Qt's Property System

A Simple Example

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.

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.

 Q_PROPERTY(QDate date READ getDate WRITE setDate)

 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;
 };

 Q_CLASSINFO("Version", "3.0.0")