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

From: Marco Costalba (mcostalba_at_[hidden])
Date: 2008-05-13 13:59:58

On Tue, May 13, 2008 at 3:01 PM, Giovanni Piero Deretta
<gpderetta_at_[hidden]> wrote:
>
>  >  >  In practice MSF acts as N different boost::function objects with a
>  >  >  think type based dispatcher.
>  >  >  I can of course fix the problem by adding another layer of indirection:
>  >  >
>  >
>  >  Have you tried to fix it with
>  >
>  >  msf_function<int(int), double(double), my_big_int(my_big_int)> x;
>  >
>  >  x = boost::ref( frob(10) );
>  >
>  >  Indeed boost::function supports ref() and cref() to avoid the copy
>  >  when is not needed or when the copy is simply wrong. MSF supports
>  >  ref() and cref() too and forwards them, still wrapped, to the
>  >  underlying boost::function objects so that no copy of the wrapped
>  >  polymorphic function object is made.
>  >
>
>  This has different semantics from what I wanted. I do not want to have
>  to keep the wrapped object in the stack to add it in a wrapper.
>  In this case I need I want MSF to copy my object (i.e. to manage its
>  lifetime) but I want it to do only one copy (usually for efficiency,
>  but sometimes like this case, for correctness).
>
>

Actually currently MSF does not copy anything, takes the functor
object as const& and forwards to boost::function

It's the default implementation of boost::function that takes the
functors by value instead of by reference.

See point 4 of boost::function FAQ

http://www.boost.org/doc/libs/1_35_0/doc/html/function/faq.html

If I have understood correctly you would like that wrapping in a
boost::ref would be done internally in MSF, something like

template<typename F>
void set_all(F const& f)
{
   F& stored_object_copy = store_a_copy_somewhere(f);

    do_set_all(boost::ref( stored_object_copy ));
}

Is it correct? And in this case what is your suggestion for an API to
fallback on the (currently default and more natural at least by an
implementation point of view) N copy behaviour?

Thanks
Marco

P.S: BTW I've tested the use of boost::ref() to fix the single state
problem: it works nicely and easily ;-)