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.