A simple (if flawed) implementation of an object in C.

c_object.c

/*
 *	A simple example of an object in ANSI C.
 *	It would be better to have initAdder simply call malloc
 *	and return a pointer to the new object (and be preferable too)
 *	as this would allow for freeing of objects too but for the sake of
 *	simplicity I've opted to pass it a pointer instead.
 *
 *	Note that with C++ the "this" variable is pretty much just implicit.
 *
*/
#include <stdio.h>

// Declare the "class"
struct adder;
typedef struct adder adder_t;

// Declare the "methods"
void myIncrementer(adder_t *this);
int myGetter(adder_t *this);

// Declare the "interface"
struct adder {
	int number;
	void (*increment)(adder_t *this);
	int (*get)(adder_t *this);
};

/*
 * Method to increment an adder's value.
*/
void myIncrementer(adder_t *this) {
	this->number++;
};

/*
 * Method to get the adder's current value.
*/
int myGetter(adder_t *this) {
	return this->number;
}

/*
 *	Initializer for an adder.  Sets the object's methods and
 *	initializes the value.
*/
void initAdder(adder_t *this) {
	this->number = 0;
	this->increment = &myIncrementer;
	this->get = &myGetter;
}

int main() {

	// Create an instance of the object
	adder_t myAdder;
	initAdder(&myAdder);
	
	// Increment the adder a few times.
	myAdder.increment(&myAdder);	// number == 1
	myAdder.increment(&myAdder);	// number == 2
	myAdder.increment(&myAdder);	// number == 3
	myAdder.increment(&myAdder);	// number == 4

	// Prints "myAdder's value is: 4"
	printf("myAdder's value is: %d\n", myAdder.get(&myAdder));

	return 0;
}