[C] OOP in C sample

Because I got a job, it was a C project.
So I am learning how to implement OOP in C now.
This is a example searched form google.
I use Xcode 3.2.3 with gcc4.2 to implement and test it.

#include <stdio.h>
#include <stdlib.h>

typedef void (*Walk_Fun)(struct Animal *a_This);
typedef struct Animal * (*Dtor_Fun)(struct Animal *a_This);

typedef struct _Animal_Vtable{
   Walk_Fun Walk;
   Dtor_Fun Dtor;
}Animal_Vtable;

typedef struct _Animal{
   Animal_Vtable vtable;
  
   char *Name;
}Animal;

typedef struct _Dog{
   Animal_Vtable vtable;
  
   char *Name; // mirror member variables for easy access
   char *Type;
}Dog;

void Animal_Walk(Animal *a_This){
   printf("Animal (%s) walking\n", a_This->Name);
}

struct Animal* Animal_Dtor(struct Animal *a_This){
   printf("animal::dtor\n");
   return a_This;
}

Animal *Animal_Alloc(){
   return (Animal*)malloc(sizeof(Animal));
}

Animal *Animal_New(Animal *a_Animal){
   a_Animal->vtable.Walk = Animal_Walk;
   a_Animal->vtable.Dtor = Animal_Dtor;
   a_Animal->Name = "Anonymous";
   return a_Animal;
}

void Animal_Free(Animal *a_This){
   a_This->vtable.Dtor(a_This);
  
   free(a_This);
}

void Dog_Walk(Dog *a_This){
   printf("Dog walking %s (%s)\n", a_This->Type, a_This->Name);
}

Dog* Dog_Dtor(Dog *a_This){
   // explicit call to parent destructor
   Animal_Dtor((Animal*)a_This);
  
   printf("dog::dtor\n");
  
   return a_This;
}

Dog *Dog_Alloc(){
   return (Dog*)malloc(sizeof(Dog));
}

Dog *Dog_New(Dog *a_Dog){
   // explict call to parent constructor
   Animal_New((Animal*)a_Dog);
  
   a_Dog->Type = "Dog type";
   a_Dog->vtable.Walk = (Walk_Fun) Dog_Walk;
   a_Dog->vtable.Dtor = (Dtor_Fun) Dog_Dtor;
  
   return a_Dog;
}


int main (int argc, const char * argv[]) {
   /* 
     base class: 
     Animal *a_Animal = Animal_New(Animal_Alloc());
     */
   Animal *a_Animal = (Animal*)Dog_New(Dog_Alloc());
  
   a_Animal->vtable.Walk(a_Animal);
  
   Animal_Free(a_Animal);
  
   return 0;
}