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# Extending QML - Inheritance and Coercion Example

Files:

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

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 elements of more than one type to a property. It specializes the Person element developed in the previous examples into two elements - 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 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 element 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(QDeclarativeListProperty<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.