RE: [boost] Re: static sized strings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-05-07 13:36:56

John Nagle wrote:
>Reece Dunn wrote:
>>John Nagle wrote:
>>
>>>Reece Dunn wrote:
>>>
>>>>There is currently a static-sized array in the Boost library that allows 
>>>>you to operate on arrays of fixed size. I was wondering if something 
>>>>similar exists for strings, in particular, providing buffer-overflow 
>>>>safe string operations.
>>>>
>>>>I have an nstring< std::size_t n > string class that provides size-safe 
>>>>copying and comparison, allowing for you to do things like:
>
>OK, thanks.  First bug reports:
>
>1.  Compile problems under VC++ 6:
>	No include brings in "std::size_t".

Fixed.

>2.  VC++ 6.x complains about references to a zero-sized array for
>    [edit]: copy( const char( & s )[ m ] )

I have disabled these for VC6 since they are causing problems with the 
compiler, even with Thorsten Ottersen's workaround :(.

>3.  "copy" function does not place a trailing null in the string.
>[snip]
>    Note that "strlen" returns a count that does NOT contain the null.

I have added this facility as a template parameter (bool null_terminate). 
The logic behind this is when you do not specifically need a null-terminated 
string, e.g.:

   struct JPEGHeader
   {
      // ...
      boost::string::char_string< 4 > magic;
      // ...
   } hdr;

   if( hdr.magic != "JPEG" ) error( "invalid format" );

but also to allow it if you need that security.

>    All the operations should guarantee that the string remains null
>terminated.  A constructor should be provided, but all it has to
>do is put a null in the first character position.

I have already provided such a constructor :).

>    As for the naming issue, the important thing for retrofit work
>is that it should be possible to write a "using" statement that makes
>"strcopy", "sprintf", for char_string etc. valid without prefixes, and
>doesn't break anything else.  You should be able to include something
>("safe_strings.hpp"?) that does as much as possible to fix old code.

I have moved the char_string class into char_string.hpp and started work on 
providing safe versions of ::strXXX. It is not possible to use the same 
names (strlen, etc.) for the char_string versions :( as can be seen in the 
example below.

   #include <iostream>
   #include <cstring>

   namespace boost { namespace string
   {
      using ::strcpy;
      inline char * strcpy( char * d, const char * s, size_t n )
      {
         return( ::strncpy( d, s, n ));
      }
   }}

   int main()
   {
      char                                str[ 100 ];
      using boost::string::strcpy;

      ::strcpy( str, "Meine Welt!" );
      std::cout << str << '\n';

      ::strcpy( str, "1234567890", 5 ); // error
      std::cout << str << '\n';

      return( 0 );
   }

Because of this, I have decided to use the c prefix (e.g. cstrlen). This 
seems the best solution, but if the c prefix is problematic, let me know and 
I'll change it.

>This is a good start, and not hard to fix.  I look forward to the
>next round.

I have included this round as attachments, but will place the next version 
in the boost sandbox. It has the signature:

   template< size_t n, bool null_terminate = true, typename CharT = char, 
class StringPolicy = std::char_traits< CharT > >
   class boost::string::char_string;

and is the initial move towards a basic_string-like interface. How about the 
name fixed_string?

Regards,
Reece

_________________________________________________________________
It's fast, it's easy and it's free. Get MSN Messenger today! 
http://www.msn.co.uk/messenger