How-to: Class extension

The typical scheme for extracting relations between classes to a separate module is as follows:

Create the MA module in which the A class will be created:

MODULE MA;

CLASS ABSTRACT A; // declaring an abstract class
a = ABSTRACT BPSTRING[10] (A); // declaring an abstract property a

Create the MB module in which the B class will be created:

MODULE MB;

CLASS B; // declaring class B
b = DATA BPSTRING[10] (B); // declaring the data property b for class B

Create the MBA module in which the relation between the A and B class will be defined:

MODULE MBA;

// specifying that the MBA module depends on the MA and MB modules so 
// that the system elements declared in them can be used in that module
REQUIRE MA, MB; 

EXTEND CLASS B : A; // inheriting class B from A
// specifying that for the abstract property a, property B should be used as an implementation
a(ba) += b(ba); 

Therefore, the MA and MB modules do not directly depend on each other and the relation between them can be enabled (disabled) by linking (unlinking) the MBA module. Note that the MBA module extends the functionality of the MB module without any changes to its code.

You can use mixin classes when using the metacode as follows:

Suppose that we have a metacode that declares a class and defines certain properties for it:

MODULE MyModule;

META defineMyClass (className) // declaring the defineMyClass metacode with the className parameter
    CLASS className; // declaring a class named className
    // adding a property named myProperty + className for the created class
    myProperty###className = DATA BPSTRING[20] (className); 
END

Note that when calling this metacode, you cannot specify the classes from which the created class must inherit anything. However, this can be implemented through a mixin of classes as follows:

CLASS MySuperClass;

@defineMyClass(MyClass); // calling the metacode that will create the class and property

// inheriting MyClass from MySuperClass, while MyClass will "will receive"
// all the properties that are declared for the MySuperClass class
EXTEND CLASS MyClass : MySuperClass;