1. Polymorphism C and Data Structures Baojian Hua bjhua@ustc.edu.cn

2. An old dream in CS Famous slogans: “ Write code once, used everywhere! ” “ Never write same code twice! ” Polymorphism Poly + morphism Elegant and amazing combination of theory and practice in CS!

3. Why polymorphism? // a list of integer: struct List_t_int { int data; struct List_t_int *next; }; // “length” int length (List_t_int l) { int size=0; List_t_int temp = l->next; while (temp) { size++; temp=temp->next; } return size; } the “ data ” field?

4. Why polymorphism? // what about if we want a list of string? struct List_t_string { char *data; struct List_t_string *next; }; // function “length” remains the same? int length (List_t_string l) { int size=0; List_t_string temp = l->next; while (temp) { size++; temp=temp->next; } return size; }

5. Outline Ad-hoc poly ’ Parametric poly ’ Subtype poly ’

6. Ad-hoc poly ’ Functions operate on different but a limited set of data type e.g., “+” in C int * int, double * double, char * * int but not: char * * char * First introduced by Strachey in 1967 Evolve into a feature called “ overloading ” in modern OO languages

7. Ad-hoc poly ’ Overloading in modern OO languages: Same printing function name with different argument types: void print (int x); void print (char *s); Compilers will dispatch function calls print (5); print (“hello”); Bad news is that C does not support overloading! We ’ d have to do some hacking. :-( Or we can write unique names, as: void Int_print (int x); void String_print (char *s);

8. Parametric poly Basic idea: decorating data types (data structure + functions) with type variables Polymorphic data structures Polymorphic functions we start with this

9. E.g. int max (int x, int y) { return x>y? x: y; } // but we’ve observed the problem: double max (double x, double y) { return x>y? x: y; } // What about factor out the type names: X max (X x, X y) { return x>y? x: y; } // but this function does not type check!

10. Old trick #define max(x, y) x>y? x: y // So we can write: int m = max(3, 4); // or double n = max (3.14, 2.71); // but this is very good! Why? // We can do better by hoisting “X” further: #define max_X(X) \ X max (X x, X y) \ { \ return x>y? x: y; \ } size

11. Client code #define max_X(X) \ X max (X x, X y) \ { \ return x>y? x: y; \ } max_X(int) int main () { max (3, 4); } Questions: 1: what about we write this? max_X (double) does this program still type check? 2: what about we write max_X (char *) is char * comparable? is this algorithm correct?

12. Client code #define max_X(X) \ X max (X x, X y, int (*m)(X, X)) \ { \ if (1==m(x, y)) \ return x; \ return y; \ } max_X(int) int int_compare (int x, int y) {…} int main () { max (3, 4, int_compare); }

13. List revisit #define List_X(X) struct List_t { X data; struct List_t *next; }; // “length” int length (List_t l) { int size=0; List_t temp = l->next; while (temp) { size++; temp=temp->next; } return size; } the “ data ” field?

14. Client code // we want a list of integer: List_X(int) // we want a list of double: List_X(double)

15. Summary so far Parametric poly: data structures and functions take extra type parameters (hence the name) instation at compile-time Monomorphinization: code eventually becomes monomorphic faithfully models the feature of template in C++

16. Summary so far Pros: very powerful and elegant no runtime overhead (code is eventually mono) of course, require some C hack Cons: code explosion (due to monomorphinization) but seldom observed in practical code the poly code itself is not type checked compilation may be slow

17. Subtype poly struct List_t { X data; struct List_t *next; }; // Key idea: what if we can invent a most general // type “X”, such that any value is compatible // with “X”. // In C, this type is “void *”.

18. List revisit struct List_t { void *data; struct List_t *next; }; // Leave it an exercise to write function “length”

19. Client code int main () { // we want a list of integer: for (int i=0; i<10; i++) { int *p = malloc (sizeof (*p)); *p = i; List_insertHead (l, p); } // we want a list of double: for (int i=0; i<10; i++) { double *p = malloc (sizeof (*p)); *p = i; List_insertHead (l, p); }

20. Subtype poly All pointer types can be treated as a subtype of void * so we ’ d have to heap-allocate all objects to accompany this type Java or C# go even further: all objects are heap-allocated the most general type is Object this models Java generic honestly

21. Subtype poly Pros: single piece of code no compilation overhead Cons: Safety issue ugly type cast, and must be veryyyyyyyyyyyyyy careful efficiency issue allocation and garbage collection complexity issue

22. Think abstractly struct List_t { int data; struct List_t *next; }; // Is it a good idea to write this function? int exists (List_t l, int x); // Or this function? int sum (List_t l, int init); // Or this one? void print (List_t l);

23. Think abstractly void print (List_t l) { List_t temp = l->next; while (temp) { printf (“%d, ”, temp->data) temp = temp->next; } return; }

24. Think abstractly // But what if we make it parametric poly? #define List_X(X) struct List_t { X data; struct List_t *next; }; void print (List_t l);

25. 1st try void print (List_t l) { List_t temp = l->next; while (temp) { printf (“%???”, temp->data); temp = temp->next; } return; }

26. Think abstractly // To make it parametric poly: #define List_X(X) struct List_t { X data; struct List_t *next; }; // so we’d have to make “print” more abstract! void print (List_t l, void (*p)(X));

27. 2nd try void print (List_t l, void (*p)(X)) { List_t temp = l->next; while (temp) { p (temp->data); temp = temp->next; } return; }

28. Client code // instantiate an “int” list: List_X(int) void f (int x) { printf (“%d, ”, x); } int main () { List_t list = …; // cook a list of int List_print (list, f); }