Inheritance describes a relationship between two (or more) types, or classes, of objects in which one is said to be a "subtype" or "child" of the other; as a result, the "child" object is said to inherit features of the parent, allowing for shared functionality. This lets programmers re-use or reduce code and simplifies the development and maintenance of software.
Inheritance is also commonly held to include subtyping, whereby one type of object is defined to be a more specialized version of another type (see Liskov substitution principle), though non sub-typing inheritance is also possible.
Inheritance is typically expressed by describing classes of objects arranged in an inheritance hierarchy (also referred to as inheritance chain), a tree-like structure created by their inheritance relationships.
For example, one might create a variable class "Mammal" with features such as eating, reproducing, etc.; then define a subtype "Cat" that inherits those features without having to explicitly program them, while adding new features like "chasing mice". This allows commonalities among different kinds of objects to be expressed once and reused multiple times.
In C++ we can then have classes that are related to other classes (a class can be defined by means of an older, pre-existing,
class ). This leads to a situation in which a new class has all the functionality of the older class, and additionally introduces its own specific functionality. Instead of composition, where a given class contains another class, we mean here derivation, where a given class is another class.
This OOP property will be explained further when we talk about Classes (and Structures) inheritance in the Classes Inheritance Section of the book.
If one wants to use more than one totally orthogonal hierarchy simultaneously, such as allowing "Cat" to inherit from "Cartoon character" and "Pet" as well as "Mammal" we are using multiple inheritance.