[boost] Re: calling bind with literals

From: Dill, John (john-dill_at_[hidden])
Date: 2004-05-07 09:30:11

"Douglas Gregor" <gregod_at_[hidden]> wrote in message news:<200405061958.52886.gregod_at_[hidden]>...
> On Thursday 06 May 2004 05:32 pm, Dill, John wrote:
> > I believe that this can be done, but doesn't justify whether it should be
> > done.  There are several issues that I see at the moment.
> >
> > A.  Changes the semantics of everyone's bind and mem_fn, no biggy there
> > ;-). B.  Pass by value is silent, not sure how many copies of the argument
> > will be generated if pass-by-value.
> >
> > But at the same time there is an advantage.
> >
> > a.  You can use literals in calling your bind functions.
> >
> > What do you all think?
> 
> I don't like the change.
> 
> Ideally, bind(f, _1, _2, _3)(x, y, z) would be exactly equivalent to f(x, y, 
> z). The current bind() implementation gives us nearly this equivalence 
> because it passes by reference, except that we get a failure at compile time 
> if one tries to pass a literal. Going to passing by value would take us 
> further from that ideal equivalence.

I see your point.  But, what if instead of argument_traits being pass by value, it by default passes by T const&?  What could be done is to have the bind_t arguments be passed by const reference, and then use reference_wrapper to do type-selection to pass by reference.  It still supports literals, and passes by T const& when it can, but everything that is passed by reference must have a ref( object ).  The compiler would fail if you try to pass a reference without a reference_wrapper because it by default tries to pass a T const&.  This wouldn't require the 2^N overloads but would require you to do a ref( object ) for everything passed by reference.  This might also be a workaround for those compilers that don't support T& and T const& overloading.

I think that this should be addressed, either through overloading, or maybe this idea might be feasible.  I think the implementation of this aspect of bind is worth doing.

Best,
John