Re: [C++0x] Range-based for statements and ADL

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 29 March 2011 21:33, Rodrigo Rivas wrote:
> On Tue, Mar 29, 2011 at 8:22 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> How about "No suitable %<begin%> and %<end%> functions found for range
>> expression of type %qT in %<for%> statement" ?
>
> Nice.
> But the problem here is that there are a lot of different error conditions:
> 1. begin member but not end member.
> 2. end member but not begin member.

I think these cases should say something like "invalid lookup results,
member 'begin' and non-member 'end'" (or vice versa) as we had in last
week's patch.

Or maybe it would be better to say "lookup for 'for' found member
'begin' but no member 'end'"

> 3. begin and end member but one or both are not callable/accesible/whatever.

I would definitely want to get an "access denied", "deleted function"
etc. error for that case.

> 3. no begin/end members, a begin global function but not end.
> 4. no begin/end members, an end global function but not begin.

Maybe something similar to the text I suggested above, but only
mentioning the lookup that failed, "no suitable 'begin' found for
range expression ..."

> 5. no begin/end members, begin and end functions but one or both not
> callable/wrong overload/etc.

In some cases I would probably want to know the actual error, not just
"cannot use range-based for"

If a begin() is found but can't be called, showing the "candidates
are..." list might be helpful to see the problem e.g. if
my::begin(my::type&) can't be called for const my::type.

> 6. no begin/end members, no begin/end global functions. Since the
> standard headers have the std::begin/std::end templates, this will not
> happen when you include (most) STL headers.

Maybe this is the only case where the text I suggested is useful, I
think a special "can't use range-based for" makes sense in this case,
instead of "begin not declared in this scope"

> What is the best message for each? Does it make sense to have up to 6
> different messages? Too many?

It would be nice for users, but it's certainly more work, and
potentially more places for bugs to creep in.