Inheritance (object-oriented programming)

In object-oriented programming, inheritance is the mechanism of basing an object or class upon another object (prototypical inheritance) or class (class-based inheritance), retaining similar implementation. In most class-based object-oriented languages, an object created through inheritance (a "child object") acquires all the properties and behaviors of the parent object (except: constructors, destructor, overloaded operators and friend functions of the base class). Inheritance allows programmers to create classes that are built upon existing classes,[1] to specify a new implementation while maintaining the same behaviors (realizing an interface), to reuse code and to independently extend original software via public classes and interfaces. The relationships of objects or classes through inheritance give rise to a directed graph.Inheritance was invented in 1969 for Simula.[2]

An inherited class is called a subclass of its parent class or super class. The term "inheritance" is loosely used for both class-based and prototype-based programming, but in narrow use the term is reserved for class-based programming (one class inherits from another), with the corresponding technique in prototype-based programming being instead called delegation (one object delegates to another).

Inheritance should not be confused with subtyping.[3][4] In some languages inheritance and subtyping agree,[a] whereas in others they differ; in general, subtyping establishes an is-a relationship, whereas inheritance only reuses implementation and establishes a syntactic relationship, not necessarily a semantic relationship (inheritance does not ensure behavioral subtyping). To distinguish these concepts, subtyping is also known as interface inheritance, whereas inheritance as defined here is known as implementation inheritance or code inheritance.[5] Still, inheritance is a commonly used mechanism for establishing subtype relationships.[6]

Inheritance is contrasted with object composition, where one object contains another object (or objects of one class contain objects of another class); see composition over inheritance. Composition implements a has-a relationship, in contrast to the is-a relationship of subtyping.

Types

Single inheritance
Multiple inheritance

There are various types of inheritance, based on paradigm and specific language.[7]

Single inheritance

where subclasses inherit the features of one superclass. A class acquires the properties of another class.

Multiple inheritance

where one class can have more than one superclass and inherit features from all parent classes.

"Multiple Inheritance (object-oriented programming) was widely supposed to be very difficult to implement efficiently. For example, in a summary of C++ in his book on objective C Brd.Cox actually claimed that adding Multiple inheritance to C++ was impossible. Thus, multiple inheritance seemed more of a challenge. Since I had considered multiple inheritance as early as 1982 and found a simple and efficient implementation technique in 1984. I couldn't resist the challenge. I suspect this to be the only case in which fashion affected the sequence of events."[8]

In JDK 1.8, Java now has support for multiple inheritance[9].

Multilevel inheritance

where a subclass is inherited from another subclass. It is not uncommon that a class is derived from another derived class as shown in the figure "Multilevel inheritance".

Multilevel inheritance

The class A serves as a base class for the derived class B, which in turn serves as a base class for the derived class C. The class B is known as intermediate base class because it provides a link for the inheritance between A and C. The chain ABC is known as inheritance path.

A derived class with multilevel inheritance is declared as follows:

Class A(...);      //Base class
Class B : public A(...);   //B derived from A
Class C : public B(...);   //C derived from B

This process can be extended to any number of levels.

Hierarchical inheritance

where one class serves as a superclass (base class) for more than one sub class.

Hybrid inheritance

a mix of two or more of the above types of inheritance.

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