[boost] type_traits: more state-of-the-union

From: David Abrahams (david.abrahams_at_[hidden])
Date: 2002-04-15 11:29:58

5. Okay, I figured out why is_empty was checking for int convertibility;
it was for Borland. I restored the nasty check for that compiler.

6. is_enum also seemed to rely on is_convertible, but there were no
tests for is_enum<noncopyable> to uncover it. Fortunately there's an
easy fix which I'm checking in.

7. The header dependencies are getting messy. One striking example is
created by the new is_class implementation, which is a standalone
template on conforming implementations but relies on a huge number of
other templates on other implementations. Add to that the fact that I
can never remember which header contains is_enum, etc., and I am coming
to the conclusion that a fine-grained header structure with one header
per template is the way to go. Otherwise, it seems to me that any user
of type traits is going to essentially get the whole wad regardless of
the fact that she's using the current "fine-grained" headers.

8. I think we need some new config macros, but I'd like someone who has
done this before to add them if possible

    BOOST_ELIPSIS_ARGS_DO_NOT_INVOKE_COPY_CONSTRUCTOR - for compilers
that can resolve overloads to functions accepting elipsis arguments
without invoking the copy constructor of the actual argument. This is
nonconforming behavior AFAICT but most compilers seem to allow it, and
it's what allows is_convertible<T,U> to work when T is noncopyable.

    BOOST_...something - to detect that the is_class implementation in
boost/type_traits/is_class works with the compiler.

-Dave

+---------------------------------------------------------------+
                  David Abrahams
      C++ Booster (http://www.boost.org)               O__  ==
      Pythonista (http://www.python.org)              c/ /'_ ==
  resume: http://users.rcn.com/abrahams/resume.html  (*) \(*) ==
          email: david.abrahams_at_[hidden]
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