[Boost-bugs] [Boost C++ Libraries] #7660: BOOST_CODECVT_DO_LENGTH_CONST is incorrectly defined

Subject: [Boost-bugs] [Boost C++ Libraries] #7660: BOOST_CODECVT_DO_LENGTH_CONST is incorrectly defined
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-11-07 17:55:24

#7660: BOOST_CODECVT_DO_LENGTH_CONST is incorrectly defined
-------------------------------+--------------------------------------------
 Reporter:  gromer@…           |       Owner:  bemandawes
     Type:  Bugs               |      Status:  new       
Milestone:  To Be Determined   |   Component:  filesystem
  Version:  Boost 1.52.0       |    Severity:  Problem   
 Keywords:                     |  
-------------------------------+--------------------------------------------
 Here's a simple way to see the problem:

 {{{
 $ gcc -c -I . libs/filesystem/src/utf8_codecvt_facet.cpp -Woverloaded-
 virtual
 In file included from ./boost/detail/utf8_codecvt_facet.ipp:13,
                  from libs/filesystem/src/utf8_codecvt_facet.cpp:23:
 /usr/include/c++/4.4/bits/codecvt.h:438: warning: ‘virtual int
 std::codecvt<wchar_t, char, __mbstate_t>::do_length(mbstate_t&, const
 char*, const char*, size_t) const’ was hidden
 ./boost/detail/utf8_codecvt_facet.hpp:171: warning:   by ‘virtual int
 boost::filesystem::detail::utf8_codecvt_facet::do_length(const mbstate_t&,
 const char*, const char*, size_t) const’
 }}}

 The problem is that the first parameter of do_length has been const-
 qualified in the derived class, whereas it is not in the base class.
 Apparently standard libraries vary in whether do_length's first parameter
 is const-qualified, because there's already a
 BOOST_CODECVT_DO_LENGTH_CONST macro controlling whether the const is
 applied. So a shallow fix would be to add libstdc++ to the list of
 libraries for which the const is disabled.

 However, the deeper problem is that the macro is defined backwards: it
 applies the const unless it detects one of a list of excluded standard
 libraries. However, the C++ standard is quite clear (in both '03 and '11)
 that this parameter should ''not'' be const-qualified. It seems far more
 practical for the macro definition to enumerate the libraries which are
 known to ''deviate'' from the standard (by const-qualifying that
 parameter), not the ones which are known to ''comply'' with it.

 See also [https://svn.boost.org/trac/boost/changeset/26758], which patches
 the same 'problem' with libc++; I note with surprise that this changeset
 is 21 months old, but is still not live in a released version of Boost.

 I've filed this in the filesystem component because boost/detail doesn't
 seem to have its own component; it could equally well be filed against
 program_options.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7660>
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