The Liskov Substitution Principle
Illustration of the gap between engineers and scientists
The Liskov Substitution Principle (LSP) can be paraphrased as:
Functions that use pointers or references to base classes must be able
to use objects of derived classes without knowing it.
Barbara Liskov first wrote it as follows [1]:
What is wanted here is something like the following substitution
property: 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 behavior of
P is unchanged when o1 is substituted for o2 then S is a subtype of T.
References:
Barbara Liskov, "Data Abstraction and Hierarchy," SIGPLAN Notices, 23,5 (May, 1988).
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