Polymorphism (in C) [duplicate]

Question

Possible Duplicate:
How can I simulate OO-style polymorphism in C?

I'm trying to better understand the idea of polymorphism with examples from languages I know; is there polymorphism in C?

Answer 1

This is Nekuromento's second example, factored in the way I consider idiomatic for object-oriented C:

animal.h

#ifndef ANIMAL_H_ #define ANIMAL_H_  struct animal {     // make vtable_ a pointer so they can be shared between instances     // use _ to mark private members     const struct animal_vtable_ *vtable_;     const char *name; };  struct animal_vtable_ {     const char *(*sound)(void); };  // wrapper function static inline const char *animal_sound(struct animal *animal) {     return animal->vtable_->sound(); }  // make the vtables arrays so they can be used as pointers extern const struct animal_vtable_ CAT[], DOG[];  #endif 

cat.c

#include "animal.h"  static const char *sound(void) {     return "meow!"; }  const struct animal_vtable_ CAT[] = { { sound } }; 

dog.c

#include "animal.h"  static const char *sound(void) {     return "arf!"; }  const struct animal_vtable_ DOG[] = { { sound } }; 

main.c

#include "animal.h" #include <stdio.h>  int main(void) {     struct animal kitty = { CAT, "Kitty" };     struct animal lassie = { DOG, "Lassie" };      printf("%s says %s\n", kitty.name, animal_sound(&kitty));     printf("%s says %s\n", lassie.name, animal_sound(&lassie));      return 0; } 

This is an example of runtime polymorphism as that's when method resolution happens.

C1x added generic selections, which make compile-time polymorphism via macros possible. The following example is taken from the C1x April draft, section 6.5.1.1 §5:

#define cbrt(X) _Generic((X), \     long double: cbrtl, \     default: cbrt, \     float: cbrtf \ )(X) 

Type-generic macros for math functions were already available in C99 via the header tgmath.h, but there was no way for users to define their own macros without using compiler extensions.