Re: [boost] Re: static sized strings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-05-27 13:54:16

Rob Stewart wrote:
>From: "Reece Dunn" <msclrhd_at_[hidden]>
> > John Nagle wrote:
> > >   The big question is whether the size specified in the
> > >declaration (as in "char_string<80>") includes the
> > >trailing null.
> > >
> > I have modified the sandbox implementation to make it easy to switch 
>between
> > the two models; it is currently using the 2nd model (adding space for 
>the
> > null). One possibility is putting the choice of models as a template
> > parameter (e.g. bool need_null -- see comments in the sandbox code for 
>an
> > implementation), that way the decision is up to the programmer and not 
>the
> > library implementor.
>
>This is overkill.

I think you misunderstand what I am trying to say. See my comments below.

>The purpose of fixed_string is to replace
>arrays of characters, otherwise, std::string would be the right
>replacement, right?

correct.

>Given that assumption, look to the syntax you're replacing:
>
>    char s[] = "1234567890";
>    assert(10 == strlen(s));
>    size_t const LENGTH = 3;
>    char t[LENGTH + 1];
>    strncpy(t, "ABCEFGHIJKLMNOP", LENGTH);
>    assert(LENGTH == strlen(t));
>
>Now do the same with the new class:

The equivalent would be:

   fixed_string< 15 > s( "1234567890" ); // [1]
   assert( 10 == strlen( s )); // C-style
   assert( 10 == s.size()); // C++-style

   const size_t LENGTH = 3;
   fixed_string< LENGTH > t; // [2]
   t = "ABCEFGHIJKLMNOP";
   assert( LENGTH == strlen( t )); // C-style
   assert( LENGTH == t.length()); // alternate C++-style

[1] You need to specify the size of the buffer. You cannot declare an object 
of type fixed_string_base (as it is abstract), you can only have references 
and pointers to it so you can operate on variable-capacity strings.

[2] This is the point that we are discussing. Should this line be:

   fixed_string< LENGTH > t;

as will work with the current sandbox implementation (my suggestion #2), or 
should it be:

   fixed_string< LENGTH + 1 > t;

as in my suggestion #1. The code calculates the correct storage and capacity 
values at the top of the fixed_string class, because buffer overrun checks 
rely on these values:

   template< size_t n, typename CharT = char, class CharStringPolicy = 
std::char_traits< CharT > >
   class fixed_string: public fixed_string_base< CharT, CharStringPolicy >
   {
      BOOST_STATIC_CONSTANT( size_t, storage_c  = n + 1 ); // = needs_null ? 
n + 1 : n
      BOOST_STATIC_CONSTANT( size_t, capacity_c = n ); // = needs_null ? n : 
n - 1
      // ...
   };

Different programmers favour the different semantics, so I ask: why not 
parameterise it, providing a default behaviour.

>(I know there is no make_fixed_string() -- yet -- but such a
>facility would be appropriate.)

Er... the fixed_string constructors! When you declare the object, you should 
know the size of the buffer you are using, e.g.:

   fixed_string< 100 > data;

but when operating on them, you want to use any capacity buffers, so you 
either need a template (not good for existing code or code that needs to be 
in a static/dynamic library), so you use fixed_string_base, or 
[w]char_string if you want to save typing :). E.g.

   inline size_t strlen( const char_string & s )
   {
      return( s.length());
   }

>I think this clearer reveals that fixed_string's size parameter
>should specify the number of characters.  Remember, one can use
>boost::array to manage a fixed size, non-string buffer.  If
>buffer overrun protection is insufficient in boost::array, that
>should be fixed (or a new class should be added to Boost).  Thus,
>fixed_string can ignore that usage.

This is not what we are discussing. See above. The need_null name was to 
indicate that an extra charcter be reserved for the null, not whether to add 
a null terminator or not. Sorry for the confusion.

I have started work on documentation, but it is currently in text form. I 
will post the documentation in BoostBook format within the next day or so.

Regards,
Reece

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