Re: [boost] static sized strings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-06-03 13:13:27

Rob Stewart wrote:
>From: "Reece Dunn" <msclrhd_at_[hidden]>
> > Martin wrote:
> > >2. Substrings
> > >-------------
> > >One of the applications I can see for fixed_string is when you want to
> > >avoid
> > >dynamic allocation but still want to use the std::string interface and
> > >algorithms. In that perspective it seems strange that substr returns a
> > >std::string instead of a fixed_string.
>
>Agreed.

I agree too. It is just that the current implementation cannot create an 
instance of boost::char_string (because of the ABC - necessary to allow 
operations on variable-capacity strings). Also, you cannot get at the 
fixed_string type/length at compile time so it is not possible to derive the 
*return type* at compile time.

> > This is related to the above problem. You cannot create an instance of
> > boost::char_string, etc. and as such cannot implement substr in the 
>usual
> > way (this is the proplem I was having with operator+). The solution 
>would be
> > to defione an alternate substr function in fixed_string, but this would 
>not
> > solve the problem for boost::char_string, etc.
> >
> > The alternative would be to use a ranged string (pointer to the 
>fixed_string
> > buffer and length), but the problem with this is null-termination.
> >
> > I do not see an easy way around this without removing the ABC design, 
>since
> > it is not possible to do fixed_string< length() > or something similar.
>
>Couldn't substr() be a pure virtual function in char_string that
>deferred to the derived class to create a copy of itself?  The
>derived class can just create a duplicate of itself, including
>the current capacity.  That capacity is guarranteed to be
>sufficient to hold any substring requested.

The return type cannot be boost::fixed_string_base because of the whole ABC 
issue. Therefore, the return type needs to be a reference or a pointer. It 
is possible to do this, and have:

   class fixed_string
   {
      inline fixed_string_base & substr_( size_type pos, size_type n )
      {
         return( this_type( *this, pos, n ));
      }
   };

This is problematic because of returning the address of a temporary - and 
leads to unpredictable output.

I am looking for a better solution to this. I am also thinking about how to 
remove the ABC from the implementation using a technique pointed out by 
someone on this thread (can't remember who):

   class fs_base
   {
      size_t len;
      szie_t capacity; // needed for buffer-safe operations
      CharT str[ 1 ];
      // string manipulation functions

      fs_base( size_t c ): capacity( c ){}
   };

   template< size_t n >
   class fixed_string: public fs_base
   {
      CharT data[ n ];
      fixed_string(): fs_base( n ){}
   };

I'll need to look into this, to see what effect this will have, to see if it 
is a possible replacement for the ABC. What do other people think?

Regards,
Reece

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