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.

GMan? [Are you kidding?](http://stackoverflow.com/users/87234/gman) — sbi, Dec 20 '10 at 18:45
I'm certain this is a different GMan than this one: http://stackoverflow.com/users/87234/gman — Fred Larson, Dec 21 '10 at 05:07
@GMan: So you're not [GMan](http://stackoverflow.com/users/242981/gman), then? `:)` This _is_ confusing... — sbi, Dec 22 '10 at 09:01

Fred Larson · Accepted Answer · 2010-12-20T17:51:00.530

6

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

edited Dec 20 '10 at 17:51

answered Dec 20 '10 at 17:31

Fred Larson

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Where can I find more information on the another level of indirection for run-time polymorphism? EDIT: And on the C++ philosophy for that matter? Thanks! – GMan Dec 20 '10 at 17:44
1

@GMan, just try to get the same behaviour using C, you will most likely end up using tables of function pointers. – doron Dec 20 '10 at 17:56
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@GMan: From the C++ origianl design doc: http://www2.research.att.com/~bs/rules.pdf That is, the zero-overhead principle: “what you don’t use, you don’t pay for” – Martin York Dec 20 '10 at 20:55
@Martin York: Great link. Thank you! – Fred Larson Dec 20 '10 at 21:14

score 2 · Answer 2 · answered Dec 20 '10 at 17:34

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.

score 2 · Answer 3 · answered Dec 20 '10 at 17:34

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.

why do we even need VPTR?

3 Answers3