[boost] Re: A misunderstanding; you went with the wrong solution

From: David Abrahams (dave_at_[hidden])
Date: 2004-08-20 18:00:30

Daryle Walker <darylew_at_[hidden]> writes:

> [Top-post since the new stuff gives a fuller context.]
>
> I think I need to explain things.  (This is what I get for not reading Boost
> e-mail for a few days.  I could have nipped this in the bud.  I didn't find
> out about this problem myself until I tried a compile....)
>
> 1.  <boost/utility_fwd.hpp> was introduced with
> <boost/utility/base_from_member.hpp>

_That_ was the first mistake.

> 1a. <boost/utility/base_from_member.hpp> #includes <boost/utility_fwd.hpp>,
> and needs to since a template value default is in the forwarding header.

It should have included boost/utility/base_from_member_fwd.hpp, if
anything.

> 2.  The other forward-able items in <boost/utility.hpp> were added to
> <boost/utility_fwd.hpp> upon first commit.

Unneccessary.  boost/utility.hpp is an obsolete construct; it's just
there for backward compatibility.

> 3.  The various items in <boost/utility.hpp> were split into several
> headers.
>
> 4.  The maintainers of <boost/noncopyable.hpp>, then _and_ now, ignored the
> fact that <boost/utility_fwd.hpp> referenced boost::noncopyable.  This was
> the mistake.
>
> 5.  What should have happened was to add "#include <boost/utility_fwd.hpp>"
> to <boost/noncopyable.hpp> at the time of the split.

Not doing that was the second mistake.

> 6.  The reason for [5]?  Headers that have a corresponding forwarding header
> should always #include that forwarding header.  Why?  So...

Yes, but noncopyable.hpp shouldn't have a forwarding header.

> 7.  If someone makes a significant change in the primary header, the
> mismatch in the forwarding header will cause a compile-time error.  (If only
> we caught the mistake at [4], then Dave's change would have been flagged,
> and we could have avoided this incident.)
>
> 8.  Unfortunately, Dave and Aleksey didn't understand this and went the
> other way.  Instead of adding "#include <boost/utility_fwd.hpp>" to
> <boost/noncopyable.hpp> (Bronek Kozicki suggested this), they excised
> boost::noncopyable from <boost/utility_fwd.hpp>.
>
> So, can we make the change go the other way?

No way, man.  Some people already argue against noncopyable on the
grounds that it introduces too many dependencies.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com