Circle-ellipse problem

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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.

The problem

It is a central tenet of object-oriented analysis and design that subtype polymorphism, which is implemented in most OO languages via inheritance, should be used to model object types which are subsets of each other; this is commonly referred to as the is-a relationship. In the present example, the set of circles is a subset of the set of ellipses; circles can be defined as ellipses whose major and minor axes are the same length. Thus, code written in an OOPL which models shapes will frequently choose to make class Circle as a sub-class of class Ellipse (i.e., inheriting from it).

A sub-class has to provide support for all behaviour supported by the super-class; subclasses must implement any mutators defined in a base class. In the present case, the method Ellipse.stretchX alters the length of one of its axes in place. If Circle inherits from Ellipse, it must also have a method stretchX, but the result of this method would be to change a circle into something which is no longer a circle. The Circle class cannot simultaneously satisfy its own invariant and the behavioural requirements of the Ellipse.stretchX method.

A related problem with this inheritance arises when we consider the implementation. An ellipse requires more state to describe than a circle does, as the former needs attributes to specify the length and rotation of the major and minor axes; whereas a circle needs only a radius. It may be possible to avoid this if the language (such as Eiffel) makes constant values of a class, functions without arguments and data members interchangeable.

Some authors have suggested reversing the relationship between circle and ellipse, on the grounds that an ellipse is a circle with additional capabilities. Unfortunately, ellipses fail to satisfy many of the invariants of circles; if Circle has a method radius, Ellipse will now have to provide it as well.

The problem is sometimes expressed in statements such as "a Circle is not a sort of Ellipse". This looks confusingly like the absurd "a circle is not a sort of ellipse", and sounds identical, so it is unhelpful. What is actually meant is "an OO-model of a circle should not be a sort of OO-model of an ellipse"

Possible solutions

One may solve the problem by changing one's model, or perhaps using a different language, which could be a (not yet implemented) extension of an existing language, or by using a different paradigm. Exactly which option is appropriate will depend on who wrote Circle and who wrote Ellipse. If the same author is designing them both from scratch, then the author will be able to define the interface to handle this situation. If the Ellipse object was already written, and cannot be changed, then the options are more limited.

Change the model

Return success or failure value

Allow the objects to return a "success" or "failure" value for each modifier or raise an Exception on failure. This is usually done in the case of file I/O, but can also be helpful here. Now, Ellipse.stretchX works, and returns 'true', while Circle.stretchX simply returns 'false'. This is in general good practice, but may require that the original author of Ellipse anticipated such a problem, and defined the mutators as returning a value. In addition it requires the client code to test the return value for support of the stretch function, which in effect is just the same as testing whether the referenced object is either a circle or an ellipse. Another way of looking at this is that it is like putting in the contract that the contract may or may not be lived up to depending on the object actually implementing the interface. Eventually it is just a clever way of bypassing the Liskov constraint by just stating up-front that the post condition might or might not be valid.

Alternately, Circle.stretchX could throw an exception (but depending on the language, this may also require that the original author of Ellipse declare that it may throw an exception).

Return the new value of X

This is a similar solution to the above, but is slightly more powerful. Ellipse.stretchX now returns the new value of its X dimension. Now, Circle.stretchX can simply return its current radius. All modifications must be done through Circle.stretch, which preserves the circle invariant.

Allow for a weaker contract on Ellipse

If the interface contract for Ellipse states only that "stretchX modifies the X axis", and does not state "and nothing else will change", then Circle could simply force the X and Y dimensions to be the same. Circle.stretchX and Circle.stretchY both modify both the X and Y size. 

Circle::stretchX(x) {xSize = ySize = x;}
Circle::stretchY(y) {xSize = ySize = y;}

Convert the Circle into an Ellipse

If Circle.stretchX is called, then Circle changes itself into an Ellipse. For example, in Common Lisp, this can be done via the CHANGE-CLASS method. This may be dangerous, however, if some other function is expecting it to be a Circle. Some languages don't allow this type of change at all, and others impose restrictions on the Ellipse class to be an acceptable replacement for Circle.

Make all instances constant

One can change the model so that instances of the classes represent constant values (i.e. they are immutable). This is exactly the implementation that is used in purely functional programming.

In this case methods such as stretchX must be changed to yield a new instance, rather than modifying the instance they act on. This means that it is no longer a problem to define Circle.stretchX, and the inheritance reflects the mathematical relationship between circles and ellipses.

A disadvantage is that changing the value of an instance then requires an assignment, which is inconvenient and prone to programming errors, e.g.
Orbit(planet[i]) := Orbit(planet[i]).stretchX.

A second disadvantage is that such an assignment conceptually involves a temporary value, which could reduce performance and be difficult to optimise.

Factor out modifiers

One can define a new class MutableEllipse, and put the modifiers from Ellipse in it. The Circle only inherits queries from Ellipse.

This has a disadvantage of introducing an extra class where all we really want to do is specify that Circle does not inherit modifiers from Ellipse.

Impose preconditions on modifiers

One can specify that Ellipse.stretchX is only allowed on instances satisfying Ellipse.stretchable, and will otherwise throw an exception. This requires anticipation of the problem when Ellipse is defined.

Factor out common functionality into an abstract base class

Create an abstract base class called RoundObject and put methods that work with both Circles and Ellipses in this class. Functions that can deal with either type of object will expect a RoundObject, and functions that call Ellipse or Circle-specific requirements will use the descendant classes. However, Circle is no more an Ellipse subclass then, leading to the "a Circle is not a sort of Ellipse" situation described above.

Drop all inheritance relationships

This solves the problem at a stroke, and may sometimes be a reasonable approach.

The disadvantage is that one now have to create a common superclass for both Circle and Ellipse, i.e. Shape and then use Shape class to describe circles and Ellipse.

Alternatively, one may provide a method Circle.asEllipse, which returns a mutable Ellipse "view" of the Circle object: an Ellipse instance initialized with the circle's current state, that "redirects" all messages to the Circle object it was obtained from. The "view" is an interface which a client that requires an ellipse may use to work with a circle, even if there is no actual subtyping relationship between the two classes. For example, the ellipse 's stretchXAxis method would be implemented to invoke stretchRadius on the Circle.

Combine class Circle into class Ellipse

Then, wherever a circle was used before, use an ellipse.

A circle can already be represented by an ellipse. There is no reason to have class Circle unless it needs some circle-specific methods that can't be applied to an ellipse, or unless the programmer wishes to benefit from conceptual and/or performance advantages of the circle's simpler model.

Inverse inheritance

Majorinc proposed a model that divides methods on modifiers, selectors and general methods. Only selectors can be automatically inherited from superclass, while modifiers should be inherited from subclass to superclass. In general case, the methods must be explicitly inherited. The model can be emulated in languages with multiple inheritance, using abstract classes.[1]

Change the programming language

This problem has straightforward solutions in a sufficiently powerful OO programming system. Essentially, the Circle-Ellipse problem is one of synchronizing two representations of type: the de facto type based on the properties of the object, and the formal type associated with the object by the object system. If these two pieces of information, which are ultimately just bits in the machine, are kept synchronized so that they say the same thing, everything is fine. It is clear that a circle cannot satisfy the invariants required of it while its base ellipse methods allow mutation of parameters. However, the possibility exists that when a circle cannot meet the circle invariants, its type can be updated so that it becomes an ellipse. If a circle which has become a de facto ellipse doesn't change type, then its type is a piece of information which is now out of date, reflecting the history of the object (how it was once constructed) and not its present reality (what it has since mutated into).

Many object systems in popular use are based on a design which takes it for granted that an object carries the same type over its entire lifetime, from construction to finalization. This is not a limitation of OOP, but rather of particular implementations only.

The following example uses the Common Lisp Object System (CLOS) in which objects can change class without losing their identity. All variables or other storage locations which hold a reference to an object continue to hold a reference to that same object after it changes class.

The circle and ellipse models are deliberately simplified to avoid distracting details which are not relevant to the Circle-Ellipse Problem. An ellipse has two semi-axes called h-axis and v-axis in the code. Being an ellipse, a circle inherits these, and also has a radius property, whose value is equal to that of the axes (which must, of course, be equal).

(defclass ellipse ()
  ((h-axis :type real :accessor h-axis :initarg :h-axis)
   (v-axis :type real :accessor v-axis :initarg :v-axis)))
 
(defclass circle (ellipse)
  ((radius :type real :accessor radius :initarg :radius)))
 
;;;
;;; A circle has a radius, but also a h-axis and v-axis that
;;; it inherits from an ellipse. These must be kept in sync
;;; with the radius when the object is initialized and
;;; when those values change.
;;;
(defmethod initialize-instance ((c circle) &key radius)
  (setf (radius c) radius)) ;; via the setf method below
 
(defmethod (setf radius) :after ((new-value real) (c circle))
  (setf (slot-value c 'h-axis) new-value
        (slot-value c 'v-axis) new-value))
 
;;;
;;; After an assignment is made to the circle's
;;; h-axis or v-axis, a change of type is necessary,
;;; unless the new value is the same as the radius.
;;;
(defmethod (setf h-axis) :after ((new-value real) (c circle))
  (unless (eql (radius c) new-value)
    (change-class c 'ellipse)))
 
(defmethod (setf v-axis) :after ((new-value real) (c circle))
  (unless (eql (radius c) new-value)
    (change-class c 'ellipse)))
 
;;;
;;; Ellipse changes to a circle if accessors
;;; mutate it such that the axes are equal,
;;; or if an attempt is made to construct it that way.
;;;
;;; EQL equality is used, under which 0 /= 0.0.
;;;
;;; The SUBTYPEP checks are needed because these methods
;;; apply to circles too, which are ellipses!!!
;;;
(defmethod initialize-instance :after ((e ellipse) &key h-axis v-axis)
  (if (eql h-axis v-axis)
    (change-class e 'circle)))
 
(defmethod (setf h-axis) :after ((new-value real) (e ellipse))
  (unless (subtypep (class-of e) 'circle)
    (if (eql (h-axis e) (v-axis e))
      (change-class e 'circle))))
 
(defmethod (setf v-axis) :after ((new-value real) (e ellipse))
  (unless (subtypep (class-of e) 'circle)
    (if (eql (v-axis e) (v-axis e))
      (change-class e 'circle))))
 
;;;
;;; Method for an ellipse becoming a circle. In this metamorphosis,
;;; the object acquires a radius, which we must initialize.
;;; There is a "sanity check" here to signal an error if an attempt
;;; is made to convert an ellipse whose axes are not equal
;;; with an explicit change-class call.
;;; The handling strategy here is to just base the radius off the
;;; h-axis and signal an error.
;;; This doesn't prevent the class change; the damage is already done.
;;;
(defmethod update-instance-for-different-class :after ((old-e ellipse)
                                                       (new-c circle) &key)
  (setf (radius new-c) (h-axis old-e))
  (unless (eql (h-axis old-e) (v-axis old-e))
    (error "ellipse ~s can't change into a circle because it's not one!"
           old-e)))

This code can be demonstrated with an interactive session, using the CLISP implementation of Common Lisp.

$ clisp -q -i circle-ellipse.lisp 
[1]> (make-instance 'ellipse :v-axis 3 :h-axis 3)
#<CIRCLE #x218AB566>
[2]> (make-instance 'ellipse :v-axis 3 :h-axis 4)
#<ELLIPSE #x218BF56E>
[3]> (defvar obj (make-instance 'ellipse :v-axis 3 :h-axis 4))
OBJ
[4]> (class-of obj)
#<STANDARD-CLASS ELLIPSE>
[5]> (radius obj)

*** - NO-APPLICABLE-METHOD: When calling #<STANDARD-GENERIC-FUNCTION RADIUS>
      with arguments (#<ELLIPSE #x2188C5F6>), no method is applicable.
The following restarts are available:
RETRY          :R1      try calling RADIUS again
RETURN         :R2      specify return values
ABORT          :R3      Abort main loop
Break 1 [6]> :a
[7]> (setf (v-axis obj) 4)
4
[8]> (radius obj)
4
[9]> (class-of obj)
#<STANDARD-CLASS CIRCLE>
[10]> (setf (radius obj) 9)
9
[11]> (v-axis obj)
9
[12]> (h-axis obj)
9
[13]> (setf (h-axis obj) 8)
8
[14]> (class-of obj)
#<STANDARD-CLASS ELLIPSE>
[15]> (radius obj)

*** - NO-APPLICABLE-METHOD: When calling #<STANDARD-GENERIC-FUNCTION RADIUS>
      with arguments (#<ELLIPSE #x2188C5F6>), no method is applicable.
The following restarts are available:
RETRY          :R1      try calling RADIUS again
RETURN         :R2      specify return values
ABORT          :R3      Abort main loop
Break 1 [16]> :a
[17]>

Challenge the premise of the problem

While at first glance it may seem obvious that a Circle is-an Ellipse, consider the following alternate representation of essentially the same problem, stated in terms of Java code.

class Person
{
  void walkNorth( int meters ) {...}   // No failure or exception allowed
  void walkEast( int meters ) {...}    // No failure or exception allowed
}

Now, a prisoner is obviously a person. So we could logically create a sub-class:

class Prisoner extends Person
{
  void walkNorth( int meters ) {...}   // No failure or exception allowed
  void walkEast( int meters ) {...}    // No failure or exception allowed
}

Just as obviously, this leads us into trouble, since a prisoner is not free to move an arbitrary distance in any direction, yet the contract of the Person class states that a Person can.

So, in fact, the class Person could better be named FreePerson. If that were the case, then the idea that class Prisoner extends FreePerson is clearly wrong.

By analogy, then, a Circle, is not an Ellipse, because it lacks the same degrees of freedom as an Ellipse.

This strongly suggests that inheritance should never be used when the sub-class restricts the freedom implicit in the base class, but should only be used when the sub-class adds extra detail to the concept represented by the base class as in 'Monkey' is-a 'Animal'.

References

  1. ↑ Kazimir Majorinc, Ellipse-Circle Dilemma and Inverse Inheritance, ITI 98, Proceedings of the 20th International Conference of Information Technology Interfaces, Pula, 1998

External links

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