blah.c

#include <stdlib.h>
#include <stdio.h>
//#include "test.h"

/**
 * The following two struct's are basically an interface.
 */

// The VTable which will only ever be a single copy (normally)
struct _SomeClass_VTable
{
  // Constructor
  struct _SomeClass_Instance*(*ctr)(void);

  // Method one will add the int to the instance value
  void(*MethodOne)(struct _SomeClass_Instance*, int);
  // Method two will call methodOne twice.
  void(*MethodTwo)(struct _SomeClass_Instance*, int);
} _SomeClass_VTable;

// The instance class, there will be one of these per instance.
struct _SomeClass_Instance
{
	struct _SomeClass_VTable* vt;
	// properties
	int val;
};

/**
 * Next three definitions are like a "class SomeClass implements <the two structs>"
 */

// Constructor
struct _SomeClass_Instance* _SomeClass_Ctr(void)
{
  struct _SomeClass_Instance* i = malloc(sizeof(struct _SomeClass_Instance));
  i->vt = &_SomeClass_VTable;
  i->val = 0;

  return i;
}

void _SomeClass_MethodOne(struct _SomeClass_Instance* _this, int x)
{
  _this->val += x;
}

void _SomeClass_MethodTwo(struct _SomeClass_Instance*_this, int x)
{
  // call "this->MethodOne(x)" twice.
  _this->vt->MethodOne(_this, x);
  _this->vt->MethodOne(_this, x);
}

/**
 * main
 */

int main(int argc, char* argv[])
{
  // setup
 _SomeClass_VTable.ctr = _SomeClass_Ctr;
 _SomeClass_VTable.MethodOne = _SomeClass_MethodOne;
 _SomeClass_VTable.MethodTwo = _SomeClass_MethodTwo; 

  // new SomeClass();
  struct _SomeClass_Instance *instanceOne = _SomeClass_VTable.ctr();
  instanceOne->vt->MethodOne(instanceOne, 5);
  instanceOne->vt->MethodTwo(instanceOne, 1);

  printf("Value of member: %i", instanceOne->val);

  return 0;
}