On Sat, 1 Dec 2007 00:31:19 +0000, Al Viro <viro@ftp.linux.org.uk> wrote:

quoted text
> 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.

quoted text
> 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
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