Document Number: WG21/N0859 = X3J16/96-0041
                             Date:            30 Jan 1996
                             Project:         C++ Standard Library
                             Reply to:        Nathan Myers
                                              <ncm@cantrip.org>
 
 
Locale codecvt<> Shift-state Predicate
--------------------------------------
 
The Standard C Library function mblen() (found in <stdlib.h>), when
called with a magic combination of argument values, reveals whether
the character set encoding has "state-dependent encodings".  The C++
locale facilities currently provide no corresponding semantics.
 
Filebuf needs this information for uses that include helping determine
whether seeking is possible on a sequence using the encoding. The
simplest way to provide this information is by adding a predicate
member to codecvt<>.
 
 
Proposed Resolution
-------------------
 
Add to [lib.locale.codecvt] (22.2.1.5), [lib.locale.codecvt.members]
(22.2.1.5.1), and [lib.locale.codecvt.byname] (22.2.1.6) the following
members:
 
  protected:
    virtual bool do_has_locking_shift() const throw();
 
    Returns: true if the encoding applied to *from* sequences has
      state-dependent encodings; false otherwise.
 
  public:
    bool has_locking_shift() const throw();
 
    Returns: do_has_locking_shift()