Class inheritance

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Purpose

Inheritance is a parent-child relation used to expresses the concept of generalization/specialization: a parent class is a generalization of a child class; a child class is a specialization of a parent class. For example a parent class could be Animal, while some child classes could be Mammal, Bird and Reptile.
Parent classes are also called super-classes. Child classes are also called sub-classes or derived classes. A class with no parent is also called a base class.

Inheritance can be applied recursively. You can have a child class of a child class. Collectively, all the classes and their inheritance relations together make a hierarchy (or taxonomy according to another terminology).

Inheritance is a common concept in object-oriented design. If class B is a child of class A, then B can be used wherever A can be used. In fact, B "inherits" all the behaviours of A. More formally, B inherits from A:

• the attributes
• the relationships
• the workflow (if present) including states and transitions
• the operations
• the permissions

Class B can also define elements of its own, which extend those inherited from A. Generally this inheritance/extension works in straightforward way. Here we will only discuss the exceptions and special cases.

Workflow

In the chapter on workflows, we discussed that a class can have at most one workflow attribute. This is true regardless of whether the attribute belongs to the class directly or to one of its parent/ancestor classes.

In other words, if a class has a workflow attribute, none of its child classes can have another workflow attribute.

Permissions

If you define class B as a child of class A and visit B's permissions page, you will find that any actors and permissions defined for A are present also in B. The only unusual thing is that those inherited actors/permission are grayed out and cannot be changed.

More formally, permissions and inheritance abide to the following rules:

• Any class, whether parent or child, can define its own actors.
• Child classes inherit their parent's actors in "read-only".
• The permissions on an attribute can only be set on the class that owns the attribute. Any child class sees those permissions in "read-only".
• A class can define the permissions on its own attributes in terms of any actor, whether its own or inherited.
• The create and delete permissions must be defined on the base class.

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