C++ – How Vtable of Virtual functions work


I have a small doubt in Virtual Table, whenever compiler encounters the virtual functions in a class, it creates Vtable and places virtual functions address over there. It happens similarly for other class which inherits. Does it create a new pointer in each class which points to each Vtable? If not how does it access the Virtual function when the new instance of derived class is created and assigned to Base PTR?

Best Solution

Each time you create a class that contains virtual functions, or you derive from a class that contains virtual functions, the compiler creates a unique VTABLE for that class.

If you don’t override a function that was declared virtual in the base class, the compiler uses the address of the base-class version in the derived class.

Then it places the VPTR into the class. There is only one VPTR for each object when using simple inheritance . The VPTR must be initialized to point to the starting address of the appropriate VTABLE. (This happens in the constructor.) Once the VPTR is initialized to the proper VTABLE, the object in effect “knows” what type it is. But this self-knowledge is worthless unless it is used at the point a virtual function is called. When you call a virtual function through a base class address (the situation when the compiler doesn’t have all the information necessary to perform early binding), something special happens. Instead of performing a typical function call, which is simply an assembly-language CALL to a particular address, the compiler generates different code to perform the function call.

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