Re: [boost] Re: static sized strings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-05-12 14:20:30

Rob Stewart wrote:
>From: John Nagle <nagle_at_[hidden]>
> >
> >    STL strings are variable-sized, with no limit.  When you
> > add characters, "size()" increases.  Should fixed-size
>                                                     ^^^^
>I presume you're referring to fixed-capacity strings which can
>have a variable size, right?

yup. The idea is to make it a buffer-overflow safe replacement for C style 
character buffers, e.g.:

   char buf[ 100 ];
   ::sprintf( buf, "...", ... );

while allowing it to behave like a basic_string.

> > strings be similar?  If the STL string operations are
> > to be meaningful, they have to be.
>
>Only if you want to apply mutating algorithms that modify size.
>IOW, it isn't unreasonable to choose either fixed-size or
>fixed-capacity strings.  If the former, then there are no
>mutating algorithms that affect the size.  If the latter, the
>size can change, but an exception occurs when trying to exceed
>the capacity.

That's the way I see it, but instead of throwing an exception, the class 
simply prevents overrun and fills the available space, e.g.:

   char_string< 5 > buf;
   buf.copy( "Meine Grosse Welt!" );
   std::cout << buf << '\n'; // prints "Mein" (null terminated)

> >    In STL strings, "size" doesn't include a trailing null.
> > Using "c_str" can result in expanding the string to add a
> > trailing null.  With fixed-size strings, we don't have that
> > option.
> >
> >    If c_str is provided, there must always be space
> > for a trailing null.  So you can't use char_string
> > when you need a specific fixed length, as for Macintosh
> > 4-character file signatures.  Perhaps "boost::array<char>"
> > is the way to go for that sort of thing.  Trying to do
> > both null-terminated and non-null-terminated strings
> > in the same class will get ugly.
>
>If the string if fixed-capacity *and* there remains sufficient
>capacity, c_str() can null terminate the buffer.  If there isn't
>sufficient remaining capacity, then throw an exception.
>
>IOW, make it a runtime error to fix the capacity too small to
>permit null termination when calling c_str().  That still leaves
>room for things like the 4-character file signature to which you
>referred, and yet prevents buffer overrun, but doesn't require
>foregoing flexibility.

That is a good idea. It will mean keeping track of the string length, but it 
does permit both versions without requiring complex internals/semasntics or 
seperate classes.

In order to keep interoperability with C style usage (i.e. operator const 
char *()), we can have the following:

   inline const char_type * c_str()
   {
      // null-terninate buffer: str[ len ] = char_type( '\0' );
      return( str )
   }
   inilne operator const char *()
   {
      return( c_str());
   }

Regards,
Reece

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