The circle-ellipse problem in software development (sometimes known as the square-rectangle problem) illustrates a number of pitfalls which can arise when using subtype polymorphism in object modelling. The issues are most commonly encountered when using object-oriented programming.
This is the L in the acronym S.O.L.I.D. which is known as the Liskov substitution principle. This problem arises as a violation of that principle.
The problem concerns which subtyping or inheritance relationship should exist between classes which represent circles and ellipses (or, similarly, squares and rectangles). More generally, the problem illustrates the difficulties which can occur when a base class contains methods which mutate an object in a manner which might invalidate a (stronger) invariant found in a derived class, causing the Liskov substitution principle to be violated.
The existence of the circle-ellipse problem is sometimes used to criticize object-oriented programming. It may also imply that hierarchical taxonomies are difficult to make universal, implying that situational classification systems may be more practical.
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