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: Michel MORIN (mimomorin_at_[hidden])
Date: 2010-12-27 15:10:15

Neil Groves wrote:
> 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.

Here is the standard:
Range-Based For Loop Wording (Without Concepts)
http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2009/n2930.html

Its summary is:
A range-based for
    for (auto x : range_expr) {
        // do something
    }
is equivalent to
    {
        // Special rule: namespace std is added to rng's associated namespaces
        auto&& rng = range_expr;

        for (
            // Note: unqualified begin/end function call
            auto it = begin(rng),
                 it_end = end(rng);
            it != it_end;
            ++it
        ) {
            auto x = *it;
            // do something
        }
    }
Note that rng has namespace std as its associated namespaces
and the unqualified begin/end function call triggers ADL.

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

That would be Great, Neil! This is an important issue.

Regards,
Michel