On Sat, 1 Dec 2007 00:31:19 +0000, Al Viro <[email protected]> wrote:
> On Sat, Dec 01, 2007 at 12:19:50AM +0100, J.A. Magall??n wrote:
> > An vtable in C++ takes exactly the same space that the function
> > table pointer present in every driver nowadays... and probably
> > the virtual method call that C++ does itself with
> >
> > thing->do_something(with,this)
> >
> > like
> > push thing
> > push with
> > push this
> > call THING_vtable+indexof(do_something) // constants at compile time
>
> This is not what vtables are. Think for a minute - all codepaths arriving
> to that point in your code will pick the address to call from the same
> location. Either the contents of that location is constant (in which case
> you could bloody well call it directly in the first place) *or* it has to
> somehow be reassigned back and forth, according to the value of this. The
> former is dumb, the latter - outright insane.
>
> The contents of vtables is constant. The whole point of that thing is
> to deal with the situations where we _can't_ tell which derived class
> this ->do_something() is from; if we could tell which vtable it is at
> compile time, we wouldn't need to bother at all.
>
Yup, my mistake (that's why I said i will learn something). I was thinking
on non-virtual methods. For virtual ones you have to fetch the vtable
start address and index from it.
> It's a tradeoff - we pay the extra memory access (fetch vtable pointer, then
> fetch method from vtable) for not having to store a slew of method pointers
> in each instance of base class. But the extra memory access is very much
> there. It can be further optimized away if you have several method calls
> for the same object next to each other (then vtable can be picked once),
> but it's still done at runtime.
--
J.A. Magallon <jamagallon()ono!com> \ Software is like sex:
\ It's better when it's free
Mandriva Linux release 2008.1 (Cooker) for i586
Linux 2.6.23-jam03 (gcc 4.2.2 (4.2.2-1mdv2008.1)) SMP Sat Nov
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
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