Define signals and slots inside main.cpp

Question

I wrote a little program with a my own class within the main.cpp. Here the code:

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

class MyWidget : public QWidget {
    //Q_OBJECT
public:
    MyWidget(QWidget* parent = 0);
    QLabel* label;
    QString string;

signals:
public slots:
    void setTextLabel();

};

void MyWidget::setTextLabel() {
    label->setText("Test");
}


MyWidget::MyWidget(QWidget* parent) 
     : QWidget(parent) {

}

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

    MyWidget widget;
    widget.show();

    return app.exec();
}

it seems work but not "completely". My slot doens't work. I suppose i have to put Q_OBJECT. BUT, doing so, I got a list of errors, like this:

undefined reference to `vtable for MyWidget'
........................................
collect2: error: ld returned 1 exit status
make: *** [mywidget] Error 1

I can I manage that? Where the problem?

Answer 1

Signals and slots in Qt are managed through the moc: meta object compiler. Basically, the moc generates additional C++ code for each class containing the Q_OBJECT macro in order to implement effectively the signals and slots mechanisms. The additional code is then linked to the original class declaration.

The problem here is that your class is declared in main.cpp: this conflicts with how the moc is working with your code. You should declare your class in a separate header.

More about the moc

Edit: as hyde pointed, an alternative is to include in your cpp the file generated by the moc: Why is important to include “.moc” file at end of a Qt Source code file?