Re: [boost] [announce] Multi signature boost::function v1.0

From: Marco Costalba (mcostalba_at_[hidden])
Date: 2008-05-14 07:22:17

On Wed, May 14, 2008 at 10:31 AM, Giovanni Piero Deretta
<gpderetta_at_[hidden]> wrote:
> On Wed, May 14, 2008 at 7:01 AM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
>  > AMDG
>  >
>  >
>  >
>  >  I suspect that there are really two tools here.  One which allows
>  >  separate function objects to be bound to each signature, more like
>  >  a fusion sequence or something.  The other binds all overloads to
>  >  a single object.  Neither one is easily implementable in terms of the other.
>  >  If all the signatures are bound at once, then it is difficult to later
>  >  change
>  >  just one.  If the functions are bound separately, then there is a
>  >  significant
>  >  size overhead compared to using a single object.
>  >
>
>  Great point Steven.
>
>  Yes, my proposal would not let one rebind signatures separately (not easily
>  at least). If this is an useful thing (really, I do not know), then
>  please Marco, disregard most of my comments,
>  probably I'm really looking for something else.
>
>  --

Yes. I fully agree with Steven and with you. But because I have
already implemented both features (indeed also a third one, see below
for passing a boost::ref(poly_obj) ) I propose to do the following:

- Rename proposed set_all() in set_polymorphic_object()

The point here is not to set all the msf slots, but to assign a poly
oject to a msf, that's the goal, the fact that doing this the slots
become all occupied is a side effect. So let the API reflect this.
Also IMHO would be misleading using operator=() in this case because
assigning a poly object opens the door to a whole set (currently 3) of
different behaviours that are not present in the other cases so let
the API reflect this.

- Default to rebind signatures separately as suggested by Daniel and
seconded by Giovanni.

The point here is that the "natural" extension is doing this and the
other behaviour is the sub-optimal one as pointed out by Steven and
Giovanni due to size overhead and possibly not intuitive behaviour as
suggested by Daniel

- Allow the bind to a single copy of poly objects as requested by Giovanni:

The point is that this feature as been requested ;-) and is possible
to implement it easily although with sub-optimal size. The user API
for this will be a boolean argument defaulted to false in
set_polymorphic_object()

- Allow to call set_polymorphic_object( boost::ref(poly_obj) )

The point here is completeness of solution because boost::function can
do this and because there are cases in which I want to modify a
poly_object state externally from MSF while poly_obj is still bounded.

I think the 3 different handling of poly objects cover this case at
least as long as msf is considered just a boost::function extension,
as I would be glad to stick with.

The case of interest for Giovanni is implemented in a performance
point of view sub-optimal way but it works and anyway the _real_ (read
benchmarked) performance penalty is still not very clear to me IMHO.

Comments?

Thanks
Marco