Liskov substitution principle

programming, theory (LSP) The principle that

object-oriented functions that use pointers or

references to a base class must be able to use objects of

a derived class without knowing it.

Barbara Liskov first wrote it as follows: If for each object

o1 of type S there is an object o2 of type T such that for all

programs P defined in terms of T, the behaviour of P is

unchanged when o1 is substituted for o2 then S is a subtype

of T.

A function that violates the LSP uses a reference to a base

class and must know about all the derivatives of that base

class. Such a function violates the open/closed principle

because it must be modified whenever a new derivative of the

base class is created.

[Liskov, B. Data Abstraction and Hierarchy, SIGPLAN

Notices. 23(5), May 1988].

(2001-09-14)