C++ virtual functions

I did this stuff while understanding virtual functions and polymorphism in C++ for job interviews. And got a chance to clean it up and comment it well (of course to be put on a blog).

This ones on virtual functions, explaining how VPTR and VTABLE work. Code is self explanatory, accompanied by a diagram.

Code example to find out VPTR and VTABLE in C++ class. And use it to probe class virtual functions. VPTR and VTABLE usage in C++ Class expalined diagrammatically below. Test Class Object      (obj)        |             VTABLE (vtable)        v                   | +------------+             v | VPTR (vptr)|------>+------------+ +------------+       |  VTEntry1  |------------> +------------+ | DATA1 (a)  |       +------------+              |  func1()   | +------------+       |  VTEntry2  |-------+      |            | | DATA2      |       +------------+       |      |            | +------------+       |      .     |       |      +------------+ |      .     |       |      .     |       | |      .     |       |      .     |       +----->+------------+ |      .     |                                   |   func2()  | |      .     |                                   |            |                                                  |            |                                                  +------------+

typedef void (*func)(void); // class member function type typedef int* ptr;           // 32 bit system pointer type class Test { public:     int a; // DATA1     Test() : a(0) {}     virtual void func1() {         cout << "Test::func1" << endl;     }     virtual void func2() {         cout << "Test::func2" << endl;     } }; int main(int argc, char * const argv[]) {     Test obj;     int  *pdata;     ptr  vptr, vtable;     func pfunc = NULL;     vptr = (ptr)&obj;     // first address of obj (class) points to vptr     vtable = (ptr)*vptr;  // dereferencing vptr to get vtable     pdata = (int*)((ptr)&obj+1); // this is 'a', the first member of class Test     *pdata = 10;     cout << "pdata (a): " << *pdata << endl;     cout << "vptr: " << vptr << "   " << "vtable: " << vtable << endl;     // dereferencing first (VTEntry1) entry in VTABLE     pfunc = (func)*(vtable+0);     pfunc();     // dereferencing second (VTEntry2) entry in VTABLE     pfunc = (func)*(vtable+1);     pfunc();     return 0; }

Output:

pdata (a): 10 vptr: 0xbfee67d4   vtable: 0x8048a80 Test::func1 Test::func2

virtualtest.cpp file link