Re: [boost] feature request for optional

From: Sascha Krissler (boost-dev_at_[hidden])
Date: 2006-08-07 22:59:04

On Mon, Aug 07, 2006 at 10:11:53PM -0400, David Abrahams wrote:
> Sascha Krissler <boost-dev_at_[hidden]> writes:
> 
> > That's what i am doing right now.
> > The messy interface differences could be smoothed out by optional<>.
> 
> It's not optional's job to make that decision.  0 is a valid pointer
> value, and it's not the same thing as "no pointer value."

The difference is that in
optional<foo *>
NULL is a valid pointer value

In
struct my_discriminator { };
optional<foo *, my_discriminator>
optional<foo *, my_discriminator>::null_value = NULL; // static member
NULL is not a valid pointer value (useful for std::type_info *, for
example, or for a -1 integer return to indicate error)

> 
> > If i had to implement all the interface of OptionalPointee again
> > that would be a lot of duplication whether changing optional would
> > be a couple of mpl::if_.  Well i will not wait for a new release of
> > boost to solve my poblem anyway it is a suggestion to improve the
> > genericity of the lib.
> 
> Unfortunately, it would reduce genericity.  Suddenly some types have
> special values that can't be represented distinctly from an empty
> optional.
> 
with increased genericity i mean: make optional a concept, rather than
(or in addition to) a class