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# Qt's Property System

Qt provides a sophisticated property system similar to those supplied by some compiler vendors. However, as a compiler- and platform-independent library, Qt cannot rely on non-standard compiler features like `__property` or `[property]`. Our solution works with any standard C++ compiler on every platform we support. It's based on the meta-object system that also provides object communication through signals and slots.

The `Q_PROPERTY` macro in a class declaration declares a property. Properties can only be declared in classes that inherit QObject. A second macro, Q_OVERRIDE, can be used to override some aspects of an inherited property in a subclass.

To the outer world, a property appears to be similar to a data member. But properties have several features that distinguish them from ordinary data members:

- A read function. This always exists.
- A write function. This is optional; read-only properties like QWidget::isDesktop() do not have one.
- An attribute "stored" that indicates persistence. Most properties are stored, but a few virtual properties are not. For example, QWidget::minimumWidth() isn't stored, since it's just a view of QWidget::minimumSize(), and has no data of its own.
- A reset function to set a property back to its context specific default value. This is very rare, but for example, QWidget::cursor needs this, since no call to QWidget::setCursor() can mean 'reset to the context specific cursor'.
- An attribute "designable" that indicates whether it makes sense to make the property available in a GUI builder (e.g. Qt Designer). For most properties this makes sense, but not for all, e.g. QAbstractButton::down. The user can press buttons, and the application programmer can make the program press its own buttons, but a GUI design tool can't press buttons.

The read, write and reset functions can be just about any member functions, inherited or not, virtual or not. The only exception is that member functions must be inherited from the first inherited class in the case of multiple inheritance.

Properties can be read and written through generic functions in QObject without knowing anything about the class in use. In the code snippet below, the QObject::setProperty() call is equivalent QButton::setDown() call is equivalent to the QObject::setProperty() call:

        QPushButton *button = new QPushButton;
        QObject *object = button;
        // button and object point to the same object

        button->setDown(true);
        object->setProperty("down", true);

Equivalent, that is, except that the first is faster, and provides much better diagnostics at compile time. When practical, the first is better. However, since you can get a list of all available properties for any QObject through its QMetaObject, QObject::setProperty() can give you control over classes that weren't available at compile time.

As well as QObject::setProperty(), there is a corresponding QObject::property() function. QMetaObject::propertyCount() returns the number of all available properties. QMetaObject::property() returns the property data for a given property index: a QMetaProperty object.

Here's a simple example that shows the most important property functions in use:

        class MyClass : public QObject
        {
            Q_OBJECT

        public:
            MyClass(QObject *parent = 0);
            ~MyClass();

            enum Priority { High, Low, VeryHigh, VeryLow };
            void setPriority(Priority priority);
            Priority priority() const;
        };

The class has a property `priority` that is not yet known to the meta object system. In order to make the property known, you must declare it with the `Q_PROPERTY` macro. The syntax is as follows:

        Q_PROPERTY(type name
                   READ getFunction
                   [WRITE setFunction]
                   [RESET resetFunction]
                   [DESIGNABLE bool]
                   [SCRIPTABLE bool]
                   [STORED bool])

For the declaration to be valid, the get function must be const and to return either the type itself, a pointer to it, or a reference to it. The optional write function must return void and must take exactly one argument, either the type itself, a pointer or a const reference to it. The meta object compiler enforces this.

The type of a property can be any QVariant supported type or an enumeration type declared in the class itself. Since `MyClass` uses the enumeration type `Priority` for the property, this type must be registered with the property system as well.

There are two exceptions to the above: The type of a property can also be either QList<QVariant> or QMap<QString,QVariant>. In these cases the type must be specified as `QList` or as `QMap` (i.e. without their template parameters).

It is possible to set a value by name, like this:

        obj->setProperty("priority", "VeryHigh");

In the case of `QList` and `QMap` properties the value passes is a QVariant whose value is the entire list or map.

Enumeration types are registered with the `Q_ENUMS` macro. Here's the final class declaration including the property related declarations:

        class MyClass : public QObject
        {
            Q_OBJECT
            Q_PROPERTY(Priority priority READ priority WRITE setPriority)
            Q_ENUMS(Priority)

        public:
            MyClass(QObject *parent = 0);
            ~MyClass();

            enum Priority { High, Low, VeryHigh, VeryLow };
            void setPriority(Priority priority);
            Priority priority() const;
        };

Another similar macro is `Q_SETS`. Like `Q_ENUMS`, it registers an enumeration type but marks it in addition as a "set", i.e. the enumeration values can be OR'd together. An I/O class might have enumeration values `Read` and `Write` and accept `Read | Write`: such an enum is best handled with `Q_SETS`, rather than `Q_ENUMS`.

The remaining keywords in the `Q_PROPERTY` section are `RESET`, `DESIGNABLE`, `SCRIPTABLE`, and `STORED`.

- `RESET` names a function that will set the property to its default state (which may have changed since initialization). The function must return void and take no arguments.
- `DESIGNABLE` declares whether this property is suitable for modification by a GUI design tool. The default is true for writable properties; otherwise false. Instead of true or false, you can specify a boolean member function.
- `SCRIPTABLE` declares whether this property is suited for access by a scripting engine. The default is true. Instead of true or false, you can specify a boolean member function.
- `STORED` declares whether the property's value must be remembered when storing an object's state. Stored makes only sense for writable properties. The default value is true. Technically superfluous properties (like QPoint pos if QRect geometry is already a property) define this to be false.

Connected to the property system is an additional macro, `Q_CLASSINFO`, that can be used to attach additional name/value-pairs to a class' meta object, for example:

        Q_CLASSINFO("Version", "3.0.0")

Like other meta data, class information is accessible at runtime through the meta object, see QMetaObject::classInfo() for details.

## Q_OVERRIDE

When you inherit a QObject subclass you may wish to override some aspects of some of the class's properties.

For example, in QWidget we have the autoMask property defined like this:

        Q_PROPERTY(bool autoMask READ autoMask WRITE setAutoMask DESIGNABLE false SCRIPTABLE false)

But we need to make the auto mask property designable in some QWidget subclasses. Similarly some classes will need this property to be scriptable (e.g., for QSA). This is achieved by overriding these features of the property in a subclass. In QCheckBox, for example, we achieve this using the following code:

        Q_OVERRIDE(bool autoMask DESIGNABLE true SCRIPTABLE true)

Another example is QToolButton. By default QToolButton has a read-only `toggleButton` property, because that's what it inherits from QButton:

        Q_PROPERTY(bool toggleButton READ isToggleButton)

But we want to make our tool buttons able to be toggled, so we write a WRITE function in QToolButton, and use the following property override to make it acessible:

        Q_OVERRIDE(bool toggleButton WRITE setToggleButton)

The result is read-write (and scriptable and designable, since we now have a "write" function) boolean property `toggleButton` for tool buttons.