Re: ptr_fun & tolower confusion

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 5 Jul 2008 00:26:33 -0700 (PDT)
Message-ID:
<2c218d2d-12d9-45a1-994e-dac3b1962d84@l64g2000hse.googlegroups.com>
On Jul 4, 10:52 pm, Greg Herlihy <gre...@mac.com> wrote:

On Jul 4, 2:34 am, Kai-Uwe Bux <jkherci...@gmx.net> wrote:


    [...]

Slightly modified from the archive:

#include <tr1/memory>
#include <cstdlib>
#include <locale>

template < typename CharT >
class to_lower {

  typedef std::ctype< CharT > char_type;

  std::tr1::shared_ptr< std::locale > the_loc_ptr;
  char_type const * the_type_ptr;

 public:

  to_lower ( std::locale const & r_loc = std::locale() )
    : the_loc_ptr ( new std::locale ( r_loc ) )
    , the_type_ptr ( &std::use_facet< char_type >( *the_loc_ptr ) )
  {}

  CharT operator() ( CharT chr ) const {
    return ( the_type_ptr->tolower( chr ) );
  }

};


TR1's shared_ptr<> class is not nearly as useful in this case
as its bind() routine. In fact, calling TR1's bind() would
eliminate the custom to_lower functor and its attendant
complexity.

After all, lowercasing a C++ string seems like it should be a
fairly straightforward task - one that should require only a
few lines of code::

    #include <iostream>
    #include <string>
    #include <algorithm>
    #include <locale>

    #include <tr1/functional>

    using std::locale;
    using std::tolower;
    using std::tr1::bind;
    using std::tr1::placeholders::_1;

    int main()
    {
        std::string s("GrEg");

        transform( s.begin(), s.end(), s.begin(),
                   bind( tolower<char>, _1, locale()));
        std::cout << s << "\n";
    }


That is, of course, the simplest solution. It hasn't been
available all that long, however, and most of us developed our
solution before bind was available. (The shared_ptr isn't
really necessary here, and even if it was, most of us had simple
implementations of shared_ptr long before it made it into TR1.)

And IMHO, there's nothing wrong with providing a general wrapped
tool (although it does lead to the mistaken belief that you can
generally use tranform for converting to lower case---in
practice, the mapping isn't one to one). And using the ctype
directly will probably be slightly faster (although I doubt that
that is an issue).

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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