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Subject: Re: [Boost-users] [attn. Dave Abrahams] workaround ideas?
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2009-03-04 13:51:32

Ovanes Markarian wrote:
> Noah,
> please see my answer below:
> On Tue, Mar 3, 2009 at 5:59 PM, Noah Roberts <roberts.noah_at_[hidden]
> <mailto:roberts.noah_at_[hidden]>> wrote:
> template < typename FIELD, typename CDR >
> typename FIELD::type & get(rec< FIELD, CDR > & r)
> {
> return r.value;
> }
> You should use the static_cast<SomeFieldSpecificType>(r) here to fix
> the problem. The only problem is, that 'r' is the entire recordset, and
> you can't cast it to itself, nor you can extract the SomeFieldSpecificType.

Not really. The get function is called with an explicit template
argument for the first argument:


The resolution should therefore cast the rec to the correct base class
so that r.value actually is the appropriate call. Putting a static cast
in the function body would actually have no effect and wouldn't resolve
the ambiguity the compiler seems to think exists.

The type passed into the function is a rec<field3, XXX> but the explicit
notation would require the function to expect a rec<field1,XXX>; the
compiler is saying it can't decide between them. I don't know where in
the standard this would be specified but I expect it says to obey the
explicit resolution over implicit.

> Your example work with MSVC 9.

Yeah, I think they fixed the bug. I tried it with express edition and
it functions. Comeau also eats it without error.

  I have rewritten it to use the
> static_cast, but did not test it with MSVC 8. Give it a try, I think
> this should work:

This solution is what the instruction comes to in the book but isn't
along the lines of my question. For one thing I wanted the rec<>
template to inherit directly from the other field recs, which my version
(and the first example in the book) does. Your code doesn't.

I am going to end up using something more like this version now, since I
found what I was doing wrong earlier, but it still doesn't address the
original problem that I'm pretty confident is a bug in the compiler. My
question was regarding the former technique since I thought I couldn't
use the latter.

Anyway, thanks for the help. I think I'm onto a solution to my original
problem if not a workaround for making the compiler swallow the book's
sample code.

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