Question

Possible Duplicate:
Undefined reference to vtable. Trying to compile a Qt project

here is the code

#include <iostream>
#include <QApplication>
#include <QTimer>


class myClass : public QObject {
    Q_OBJECT

public:
    QTimer *timer;
    myClass(){
        timer = new QTimer(this);
        connect(timer, SIGNAL(timeout()), this, SLOT(mySlot()));
        timer->start(1000);
    }
public slots:
    void mySlot() {
        std::cout << "Fire" << std::endl;
    }
};

int main() {
    std::cout << "Hello, world";
    myClass atimer;
    return 0;
}

Apart from the error, there are two more things I don’t understand:

1. Why there isn’t any semicolon after macros, in this case Q_OBJECT. It doesn’t seem to obey C++ syntax but yet people write code like that.

2. The “public slots” is a modifier created by Qt, but how come gcc compiler can still understand it. How could an IDE like Qt modify the standard syntax of a language?

Answer 1

You didn’t give the exact error message, but I suspect what’s happening is that you didn’t run moc on your code, or you didn’t compile the code generated by moc, or you didn’t link the code into your executable/library.

As for your other questions:

1. You don’t need to have a semicolon after macros; the preprocessor doesn’t care about semicolons – only the compiler does. So whether or not you need to add a semicolon manually depends on what your macro (Q_OBJECT) in this case expands to, and where you use it. In your case, no semicolon is needed.

2. slots is an macro which expands to an emtpy string, so any C++ compile can process it. However, slots is also recognized as a special key word by moc. The same goes for signals, by the way (it’s a macro expanding to protected:).