Ng, theory> (LSP) The priNciple that object-orieNtedfuNctioNs 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, SIGPLANNotices. 23(5), May 1988]. (2001-09-14)
