Q_OBJECT macro generation

michalt38

I have read that when Q_OBJECT macro is placed inside a class, some meta-object code is generated. Is this code generated as body of the Q_OBJECT macro? What's more does the generated code provide a signal/slot mechansim for a class in general or only for user-defined signals and slots?

jsulm

@michalt38 said in Q_OBJECT macro generation:

Is this code generated as body of the Q_OBJECT macro?

No, moc generates additional source code files, see https://doc.qt.io/qt-5/why-moc.html

michalt38

So what is the body of the Q_OBJECT doing?

mrjj

@michalt38
It expands to this
Basically it inserts a static MetaObject.

#define Q_OBJECT \
public: \
    QT_WARNING_PUSH \
    Q_OBJECT_NO_OVERRIDE_WARNING \
    static const QMetaObject staticMetaObject; \
    virtual const QMetaObject *metaObject() const; \
    virtual void *qt_metacast(const char *); \
    virtual int qt_metacall(QMetaObject::Call, int, void **); \
    QT_TR_FUNCTIONS \
private: \
    Q_OBJECT_NO_ATTRIBUTES_WARNING \
    Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \
    QT_WARNING_POP \
    struct QPrivateSignal {}; \
    QT_ANNOTATE_CLASS(qt_qobject, "")

michalt38

Thanks. Is the code that moc generates used to handle the user-defined signals and slots only or the whole signal/slot mechanism in general?

mrjj

@michalt38
For signal and slot in general.

However, its now a slight grey area with the new pointer to method based connect syntax as
i think you can skip the Q_OBJECT now if you dont have any custom signals.
And you dont have to flag a slot with the slot macro anymore to use it as any member method can now be used as a slot. with the "new" connect syntax.