Implications of Substitution

1. Implications of Substitution CMPS 2143

2. Overview • Memory Layout and methods of space allocation • Assignment • Copies and Clones • Equality and Identity

3. Inheritance and Substitution • Subtle, yet pervasive impact on MANY aspects of a programming language • Type system • Meaning of assignment, testing for equivalence • Creation of copies • Storage allocation • Is-a relationship: associate a type (of a variable) with a set of values • Window win = new Window(); • Window win = new TextWindow();

4. Storage allocation • Commonly believed variables allocated on the run-time stack (in activation records) are more efficient than variables allocated on the heap. • Language designers go to great lengths to make this happen • Major problem: storage requirements must be determined at statically (at compile-time) or at least at procedure-entry time. • Values the variables hold are determined at runtime.

5. Subclasses and storage requirements • Subclasses may have more data (not present in superclass) • Ex. textWindow may have more data than window, • eg. a string and cursorLocation • When allocating for a Window object, should we really allocate for a TextWindow object, just in case?

6. THREE SOLUTIONS • Minimum space allocation: Allocate the amount of space necessary for the base class. • Maximum space allocation: Allocate the maximum amount of space necessary to hold any legal value (from any of the classes in the hierarchy) • Dynamic memory allocation: Allocate only the amount of space necessary to hold a single pointer/reference. (Allocate the space necessary for the value at runtime on the heap.)

7. Minimum space allocation • C was designed to be efficient, C++ retains many concepts of nondynamic and dynamic variables Window win; //puts an object on the stack Window * tWinPtr; //puts an pointer on the stack tWinPtr = new TextWindow(); //allocates on the heap • What happens if …? win = *tWinPtr; • Space allocated to win on the stack is not large enough!!!

8. Minimum Space Allocation • Default: not all fields copied/only the corresponding fields. (termed slicing) • C++ can override the meaning of assignment, but how without losing some information? • Do we care? • Can only call methods in win (Window) • What if methods are overridden in the subclass and they access extra data?

9. Minimum Space Allocation • C++: 2 solutions • For pointers (and references) – member function selected is determined by dynamic value of receiver • For other variables – binding determined by static class (the class at declaration) • Given this modified rule – not possible for a method to access fields that are physically present in memory in the object

10. Maximum Space Allocation • Might seem to be ideal (although possibly wasteful) • Size of object must be known by scanning ENTIRE program • Need 2 pass compiler • Need to see all libraries, linked in code, etc. • No major programming language uses this approach

11. Dynamic Memory Allocation • Values of objects not stored on the stack – only the pointer or reference is • Values stored in the heap. • Java, Smalltalk, Object Pascal, Objective-C use this approach • Must use new to allocate space in the heap. • Will have consequences with assignment semantics!!!

12. Garbage collection • When data stored in heap, must be recovered • Use delete in C++ • Use auto garbage collector

13. Assignment – 2 interpretations • Copy semantics – assignment copies entire value of right side, assigning it to left, thereafter the two values are independent (changes in one not reflected in other) • Sometimes used in C++, sometimes not • Pointer semantics – Assignment changes the reference of left side to be of right side (aliasing). Two pointers refer to the same memory location. (Changes in one, will affect the other.) • Java, CLOS, Object Pascal • If used, languages usually provide a means for true copy

14. Copies and Clones • How to copy a value that references other objects? • Shallow copy • assignment of references • Deep copy • Overload assignment • Write copy constructor • Overload clone method

15. Copy constructors • Define what you want to copy • Argument is a reference parameter of same type public class MyClass { public: MyClass(constMyClass source) { ….}; • Considered GOOD practice to ALWAYS include a copy constructor

16. Cloning in Java • Implement Cloneable interface • Override method clone class PlayingCard implements Cloneable { : public Object clone () throws CloneNotSupportedException { Object newCard = super.clone); return newCard; } } • Default behavior creates a shallow copy. Add additional code to create a deep copy

17. Equality • Identity – are two objects the same entity • Equality – two objects that contain the same values char * a = “abc”; char * b = “abc”; If (a==b) // T/F?

18. Equality Testing • Override == • Override or write equals method • There is an equals method in Java’s Object class

19. Paradoxes of equality testing • Equality can be done at various levels • Let p1 and p2 be parent classes and c1 and c2 be child classes Is p1.equals(c1) same as c1.equals(p1) ? If p1.equals(c1) and p1.equals(c2), does c1.equals(c2)? If p1.equals(c1) and c1.equals(p2), does p1.equals(p2)? • HMMM?

20. Study questions • Pg. 286: 2-5, 9