A simple implementation of an object in C using malloc an free for creation and destruction.

object_malloc.c

/*
 *	A simple example of an object in ANSI C.
 *	This example uses malloc to create the new object.
 *
 *	Note that with C++ the "this" variable is pretty much just implicit.
 *
*/
#include <stdio.h>
#include <stdlib.h>

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

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

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

/*
 *	Constructor for an adder.  Sets the object's methods and
 *	initializes the value.
*/
adder_t *newAdder() {
	adder_t *a = (adder_t*)malloc(sizeof(adder_t));
	a->number = 0;
	a->increment = &myIncrementer;
	a->get = &myGetter;
	a->destroy = &destroyAdder;
	return a;
}

/*
 *	Destructor for the adder object.
*/
void destroyAdder(adder_t *this) {
	free(this);
	this = NULL;
}

int main() {

	// Create an instance of the object
	adder_t *myAdder = newAdder();
	
	// 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));

	// Destroy the object
	myAdder->destroy(myAdder);

	return 0;
}