Stoop 416-lsp


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Stoop 416-lsp

  1. 1. S.Ducasse 1 QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture. Stéphane Ducasse LSP Stéphane Ducasse --- 2005
  2. 2. S.Ducasse 2 License: CC-Attribution-ShareAlike 2.0
  3. 3. S.Ducasse 3 A step deeper in Subtyping...
  4. 4. S.Ducasse 4 Liskov Substitution Principle (LSP) if for each object o1 of type S there is another 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. Barbara Liskov, "Data Abstraction and Hierarchy," SIGPLAN Notices, 23,5 (May 1988)
  5. 5. S.Ducasse 5 A Simple Bag We will define a Bag as an object that accepts two messages: put and count (send a-Bag 'put x) puts an element x into the Bag, and (send a-Bag 'count x) gives the count of occurrences of x in the Bag (without changing a-Bag's state).
  6. 6. S.Ducasse 6 A Simple Set Likewise, a Set is defined as an object that accepts two messages: (send a-Set 'put x) puts an element x into a-Set unless it was already there, (send a-Set 'count x) gives the count of occurrences of x in a-Set (which is always either 0 or 1).
  7. 7. S.Ducasse 7 A Bag Function (define (fnb bag) (send bag 'put 5) (send bag 'put 5) (send bag 'count 5)) The behavior of this function can be summed as follows: given a Bag, the function adds two elements into it and returns (+ 2 (send orig-bag 'count 5))
  8. 8. S.Ducasse 8 The problem Technically you can pass to fnb a Set object as well. Just as a Bag, a Set object accepts messages put and count. However applying fnb to a Set object will break the function's post-condition Therefore, passing a set object where a bag was expected changes behavior of some program. According to the Liskov Substitution Principle (LSP), a Set is not substitutable for a Bag -- a Set cannot be a subtype of a Bag.
  9. 9. S.Ducasse 9 Another function (define (fns set) (send set 'put 5) (send set 'count 5)) The behavior of this function is: given a Set, the function adds an element into it and returns 1.
  10. 10. S.Ducasse 10 Problem two If you pass to this function a bag (which -- just as a set -- replies to messages put and count), the function fns may return a number greater than 1. This will break fns's contract, which promised always to return 1.
  11. 11. S.Ducasse 11 Finally Therefore, from the OO point of view, neither a Bag nor a Set are a subtype of the other. Bag and Set only appear similar. The interface or an implementation of a Bag and a Set appear to invite subclassing of a Set from a Bag (or vice versa). Doing so however will violate the LSP -- and you have to brace for very subtle errors. Sets and Bags are very simple types, far simpler than the ones you deal with in a production code. either. It's manual work -- you have to see the problem
  12. 12. S.Ducasse 12 Watch out Alas, LSP when considered from an OOP point of view is undecidable. You cannot count on a compiler for help in pointing out an error. You cannot rely on regression tests
  13. 13. S.Ducasse 13 In C++ typedef int const * CollIterator; // Primitive but will do class CBag { public: int size(void) const; // The number of elements in the bag virtual void put(const int elem); // Put an element into the bag int count(const int elem) const; // Count the number of occurrences // of a particular element in the bag virtual bool del(const int elem); // Remove an element from the bag // Return false if the element // didn't exist CollIterator begin(void) const; // Standard enumerator interface CollIterator end(void) const; CBag(void); virtual CBag * clone(void) const; // Make a copy of the bag private: // implementation details elided };
  14. 14. S.Ducasse 14 CSet class CSet : public CBag { public: bool memberof(const int elem) const { return count(elem) > 0; } // Overriding of CBag::put void put(const int elem) { if(!memberof(elem)) CBag::put(elem); } CSet * clone(void) const { CSet * new_set = new CSet(); *new_set += *this; return new_set; } CSet(void) {} };
  15. 15. S.Ducasse 15 Summary Subtyping...