[boost] runtime rvalue detection for Visual C++ 6.0

From: Peder Holt (peder.holt_at_[hidden])
Date: 2005-02-26 14:12:46

I was looking through the FOR_EACH - implementation, and noticed that
it lacks rvalue detection for Visual C++ 6.0.
After some investigation, I have found one very sleazy way of
detecting rvalue-ness runtime without evaluating the argument more
than once:

# define BOOST_FOREACH_TYPEOF(COL)                                    
         \
    *(true ? 0 : &::boost::for_each::wrap(COL))

template<typename T>
inline static_any<simple_variant<T const> > contain(int const&
position_test,T const &t, bool const &, mpl::false_)
{
    int rvalue_test=int(&t)-int(&position_test);
    bool rvalue=(rvalue_test<=8);

    return rvalue ? simple_variant<T const>(t) : simple_variant<T const>(&t);
}

Since this relies on the position of objects on the stack, there are
probably cases where it will fail.
Pointers (e.g. string constants) must be handled separately.

Regards
Peder