CareerCup

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

struct PARENT_vtable { 
	int (*Init)(); 
	int (*Add)();
}; 

struct CHILD_vtable { 
	int (*Init)();//child will override 
	int (*Add)();//parent will define
	int (*Sub)();//child will define
}; 


int parent_doInit() 
{
	printf("parent init\n");
}

int child_doInit() 
{
	printf("child init\n");
}

int parent_doAdd() 
{
	printf("parent add\n");
}


int child_doSub() 
{
	printf("child sub\n");
}

struct PARENT_vtable PARENT_vtbl = {&parent_doInit ,&parent_doAdd }; 

struct CHILD_vtable CHILD_vtbl = {&child_doInit, &parent_doAdd ,&child_doSub}; 


struct PARENT_data 
{ 
	int x; 
}; 

struct CHILD_data { 
	int a;
}; 


struct PARENT 
{ 
	PARENT_vtable * vptr; 
	struct PARENT_data parent; 
}; 

struct CHILD { 
	CHILD_vtable * vptr; 
	struct PARENT_data parent; 
	struct CHILD_data child; 
}; 

struct PARENT * CreateNewInstance() 
{ 

	struct PARENT * Cthis = (struct PARENT*)malloc(sizeof(CHILD)); 

	Cthis->vptr =(PARENT_vtable*) &CHILD_vtbl; 

	return Cthis; 
} 


main() 
{ 

	struct PARENT * Kids1=CreateNewInstance(); 

	((CHILD_vtable*)Kids1->vptr)->Init();
	((CHILD_vtable*)Kids1->vptr)->Add();
	((CHILD_vtable*)Kids1->vptr)->Sub();

}

just use the key word struct instead of class.

struct and class are exactly similar (they both can have constructors). But struct's members are public by default and class members are private by default.

so declare a struct and have its constructor in its private region.

I am bit confused!!
"Using struct in C"
We have no concept public/private member in C struct.
Moreover, member functions are undefined in C struct.
I might understand the question wrongly so let me clear

Can somebody clarify this question? In C, struct is public, there are no access specifiers in C because data abstraction is an OO concept. So does this imply that we need to mimic OO behavior using struct in C?
@hyan, what did the interviewer say?
@hemant - can you explian your solution?

this is so confusing, someone please clarify this...

This is not confusing. C struct does not allow c++ class features. However, C allows function pointer. Therefore, the point is how to use function pointer to implement c++ constructor concept.

i saw Hemant Jain's code somewhere before. But I am not very convinced by that. Somebody else has better idea?

Guys, see this link. It is helpful
http://www.keyongtech.com/4831974-how-to-implement-c-class

Sounds perfect Hyan. for more of C++ flavor into this, we could also add this pointer into it :).
polymorphic C++ classes includes vptr and vtable that you have already done nicely.
If we add this pointer to all member functions that would add more of C++ flavor


int parent_doAdd(struct PARENT *this)
{
printf("parent add\n");
}



int child_doSub(struct CHILD *this)
{
printf("child sub\n");
}