Liskov substitution principle definition 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
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].