Question

I was recently on an interview for a position where C/C++ is the primary language and during one question I was told that it’s possible to use the vtable to determine which class in a hierarchy a base pointer actually stores.

So if, for example you have

    class A  
    {  
    public:  
    A() {}  
    virtual ~A() {}  
    virtual void method1() {}  
    };

    class B : public A  
    {  
    public:  
    B() {}  
    virtual ~B() {}  
    virtual void method1() {}  
    };

and you instantiate A * pFoo = new B(), is it indeed possible to use the vtable to determine whether pFoo contains a pointer to an instance of A or B?

Answer 1

This is obviously implementation dependent but, on most implementations, the in-memory representation of an object of class A or B will start with a pointer to the vtable. You can look at this vtable pointer, compare it to vtable pointers for objects that you know to be of class A or B, and determine the class of the object in that way.

To illustrate (of course this is anything but good style):

A *pFoo=new B(); // pointer to object of unknown class (either A or B)
A a;  // Object known to be of class A
B b;  // Object known to be of class B
void *vptrA=*((void **)&a);  // Pointer to vtable of class A
void *vptrB=*((void **)&b);  // Pointer to vtable of class B
void *vptrFoo=*((void **)pFoo);  // Pointer to vtable of unknown object
if(vptrFoo==vptrA)
    printf("Class A\n");
else
    printf("Class B\n");

Important: This is only an illustration of how most implementations work; besides being implementation dependent, this technique breaks down in the presence of multiple inheritance. You should never do anything like this in production code; use RTTI instead.