Meta-Object System

How Nt describes classes at run time: introspection, properties and invoking by name.

A class that contains Q_OBJECT (or Q_GADGET) and goes through the ntmoc tool has a QMetaObject. ntmoc reads the class declaration and records:

• the class name and the base class;

• signals, slots and functions marked Q_INVOKABLE, with their parameter types and names, and their default arguments;

• constructors marked Q_INVOKABLE;

• properties declared with Q_PROPERTY();

• enumerations declared with Q_ENUM() and Q_FLAG();

• class information declared with Q_CLASSINFO().

Using it

class Counter : public QObject {
    Q_OBJECT
    Q_PROPERTY(int value READ value WRITE setValue NOTIFY valueChanged)
    Q_CLASSINFO("Version", "1.0")
public:
    enum Mode { Up, Down };
    Q_ENUM(Mode)
    int value() const { return m_value; }
    Q_INVOKABLE void reset() { setValue(0); }
public slots:
    void setValue(int v);
signals:
    void valueChanged(int value);
private:
    int m_value = 0;
};

Counter c;
c.setProperty("value", 5);                         // via the property system
QMetaObject::invokeMethod(&c, "reset");            // by name
c.metaObject()->className();                       // "Counter"
QObject::connect(&c, SIGNAL(valueChanged(int)), &other, SLOT(setValue(int)));

Properties

QObject::property() and QObject::setProperty() read and write properties by name, converting values through QVariant. A name that is not a declared property creates a dynamic property on the object; its changes are reported with a QDynamicPropertyChangeEvent.

Value types

QMetaType describes the types that properties, method arguments and QVariant hold. Types that Nt does not know need Q_DECLARE_METATYPE() or qRegisterMetaType() before they can be queued, stored in a QVariant or looked up by name.

Limits

The meta-object of a class exists only if ntmoc processed its header. Code that does not use ntmoc, such as a class that only declares signals with emitSignal() bodies, keeps working but has no QMetaObject of its own: QObject::metaObject() returns that of its nearest base class that has one. ntmoc is a text scanner and does not evaluate the preprocessor, so declarations produced by macros are not seen.

See also QMetaObject, QMetaType, QVariant and Signals and Slots.