Re: [boost] cref(_1)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-03-08 13:46:35

David Abrahams wrote:
> Hi,
>
> I have this code that isn't compiling because of the "forwarding
> problem" limitation of boost::bind:
>
>         std::for_each(
>             make_difference_iterator(p->second.begin())
>           , make_difference_iterator(p->second.end())
>           , boost::bind(&fu::writeVInt, &prox, _1));

Nice namespace name.

> The problem is that an implicit conversion is required between the
> reference type of the difference_iterator and the argument type of
> writeVInt.
>
> It seems to me that, since I know that binding to an rvalue is
> required, we ought to have a way to say "force this to be a const
> ref".  I thought of something like
>
>         std::for_each(
>             make_difference_iterator(p->second.begin())
>           , make_difference_iterator(p->second.end())
>           , boost::bind(&fu::writeVInt, &prox, boost::cref(_1)));
>                                                ^^^^^^^^^^^^  ^
>
> I know it's not pretty, but it sorta does what we want.  Thoughts?

Have you tried to implement it? ;-)

The current bind way to work around the above problem is to use an adaptor
that fixes its signature to, for example, void(fu const &). Choices are
function<void(fu const &)> and make_adaptable<void, fu const &> (in
boost/bind/make_adaptable.hpp).

Lambda also provides its own mechanisms.