trouble with signals and slots

[Dmon]

Hi I am new to Qt and I can't figure out how to get a signal and slot to work. I am trying to set a object called f1 of type Fraction to values in a QDoubleSpinBox called num when the button setbutton is clicked. This is main:

#include <QtGui/QApplication>
#include <QtGui>
#include "mainwindow.h"
#include "fraction.h"
#include <windows.h>
#include <QObject>
 
int main(int argc, char *argv[])
{
    QTextStream cout(stdout);
    QApplication a(argc, argv);
    QWidget *window = new QWidget;
    QLabel* first = new QLabel("Enter a fraction");
    QDoubleSpinBox* num = new QDoubleSpinBox;
    QPushButton* setbutton = new QPushButton ("Set Fraction");
    QDialogButtonBox* box = new QDialogButtonBox;
    box->addButton(QDialogButtonBox::Ok);
    box->addButton(QDialogButtonBox::Cancel);
    QVBoxLayout* buttonlayout = new QVBoxLayout;
    buttonlayout->addWidget(first);
    buttonlayout-> addWidget(num);
    buttonlayout-> addWidget(setbutton);
    buttonlayout->addWidget(box);
    window->setLayout(buttonlayout);
    window->show();
    Fraction f1, f2;
    QObject::connect(setbutton,SIGNAL(clicked()),num,SLOT(set(int nn, int nd)));
 
    //f1.set(1,7);
    //f2.set(11,12);
    //cout << "The first fraction is: " << f1.toString() << endl;
    //cout << "\nThe second fraction, as a double is: " << f2.toDouble() << endl;
    //cout.flush();
    //Sleep(10000);
    //return 0;
    return a.exec();
}

#include <QtGui/QApplication>
#include <QtGui>
#include "mainwindow.h"
#include "fraction.h"
#include <windows.h>
#include <QObject>

int main(int argc, char *argv[])
{
    QTextStream cout(stdout);
    QApplication a(argc, argv);
    QWidget *window = new QWidget;
    QLabel* first = new QLabel("Enter a fraction");
    QDoubleSpinBox* num = new QDoubleSpinBox;
    QPushButton* setbutton = new QPushButton ("Set Fraction");
    QDialogButtonBox* box = new QDialogButtonBox;
    box->addButton(QDialogButtonBox::Ok);
    box->addButton(QDialogButtonBox::Cancel);
    QVBoxLayout* buttonlayout = new QVBoxLayout;
    buttonlayout->addWidget(first);
    buttonlayout-> addWidget(num);
    buttonlayout-> addWidget(setbutton);
    buttonlayout->addWidget(box);
    window->setLayout(buttonlayout);
    window->show();
    Fraction f1, f2;
    QObject::connect(setbutton,SIGNAL(clicked()),num,SLOT(set(int nn, int nd)));

    //f1.set(1,7);
    //f2.set(11,12);
    //cout << "The first fraction is: " << f1.toString() << endl;
    //cout << "\nThe second fraction, as a double is: " << f2.toDouble() << endl;
    //cout.flush();
    //Sleep(10000);
    //return 0;
    return a.exec();
}

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here is fraction.h:

#ifndef FRACTION_H
#define FRACTION_H
#include <QString>
#include <QObject>
 
class Fraction: public QObject {
Q_OBJECT
public slots:
    void set(int nn, int nd);
public:
    Fraction();
    Fraction(int nn, int nd);
    QString toString() const;
    double toDouble() const;
    Fraction add(const Fraction& other);
    Fraction subtract(const Fraction& other);
    Fraction multiply(const Fraction& other);
    Fraction divide(const Fraction& other);
 
private:
    int m_Numerator;
    int m_Denominator;
};
 
 
#endif // FRACTION_H

#ifndef FRACTION_H
#define FRACTION_H
#include <QString>
#include <QObject>

class Fraction: public QObject {
Q_OBJECT
public slots:
    void set(int nn, int nd);
public:
    Fraction();
    Fraction(int nn, int nd);
    QString toString() const;
    double toDouble() const;
    Fraction add(const Fraction& other);
    Fraction subtract(const Fraction& other);
    Fraction multiply(const Fraction& other);
    Fraction divide(const Fraction& other);

private:
    int m_Numerator;
    int m_Denominator;
};


#endif // FRACTION_H

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this is the error I get when I try to run:
'QObject::QOject(const QObject&)' is private

[wysota]

You are trying to copy an instance of your Fraction object in lines 15-18 of your last snippet.

[Santosh Reddy]

QObject::connect(setbutton,SIGNAL(clicked()),num,SLOT(set(int nn, int nd)));

QObject::connect(setbutton,SIGNAL(clicked()),num,SLOT(set(int nn, int nd)));

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This is not legal in Qt. The slot should have less than or equal number of arguments as signal.

Fraction add(const Fraction& other);
Fraction subtract(const Fraction& other);
Fraction multiply(const Fraction& other);
Fraction divide(const Fraction& other);

Fraction add(const Fraction& other);
Fraction subtract(const Fraction& other);
Fraction multiply(const Fraction& other);
Fraction divide(const Fraction& other);

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All these function declaration are not possible int Qt, because Fraction is derived QObject, instead use somthing like

void add(const Fraction& other);
void subtract(const Fraction& other);
void multiply(const Fraction& other);
void divide(const Fraction& other);

void add(const Fraction& other);
void subtract(const Fraction& other);
void multiply(const Fraction& other);
void divide(const Fraction& other);

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Also typically all QObject derived class objects follow parent-child relationships, i.e the parent is passed during QObject construction. The best way to define Fraction class would be to (If you have decided to make it QObject)

class Fraction: public QObject {
Q_OBJECT
public:
    Fraction(int nn, int nd, QObject * parent = 0);
}

class Fraction: public QObject {
Q_OBJECT
public:
    Fraction(int nn, int nd, QObject * parent = 0);
}

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[wysota]

In my opinion it makes completely no sense to derive Fraction from QObject.

[Dmon]

Thanks for the help. So what you are saying is that the SLOT needs to have no variables because the SIGNAL has none?

[alizadeh91]

The arguments in Slot and signal has to be same or at least the slot can have less arguments than signal(but the type of argument and ordering is mandatory)