- Qt 5.3
- Qt QML
- Extending QML - Inheritance and Coercion Example
- Qt 5.3.2 Reference Documentation

# Extending QML - Inheritance and Coercion Example

C++ Inheritance and Coercion

This example builds on:

- Extending QML - Object and List Property Types Example
- Extending QML - Adding Types Example

The Inheritance and Coercion Example shows how to use base classes to assign types of more than one type to a property. It specializes the Person type developed in the previous examples into two types - a `Boy` and a `Girl`.

    BirthdayParty {
        host: Boy {
            name: "Bob Jones"
            shoeSize: 12
        }
        guests: [
            Boy { name: "Leo Hodges" },
            Boy { name: "Jack Smith" },
            Girl { name: "Anne Brown" }
        ]
    }

## Declare Boy and Girl

    class Boy : public Person
    {
        Q_OBJECT
    public:
        Boy(QObject * parent = 0);
    };

    class Girl : public Person
    {
        Q_OBJECT
    public:
        Girl(QObject * parent = 0);
    };

The Person class remains unaltered in this example and the Boy and Girl C++ classes are trivial extensions of it. As an example, the inheritance used here is a little contrived, but in real applications it is likely that the two extensions would add additional properties or modify the Person classes behavior.

### Define People as a base class

The implementation of the People class itself has not changed since the previous example. However, as we have repurposed the People class as a common base for Boy and Girl, we want to prevent it from being instantiated from QML directly - an explicit Boy or Girl should be instantiated instead.

    qmlRegisterType<Person>();

While we want to disallow instantiating Person from within QML, it still needs to be registered with the QML engine, so that it can be used as a property type and other types can be coerced to it.

### Define Boy and Girl

The implementation of Boy and Girl are trivial.

    Boy::Boy(QObject * parent)
    : Person(parent)
    {
    }

    Girl::Girl(QObject * parent)
    : Person(parent)
    {
    }

All that is necessary is to implement the constructor, and to register the types and their QML name with the QML engine.

## Running the example

The BirthdayParty type has not changed since the previous example. The celebrant and guests property still use the People type.

        Q_PROPERTY(Person *host READ host WRITE setHost)
        Q_PROPERTY(QQmlListProperty<Person> guests READ guests)

However, as all three types, Person, Boy and Girl, have been registered with the QML system, on assignment QML automatically (and type-safely) converts the Boy and Girl objects into a Person.

The main.cpp file in the example includes a simple shell application that loads and runs the QML snippet shown at the beginning of this page.

Files:

- referenceexamples/coercion/birthdayparty.cpp
- referenceexamples/coercion/birthdayparty.h
- referenceexamples/coercion/example.qml
- referenceexamples/coercion/person.cpp
- referenceexamples/coercion/person.h
- referenceexamples/coercion/main.cpp
- referenceexamples/coercion/coercion.pro
- referenceexamples/coercion/coercion.qrc