Inheritance (object-oriented programming)

In object-oriented programming, inheritance is the mechanism of basing an object or class upon another object (prototype-based inheritance) or class (class-based inheritance), retaining similar implementation. Also defined as deriving new classes (sub classes) from existing ones such as super class or base class and then forming them into a hierarchy of classes. In most class-based object-oriented languages like C++, an object created through inheritance, a "child object", acquires all the properties and behaviors of the "parent object", with the exception of: constructors, destructors, overloaded operators and friend functions of the base class. Inheritance allows programmers to create classes that are built upon existing classes, 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 acyclic graph.

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). Class-modifying inheritance patterns can be pre-defined according to simple network interface parameters such that inter-language compatibility is preserved.

Inheritance should not be confused with subtyping. In some languages, generally statically-typed class-based OO languages, like C++, C#, Java, and Scala, inheritance and subtyping agree, 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 sometimes referred to as interface inheritance (without acknowledging that the specialization of type variables also induces a subtyping relation), whereas inheritance as defined here is known as implementation inheritance or code inheritance. Still, inheritance is a commonly used mechanism for establishing subtype relationships.

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. In contrast to subtyping’s is-a relationship, composition implements a has-a relationship.

Mathematically speaking, inheritance in any system of classes induces a strict partial order on the set of classes in that system.

In 1966, Tony Hoare presented some remarks on records, and in particular, the idea of record subclasses, record types with common properties but discriminated by a variant tag and having fields private to the variant. Influenced by this, in 1967 Ole-Johan Dahl and Kristen Nygaard presented a design that allowed specifying objects that belonged to different classes but had common properties. The common properties were collected in a superclass, and each superclass could itself potentially have a superclass. The values of a subclass were thus compound objects, consisting of some number of prefix parts belonging to various superclasses, plus a main part belonging to the subclass. These parts were all concatenated together. The attributes of a compound object would be accessible by dot notation. This idea was first adopted in the Simula 67 programming language. The idea then spread to Smalltalk, C++, Java, Python, and many other languages.

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

"Multiple inheritance ... was widely supposed to be very difficult to implement efficiently. For example, in a summary of C++ in his book on Objective C, Brad 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."