why do we even need VPTR?

Question

And why don't we use the same method for non virtual functions?

I mean, why do we use virtual functions in that way? Can't we just use them as non-virtaul ones and override them?

And if this method is saving us time/space or what ever, why don't we use the same method for non-virtual functions? I mean it would make sense that there would be one table of functions for a specific class.

Anyway, thanks in advance, I am just a bit confused.

Answer 1

You can't have run-time polymorphism without using a level of indirection. That's what the vptr is for.

The vptr is not used for non-polymorphic functions because that indirection costs something. The C++ philosophy is that you don't pay for what you don't use.

EDIT:

Here's some info on how virtual tables work: http://en.wikipedia.org/wiki/Virtual_table

Answer 2

The compiler essentially generates a direct call to non-virtual methods. With a virtual method call, the compiler generates code to lookup the address of the method and then makes a call to that address. Thus, it is, in theory, at least one more lookup when calling a virtual function. There would be no reason to incur that cost otherwise.

Answer 3

Using vptr allows method resolution based on object type rather than variable type. Not using vptr makes method calls faster. The C++ designers decided to allow the convenience of virtual functions but not require the performance penalty for other functions.