connect() problem

[harmodrew]

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:

#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

[squidge]

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.

[harmodrew]

Why not use the reference you already have?

What reference?

[leoalvesmachado]

To use Message Box, use the following example, for instance
QMessageBox msgBox;
msgBox.setText("The document has been modified.");
msgBox.exec();

[harmodrew]

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...

[squidge]

What reference?



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




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.

[harmodrew]

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

mainwindow.cpp:


#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:


#include "actions.h"
#include <QMessageBox>

void actionsObj ::ButtonClicked()
{
QMessageBox msg;
msg.setText("Hello. You've clicked the button");
msg.exec();
}


actions.h:


#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:


#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:


#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:

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

[Lykurg]

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;)

[harmodrew]

I've changed the code and now it works!


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?

[harmodrew]

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 :)

[harmodrew]

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


#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:

collect2: Id returned 1 exit status
what it refers to??
in addition if I comment the Q_OBJECT line it compiles without errors!

[Lykurg]

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

[harmodrew]

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?

[squidge]

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.

[harmodrew]

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