Programming in XPCE/Prolog: Section 8.4

## 8.4 Class variables in User Defined Classes

Class-variables are declared, similar to instance-variables. through macro-expansion inside the :- pce_begin_class/[2,3] ... :- pce_end_class/0 definition of a class. The syntax is:

> `class_variable(`<name>`,` <type>`,` <default>`,` [<summary>]`)`.

<default> defines the value if not overruled in the `Defaults` file. It is a Prolog term describing an object similar to the arguments of send/[2-12].

In the following example. there is a class with the property `expert_level'. The program defines the default level to be novice. The user may change that in his/her personal `Defaults` file or change it while the application is running. As the value may change at runtime, there should be an instance- as well as a class-variable. Below is the skeleton for this code:

    variable(expert_level, {novice,advanced,expert}, get,
             "Experience level of the user").

    class_variable(expert_level, @default, novice).

    expert_level(Obj, Level:{novice,advanced,expert}) :->
            send(Obj, slot, expert_level, Level),
            <handle changes>.

            ...,
            (   get(Obj, expert_level, expert)
            ->  ...
            ;   ...
            ),
            ...