Re: [boost] Re: static sized strings

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-05-21 10:56:16

Rob Stewart wrote:
>From: "Reece Dunn"
> > Rob Stewart wrote:
> > >From: "Reece Dunn"
> > > >
> > > > Currently, this implementation is missing iterator support (and thus 
>all
> > > > basic_string functionality that relies on begin(), end(), etc). This 
>is
> > > > because I am wondering how to map them from the basic_string adaptor 
>to
> > >the
> > > > implementation (knowing that you cannot have const and non-const 
>virtual
> > > > functions).
> > >
> > >Since when can you not have const and non-const virtual
> > >functions?
> >
> > Let me rephrase:
> >
> >    class char_string
> >    {
> >       virtual iterator begin() = 0;
> >       virtual const_iterator begin() = 0; // oops! begin already in 
>vtable!!
> >    };
>
>I'm still missing something.  Why would you want a non-const mf
>to return the const iterator type?

That was a typeo!

>    class char_string
>    {
>       virtual iterator begin() = 0;
>       virtual const_iterator begin() const = 0;
>    };

That is what I was referring to. You need to change the name of the const 
version if you are to allow virtual functions.

> > > > I have two possible solutions:
> > > > [1] name the const versions cXXX (cbegin(), crend(), etc.) -- the
> > >problem
> > > > with this is that you have 8 virtual functions!
> >
> > I was meaning to modify the above to:
> >
> >    class char_string
> >    {
> >       virtual iterator begin() = 0;
> >       virtual const_iterator cbegin() = 0; // ok - cXXX variant
> >    };
> >
> > >You'd have those same eight variations with const and non-const
> > >virtual functions.
> >
> > That's the problem -- too many virtual functions.
>
>But if you need all eight variations, then you need eight virtual
>functions.  Is it that you're trying to minimize what the derived
>class must implement for the adaptor to work?  Even if that's the

That is indeed what I am trying to do.

>case, because the const and non-const functions return different
>types, you can't have the derived class implement a const
>implementation function that the adaptor can use to provide both
>const and non-const interface functions.  So what am I missing?

Nothing. You do indeed need two functions:
   iterator iter_offset( difference_type );
   const_iterator const_iter_offset( difference_type ) const;

these can be used to implement the 8 iterator functions like this:
   begin() const = const_iter_offset( 0 );
   begin() = iter_offset( 0 );
   end() const = const_iter_offset( size());
   end() = iter_offset( size());

   rbegin() const = const_reverse_iterator( const_iter_offset( size()));
   rbegin() = reverse_iterator( iter_offset( size()));
   rend() const = const_reverse_iterator( const_iter_offset( -1 ));
   rend() = reverse_iterator( iter_offset( -1 ));

Regards,
Reece

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