Qt: Signal and slots and Q_OBJECT

Question

I have written a program in Qt, it works fine, but clicking on push buttons does not work. When I use Q_OBJECT just after the class declaration, it gives me compilation errors.

Here is my code:

    #include <QApplication>
    #include <QPushButton>
    #include <QLabel>
    #include <QWidget>

    class Communicate : public QWidget
    {
     // Q_OBJECT

      public:
        Communicate(QWidget *parent = 0);

      private slots:
        void OnPlus();
        void OnMinus();

      private:
        QLabel *label;

    };

    Communicate::Communicate(QWidget *parent)
        : QWidget(parent)
    {
      QPushButton *plus = new QPushButton("+", this);
      plus->setGeometry(50, 40, 75, 30);

      QPushButton *minus = new QPushButton("-", this);
      minus->setGeometry(50, 100, 75, 30);

      label = new QLabel("0", this);
      label->setGeometry(190, 80, 20, 30);

      connect(plus, SIGNAL(clicked()), this, SLOT(OnPlus()));
      connect(minus, SIGNAL(clicked()), this, SLOT(OnMinus()));
    }

    void Communicate::OnPlus()
    {
      int val = label->text().toInt();
      val++;
      label->setText(QString::number(val));
    }

    void Communicate::OnMinus()
    {
      int val = label->text().toInt();
      val--;
      label->setText(QString::number(val));
    }

    int main(int argc, char *argv[])
    {
      QApplication app(argc, argv);

      Communicate window;

      window.setWindowTitle("Communicate");
      window.show();

      return app.exec();
    }

score 1 · Answer 1 · answered Feb 22 '13 at 22:20

1

You need to run files that contain the Q_OBJECT macro through moc, and compile and link it's output together with the rest of your applications. Depending on your buildsystem this happens automatically (qmake) or by adding one line or so (cmake)

answered Feb 22 '13 at 22:20

wer

116
1

score 0 · Accepted Answer · answered Feb 22 '13 at 22:20

0

Use O_QBJECT macro and to correct the errors you get you can either:

1) Declare your QWidget/QObject derived class in it's own .h file and define it in it's own .cpp file.

2) include the .moc file generated by moc after your class definition in the .cpp you have (add something like #include "xxxx.moc")

LE: also you want to use layouts instead of use the setGeometry to place your widgets into a window, read more here

answered Feb 22 '13 at 22:20

Zlatomir

6,964
3
26
32

i found it running correctly when i use seperate .h and .cpp files, in fact its a tutorial code i run on my system, that told to make seperate files. I modified for myself, and want to know why does this happen, why can't we write everything in main.cpp?? also i dont understand ur second method, what's moc? – shivshnkr Feb 23 '13 at 07:38
moc is a Qt tool that generates C++ code for QObject derived classes functionality, like: signals and slots. Anyway the recommended method is the first and don't ignore the advice to use layouts instead of manually set the widgets to position on parent. – Zlatomir Feb 23 '13 at 07:41
why can't we write everything in main.cpp?? – shivshnkr Feb 23 '13 at 09:15
you can, but then you have to include "main.moc"*** (so that the C++ compiler you use see the generated stuff) ***name might be different, it might depend on moc version and so on (this is another reason to use h/cpp files for classes and let the tools do their job) – Zlatomir Feb 23 '13 at 10:34

Qt: Signal and slots and Q_OBJECT

2 Answers2