Inheritance

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In object-oriented programming (OOP), inheritance is a way to form new classes (instances of which are called objects) using classes that have already been defined. Inheritance is intended^{[by whom?]} to help reuse existing code with little or no modification. The new classes, known as derived classes, inherit attributes and behavior of the pre-existing classes, which are referred to as base classes (or ancestor classes). The inheritance relationship of sub- and superclasses gives rise to a hierarchy. The inheritance concept was invented in 1967 for Simula.^[1]

Inheritance should not be confused with (subtype) polymorphism, commonly called just polymorphism in object-oriented programming. Inheritance is a relationship between implementations, whereas subtype polymorphism is relationship between types (interfaces in OOP).^[2] (Compare connotation/denotation.) In some, but not all OOP languages, the notions coincide because the only way to declare a subtype is to define a new class that inherits the implementation of another. Inheritance does not entail behavioral subtyping either. It is entirely possible to derive a class whose object will behave incorrectly when used in a context where the parent class is expected; see the Liskov substitution principle.

Complex inheritance, or inheritance used within an insufficiently mature design, may lead to the Yo-yo problem.

Applications of inheritance

Using inheritance offers several advantages. Sometimes it's desirable to distinguish these pluses, as it's not necessarily obvious from context.

Specialization

One can commonly use inheritance to create specializations of existing classes or objects. In specialization, the new class or object has data or behavior aspects that are not part of the inherited class. For example, a "Bank Account" class might have data for an "account number", "owner", and "balance". An "Interest Bearing Account" class might inherit "Bank Account" and then add data for "interest rate" and "interest accrued" along with behavior for calculating interest earned.

Another form of specialization occurs when a base class specifies that it has a particular behavior but does not actually implement the behavior. Each non-abstract, concrete class which inherits from that abstract class must provide an implementation of that behavior. This providing of actual behavior by a subclass is sometimes known as implementation or reification.

Overriding

Many object-oriented programming languages permit a class or object to replace the implementation of an aspect—typically a behavior—that it has inherited. This process is usually called overriding. Overriding introduces a complication: which version of the behavior does an instance of the inherited class use—the one that is part of its own class, or the one from the parent (base) class? The answer varies between programming languages, and some languages provide the ability to indicate that a particular behavior is not to be overridden and behave.

Code re-use

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One of the earliest motivations for using inheritance was the re-use of code which already existed in another class. This practice is usually called implementation inheritance.^{[dubious – discuss]}

In most quarters, class inheritance for the sole purpose of code re-use has fallen out of favor.^{[citation needed]} The primary concern is that implementation inheritance does not provide any assurance of polymorphic substitutability—an instance of the re-using class cannot necessarily be substituted for an instance of the inherited class. An alternative technique, delegation, requires more programming effort but avoids the substitutability issue. In C++ private inheritance can be used as form of implementation inheritance without substitutability. Whereas public inheritance represents an "is-a" relationship and delegation represents a "has-a" relationship, private (and protected) inheritance can be thought of as an "is implemented in terms of" relationship[1].

Object Oriented-Software Construction, 2^nd edition by Bertrand Meyer, the creator of the object-oriented programming language Eiffel, lists twelve different uses of inheritance ^[3], most of which involve some amount of implementation inheritance.^{[dubious – discuss]}

Inheritance vs subtype polymorphism

This section requires expansion.

Limitations and alternatives

When using inheritance extensively in designing a program, one should note certain constraints that it imposes.

For example, consider a class Person that contains a person's name, address, phone number, age, gender, and race. We can define a subclass of Person called Student that contains the person's grade point average and classes taken, and another subclass of Person called Employee that contains the person's job-title, employer, and salary.

In defining this inheritance hierarchy we have already defined certain restrictions, not all of which are desirable:

Constraints of inheritance-based design

• Singleness: using single inheritance, a subclass can inherit from only one superclass. Continuing the example given above, Person can be either a Student or an Employee, but not both. Using multiple inheritance partially solves this problem, as one can then define a StudentEmployee class that inherits from both Student and Employee. However, it can still inherit from each superclass only once; this scheme does not support cases in which a student has two jobs or attends two institutions.

• Static: the inheritance hierarchy of an object is fixed at instantiation when the object's type is selected and does not change with time. For example, the inheritance graph does not allow a Student object to become a Employee object while retaining the state of its Person superclass. (Although similar behavior can be achieved with the decorator pattern.) Some have criticized inheritance, contending that it locks developers into their original design standards.^[4]

• Visibility: whenever client code has access to an object, it generally has access to all the object's superclass data. Even if the superclass has not been declared public, the client can still cast the object to its superclass type. For example, there is no way to give a function a pointer to a Student's grade point average and transcript without also giving that function access to all of the personal data stored in the student's Person superclass.

Does not guarantee behavioral subtyping

It is intuitive to assume that inheritance creates a semantic "is a" relationship, and thus to infer that objects instantiated from subclasses can always be safely used instead of those instantiated from the superclass. But this intuition does not hold in most OOP languages, in particular in all those that allow mutable objects. Subtype polymorphism as enforced by the type checker in OOP languages (with mutable objects) cannot guarantee behavioral subtyping in a any context. Behavioral subtyping is undecidable in general, so it cannot be implemented by a program (compiler). Class or object hierarchies need to be carefully designed considering possible incorrect uses that cannot be detected syntactically. This issue is known as the Liskov substitution principle.

Roles and inheritance

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Sometimes inheritance-based design is used instead of roles. A role, say Student role of a Person describes a characteristic associated to the object that is present because the object happens to participate in some relationship with another object (say the person in student role -has enrolled- to the classes). Some object-oriented design methods do not distinguish this use of roles from more stable aspects of objects. Thus there is a tendency to use inheritance to model roles, say you would have a Student role of a Person modelled as a subclass of a Person. However, neither the inheritance hierarchy nor the types of the objects can change with time. Therefore, modelling roles as subclasses can cause the roles to be fixed on creation, say a Person cannot then easily change his role from Student to Employee when the circumstances change. From modelling point of view, such restrictions are often not desirable, because this causes artificial restrictions on future extensibility of the object system, which will make future changes harder to implement, because existing design needs to be updated. Inheritance is often better used with a generalization mindset, such that common aspects of instantiable classes are factored to superclasses; say having a common superclass 'LegalEntity' for both Person and Company classes for all the common aspects of both. The distinction between role based design and inheritance based design can be made based on the stability of the aspect. Role based design should be used when it's conceivable that the same object participates in different roles at different times, and inheritance based design should be used when the common aspects of multiple classes (not objects!) are factored as superclasses, and do not change with time.

One consequence of separation of roles and superclasses is that this cleanly separates compile-time and run-time aspects of the object system. Inheritance is then clearly a compile-time construct. Inheritance does influence the structure of many objects at run-time, but the different kinds of structure that can be used are already fixed at compile-time.

To model the example of Person as an employee with this method, the modelling ensures that a Person class can only contain operations or data that are common to every Person instance regardless of where they are used. This would prevent use of a Job member in a Person class, because every person does not have a job, or at least it is not known that the Person class is only used to model Person instances that have a job. Instead, object-oriented design would consider some subset of all person objects to be in an "employee" role. The job information would be associated only to objects that have the employee role. Object-oriented design would also model the "job" as a role, since a job can be restricted in time, and therefore is not a stable basis for modelling a class. The corresponding stable concept is either "WorkPlace" or just "Work" depending on which concept is meant. Thus, from object-oriented design point of view, there would be a "Person" class and a "WorkPlace" class, which are related by a many-to-many associatation "works-in", such that an instance of a Person is in employee role, when he works-in a job, where a job is a role of his work place in the situation when the employee works in it.

Note that in this approach, all classes that are produced by this design process form part of the same domain, that is, they describe things clearly using just one terminology. This is often not true for other approaches.

The difference between roles and classes is especially difficult to understand if one assumes referential transparency, because roles are types of references and classes are types of the referred-to objects.

Notes

References

• John C. Mitchell, Concepts in programming languages, Cambridge University Press, 2003, ISBN 0521780985, chapter 10 "Concepts in object-oriented languages"

See also

• Interface inheritance
• Circle-ellipse problem
• Class (object-oriented programming)
• Composition in object-oriented programming
• Interface (computer science)
• Polymorphism in object-oriented programming
• Multiple inheritance
• Override (object-oriented programming)
• Virtual inheritance
• Differential inheritance
• The Third Manifesto
• Role-Oriented Programming
• Defeasible reasoning