the message doesn't show!

What could be the problem?

Thanks a lot, harmodrew

In your constructor you create a local variable which is destroyed when the constructor has finished.

Why not use the reference you already have?

Secondly, the class declaration should be in a seperate header file to the definition so it can be easily compiled by the meta object compiler.

Quote:

---

Why not use the reference you already have?

---

What reference?

To use Message Box, use the following example, for instance

Code:

---

QMessageBox msgBox;
 msgBox.setText("The document has been modified.");
 msgBox.exec();

---

searching around the Net I've found that all signal/slot classes must include in the private section the macro Q_OBJECT.
Now the problem is that if I include this macro I have the error:
"collect2: Id returned 1 exit status"
and clicking the message of the error it doesn't bring me to the row affected...

Quote:

---

Originally Posted by harmodrew

What reference?

---

Code:

---

act a;
connect(PB, SIGNAL(clicked()), &a,SLOT(buttonClicked()));

---

Code:

---

connect(PB, SIGNAL(clicked()), this,SLOT(buttonClicked()));

---

Secondly, when you are finished, choose "run qmake" in QtCreator before trying to run. This will generate the rules to run MOC.

So, here are my project files (I've split the class "actionObj" from the mainwindow file).

mainwindow.cpp:

Code:

---

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPushButton>
#include "actions.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QPushButton *NewButton= new QPushButton(this);
    NewButton->setText("Button");
 
    actionsObj *ao;
    ao = new actionsObj;
    connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));
    NewButton->setGeometry(50, 40, 300, 200);
    NewButton->show();
 //  ui->setupUi(this); if I uncomment this, the button isn't clickable!
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

---

actions.cpp:

Code:

---

#include "actions.h"
#include <QMessageBox>
 
void actionsObj ::ButtonClicked()
{
    QMessageBox msg;
    msg.setText("Hello. You've clicked the button");
    msg.exec();
}

---

actions.h:

Code:

---

#ifndef ACTIONS_H
#define ACTIONS_H
#include <qobject.h>
 
class actionsObj : public QObject
{
private:
    Q_OBJECT
public slots:
    void ButtonClicked();
};
 
#endif // ACTIONS_H

---

mainwindow.h:

Code:

---

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
    class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
};
 
#endif // MAINWINDOW_H

---

main.cpp:

Code:

---

#include <QtGui/QApplication>
#include "mainwindow.h"
#include <QAction>
#include <QString>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QString title="MainWindow";
 
    MainWindow w;
    w.setGeometry(100, 100, 500, 500);
    w.show();
    return a.exec();
}

---

But now I have the following error:

Quote:

---

collect2: Id returned 1 exit status

---

and the IDE doesn't bring me to the line affected...what could be the problem? it's possible that a simple program like this, even if I'm a starter, is so complicated and full of problems??

Thanks a lot

Code:

---

connect(NewButton, SIGNAL(clicked()), this,SLOT(ButtonClicked()));

---

.
"this" is wrong because it means your main window, but your slot is in your actionsObj. Also you are producing memory leaks! (ao = new actionsObj;)

I've changed the code and now it works!

Code:

---

QPushButton *NewButton= new QPushButton(this);
NewButton->setText("Button");
 
actionsObj *ao= new actionsObj;
connect(NewButton, SIGNAL(clicked()),ao,  SLOT(ButtonClicked()));
NewButton->setGeometry(50, 40, 300, 200);
NewButton->show();
//ui->setupUi(this);

---

the only problem now is that if I uncomment the last row the button isn't clickable anymore. What could be the problem?

ok, I've done it!
simply, the ui->setupUi(this); row is to be written before the other rows, not in the bottom :)
thanks you all :)

here I am again for the same problem...
mainwindow.cpp:

Code:

---

#include "mainwindow.h"
#include "ui_mainwindow.h"
 #include <QObject>
 
class act :public QObject
{
    Q_OBJECT
 
};
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    act *ao= new act;
}
 
MainWindow::~MainWindow()
{
    delete ui;
}

---

this little code returns me the same error:

Quote:

---

collect2: Id returned 1 exit status

---

what it refers to??
in addition if I comment the Q_OBJECT line it compiles without errors!

put your class act in a single file or make sure moc runing right for your class. Simpliest is every class a single file!

thanks for the reply, it helped! but, why are we obliged to write the class in an another file to make it work? what does it change?

Its to do with the meta object compiler. You can have classes in the same file as your .cpp if you want, but if they include the Q_OBJECT macro, then you need to run MOC, so ideally those classes should be in .h files instead.

oh, it's clear now! :) thank again! :)