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)
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