A virtual table is a mechanism used for dynamic binding (run-time binding).
If a class has at least one virtual function a virtual table is created for that class.
Only one virtual table will be created for all instances of that class.
Each of the instances will have a pointer to that virtual table.
The virtual table pointer is called _vfptr, which will not be directly accessible from the code.
But, we can see that from the watch window while debugging.
class CFooBase
{
private:
int data;
public:
virtual void fun1()
{
printf("fun1 from base\n");
}
virtual void fun2()
{
printf("fun2 from base\n");
}
};
class CFooDerived: public CFooBase
{
private:
int data;
public:
void fun1()
{
printf("fun1 from derived\n");
}
};
CFooDerived obj1, obj2;
The representation of CFooDerived objects will be like the following
Note: There is no implementation of fun2 in derived class CFooDerived. So, the virtual table stores the pointer of CFooBase::fun2()
we can access virtual functions directly from virtual table.
typedef void (*Fun)();
int _tmain(int argc, _TCHAR* argv[])
{
CFooDerived obj;
Fun fun = (Fun)*((int*) (*(int*)(&obj)) + 0);
fun();
fun = (Fun)*((int*) (*(int*)(&obj)) + 1);
fun();
getche();
return 0;
}
we assumed here that the _vfptr placed at the beginning of the class object. but, the C++ standards allows the compailer that the freedom to insert that anywhere in the object.
In case of multiple inheritance, multiple virtual tables will be created corresponds to each base classes, which has at least one virtual function.
class CFooBase1
{
private:
int base1_data;
public:
virtual void fun1_base1()
{
printf("fun1 from base1\n");
}
virtual void fun2_base1()
{
printf("fun2 from base1\n");
}
};
class CFooBase2
{
private:
int base2_data;
public:
virtual void fun1_base2()
{
printf("fun1 from base2\n");
}
virtual void fun2_base2()
{
printf("fun2 from base2\n");
}
};
class CFooDerived: public CFooBase1, public CFooBase2
{
private:
int der_data;
public:
void fun1_base1()
{
printf("fun1_base1 from derived\n");
}
void fun2_base1()
{
printf("fun2_base1 from dervied\n");
}
void fun1_base2()
{
printf("fun1_base2 from derived\n");
}
};
CFooDerived obj;
the representation of obj will be like the following
Cheers,
