Virtual table pointer and Virtual table simulation - c++

Question

I am trying to simulate the vptr/vtable concept. I have 3 questions regarding the following:
1. I have not defined any constructor for the class FOO , which should be fine as can be seen inside main(Foo *ptr= new FOO is OK ).
But when i create the object on heap for the same inside FooVtable::bar() , it complains about no appropriate default constructor found. Why?
2. I see that vptr points to some address(even though there is no new FooVTable done for vptr,i guess ctor by default does it. is it expected?), FooVtable::bar is getting called but cout doesnt print the statement. Why?
3. Inside callsomeFun(), i am calling foo->vptr->bar . What i am trying to achieve here is that vptr->bar part of foo->vptr->bar should actually call a pointer to member function Foo::bar() , which i guess isnt directly possible, hence i declared void bar(Foo* foo) inside which i am thinking to call the Foo:bar through ptr. Am i approaching it right, or is there a better way to approach this?
class Foo;
struct FooVtable {
void bar(Foo* foo)
{
//Foo *ptr = new Foo; //Why does this throw error as No Default constructor?
cout<"FooVTable : bar"; //Doesnt get printed even though gets called
}
};
struct Foo {
FooVtable* vptr;
void bar(Foo *foo)
{
cout<<"Foo : bar";
}
};
int callSomeFun(Foo* foo) {
foo->vptr->bar(foo);
return 0;
}
int main()
{
Foo *ptr=new Foo;
callSomeFun(ptr);
return 0;
}

Answer 1

when i create the object on heap for the same inside FooVtable::bar() , it complains about no appropriate default constructor found. Why?
That's a poor error message for a situation where you're trying to use a class before it has been fully declared. On a different compiler the message could be more like "error: invalid use of incomplete type 'class Foo'".
One way to solve it would be to put the full struct Foo stuff above FooVtable and then use a forward declaration of FooVtable instead of Foo.
I see that vptr points to some address(even though there is no new FooVTable done for vptr,i guess ctor by default does it. is it expected?), FooVtable::bar is getting called but cout doesnt print the statement. Why?
That vptr is uninitialized. Primitive types like pointers aren't initialized to any kind of default value by just a new. So whatever is in there is garbage. That much is expected.
Whatever happens after you call a function via a pointer with a garbage value is Undefined Behavior. So just about anything could happen and there wouldn't be a guaranteed explanation.
However in this case there's also another problem: You've made a typo. cout<"FooVTable : bar"; has a single < and thus would do a strange and useless comparison instead of outputting anything.
foo->vptr->bar should actually call a pointer to member function Foo::bar() , which i guess isnt directly possible
Pointers to member functions are possible. You could potentially use one in the situation you've created so far. However once you start bringing inheritance into the scenario (which is where virtual functions are finally useful), it could get interesting to try to make this work out between the different types. (I use member function pointers so rarely that I can't say offhand.)
Am i approaching it right, or is there a better way to approach this?
It really depends on how much of a real implementation you're trying to "simulate" and what you're hoping to achieve by it.