RE: [asio-users] ..._buffer_container_1

[Alterman, E. <Eugene.Alterman@Staples.com>]

Hi Chris,
Thank you for reply.


> -----Original Message-----
> From: Christopher Kohlhoff [mailto:ch...@ko...] 
> Sent: Monday, November 21, 2005 8:09 PM
> To: Alterman, Eugene; 'asi...@li...'
> Subject: Re: [asio-users] ..._buffer_container_1
> 
> 
> Hi Eugene,
> 
> --- "Alterman, Eugene" <Eugene.Alterman@Staples.com> wrote:
> 
> > I think it is better to have (mutable_/const_)buffer_conteiner_1
> > derived
> > from boost::array<..._buffer, 1>. Then the only thing you need to do 
> > is to add construction from a buffer:
> > 
> > class mutable_buffer_container_1
> >   : public boost::array<mutable_buffer, 1>
> > {
> > public:	
> >  /// Construct to represent a single modifiable buffer.
> >   explicit mutable_buffer_container_1(const mutable_buffer& b)
> >   {
> >     *begin() = b;
> >   }
> > };
> 
> The problem was that I also wanted the return value of asio::buffer() to
be
> assignable directly into a const_buffer or mutable_buffer, e.g.:
> 
>   asio::const_buffer b = asio::buffer(data, size);
> 
> or
> 
>   asio::mutable_buffer b = asio::buffer(data, size);
> 
> Furthermore, in the case of asio::buffer() returning a
> mutable_buffer_container_1, I want the "conversion" to mutable_buffer to
be
> a better conversion than to const_buffer.
> 
> Otherwise you get ambiguities with code like:
> 
> void foo(const mutable_buffer&);
> void foo(const const_buffer&);
> 
> char data[1024];
> foo(asio::buffer(data));
> 
> Using the inheritance here was the only way I could find to make this the
> "better" conversion. I'd be happy to know about alternative ways of doing
> it.

I see.
I think you can use Andrei Alexandrescu's mojo trick:
Have mutable_buffer derived from const_buffer, and in
mutable_buffer_container_1 declare operators:
  operator const_buffer() const;
  operator mutable_buffer();

This will give you the desired effect. See the discussion:
http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/
c6fe15957ab6a261


> > Aside from more code, there is another problem with the current 
> > approach - 'end()' is defined as 'begin() + 1', but you can use 
> > pointer arithmetic
> > legally only within an array.
> 
> According to the C++ standard, in section 5.7/4 which discusses pointer
> arithmetic:
> 
> "For the purposes of these operators, a pointer to a nonarray object
behaves
> the same as a pointer to the first element of an aray of length one with
the
> type of the object as its element type."
> 
> which i took to mean it was legal. I could be wrong though :)

Of course I was wrong here :(

Eugene.

P.S. Great library! Thanks.