g++ seems to want a const constructor.  What's that?
====================================================

   gcc-2.7.1 introduced a bug that causes the compiler to ask for a
const constructor (there's no such thing in C++) in certain situations
where a const object appears in a template class.  Most cases have been
fixed in gcc-2.7.2, but unfortunately not all.  Still, if you're running
gcc-2.7.1 and have this problem, upgrade to 2.7.2; it is a vast
improvement.

   The default constructor for the template `pair' in ObjectSpace's
implementation of STL triggers the bug in one place, for gcc 2.7.2.  If
you're using ObjectSpace<STL> and having this problem, simply change
the default constructor from

     os_pair () : first (T1 ()), second (T2 ()) {}

   to just

     os_pair () {}

   Once this is done, ObjectSpace<STL> works fairly well.