Re: [Boost-users] ambiguity between std::begin/end and boost::begin/end in gcc 4.6

Subject: Re: [Boost-users] ambiguity between std::begin/end and boost::begin/end in gcc 4.6
From: Neil Groves (neil_at_[hidden])
Date: 2010-12-27 12:18:52

On Mon, Dec 27, 2010 at 3:37 PM, Michel MORIN <mimomorin_at_[hidden]> wrote:

> Neil Groves wrote:
> > You might get a quicker response from the GCC team, but I'm happy to look
> > into this issue if you provide the exact version of GCC and library
> version
> > you are using. It will, of course, take me a while since I'll have to
> setup
> > the compiler and environment.
> >
> > You might want to take a look at the relevant standard library, or the
> > compiler implementation of the new 'for' syntax. I suspect there might be
> an
> > unqualified call to begin() and/or end().
>
> For this issue, there is some information in the following thread:
> (a patch and a test case are also provided in the thread.)
> http://article.gmane.org/gmane.comp.lib.boost.devel/211863
>
> If we are allowed to break existing codes that rely on ADL to find
> boost::begin/end,
> this issue can be easily resolved by putting boost::begin/end into an
> ADL-barrier namespace
> and bringing them into namespace boost with a using directive.
>
>
None of our boost code should be using unqualified calls to begin/end. The
Boost.Range design explicitly provided range_begin and range_end as the
function names to be found via ADL so that everyone could use qualified
boost::begin and boost::end calls. boost::begin and boost::end are
explicitly qualified throughout Boost.Range and hence the ambiguous call
isn't coming from Boost.Range. Please see
http://www.boost.org/doc/libs/1_45_0/libs/range/doc/html/range/reference/extending/method_2.htmlfor
the relevant documentation.

Hence we can certainly break code that is erroneously relying on ADL for
begin/end to be found via ADL.

However in this case I have yet to understand the nature of the underlying
problem, because there isn't an obvious unqualified call to begin/end. The
reverse adaptor uses qualified calls that can't possibly be ambiguous with
std::begin/end. I haven't yet checked to see if unqualified calls to
begin/end are intended to be used as an extension mechanism for the next C++
standard.

I shall put begin and end in an ADL barrier namespace anyhow, since these
are explicitly designed to be qualified when called.

> Regards,
> Michel
>
>
Regards,
Neil Groves