[boost] bug: cyclic header dependencies in type_traits

From: Scott McPeak (smcpeak_at_[hidden])
Date: 2006-04-05 19:47:57

In Boost 1.33.1, the following headers are involved in an #inclusion
cycle:

   boost/type_traits/is_scalar.hpp
   boost/type_traits/is_enum.hpp
   boost/type_traits/is_convertible.hpp
   boost/type_traits/is_abstract.hpp
   boost/type_traits/is_class.hpp

This causes a problem for standards-compliant compilers because,
depending on where you enter the cycle, some template's declaration
will not be seen before it is used.  For example, entering at
is_scalar.hpp (which happens when you include optional.hpp) causes the
definition of is_class<> to be seen before the declaration of
is_scalar<>, but the former uses the later.

The problem was introduced in the change from 1.32 to 1.33, when
is_abstract.hpp started being #included by is_convertible.hpp.

For reasons I still don't fully understand, this doesn't always
actually cause a problem.  I think it has to do with the mysterious
BOOST_NO_IS_ABSTRACT macro; my understanding of the intricacies of
Boost configuration is very limited.  The place I ran into this was
using an EDG-based front end to parse the preprocessed output of Sun
CC 5.7 on solaris-sparc; input is "#include <boost/optional.hpp>";
using Boost 1.33.0 or 1.33.1.  I can't repro on linux or with gcc, nor
with the latest CVS version of Boost.

But regardless, the cycle is still present in the latest CVS, and
cyclic header dependencies are usually a bug.

-Scott