Hi all.

On several occasions I have needed to associate a boolean flag with a
pointer, much in the manner of the last version of auto_ptr. On most
architectures it is possible to overlay a bit on the pointer, and thus
get a [much] smaller footprint. Library implementors will most likely
implement this optimization for auto_ptr.

This prompts me to suggest that a new template class, say

template <class T> class flag_ptr {
public:
  T & operator *() const;
  T * operator->() const;
  void flag(bool);
  bool flag() const;
  ...
};

be introduced for public use. It might serve as a private base class
for auto_ptr. Some details need to be sorted out (e.g. assignment and
copy semantics) but I believe such a class would be genuinely useful.

(note that since ownership is not public in auto_ptr, one cannot kludge
flag_ptr around auto_ptr).

What do you think ?
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