Re: Smart pointer status, directions, thoughts

From: williamkempf_at_[hidden]
Date: 2001-05-26 08:47:54

--- In boost_at_y..., "Peter Dimov" <pdimov_at_m...> wrote:
> From: <williamkempf_at_h...>
> 
> > > > C)  There are plenty of other cases where this holds true.  A
> > classic
> > > > example is std::type_info.  Basically any type that doesn't 
not
> > have
> > > > full ordering but *can* have consistent partial ordering.
> > >
> > > But std::type_info is totally ordered. :-)
> >
> > I used the wrong terminology here.  I should have said strict-weak
> > ordered.  The point still stands.
> 
> Unfortunately the point does not stand.
> 
> std::type_info is not partially ordered; it provides neither 
operator< nor
> std::less; and it cannot be put in standard containers.

Notice that when using the correct terminology I don't say "partially 
ordered".  As for the standard not providing a std::less, that's 
true, but it's likely just an oversight.  A version of std::less can 
be trivially added using typ_info::before() which is in keeping with 
how things work for pointers that are not pointing within an array.  
Adding an operator< for std::type_info would not be in keeping with 
this concept, however.  Scott Meyers defines a std::less for 
type_info in MEC++ for this very reason.  I believe the example 
*DOES* stand.

> Try to provide another example of a type that should have std::less 
but not
> operator<, having in mind 20.3.3/5 and 17.4.3.1/1.
> 
> > > Did you read the example in my original message?
> >
> > Yes, I did.
> 
> OK. Do you agree that it is a real problem? How would you solve it? 
(I mean
> the problem with serializing a shared_ptr.)

Using the same techniques I'd use if raw pointers were being used 
instead.  Seriously, the example given is a very non-trivial example 
and knowing if an object has been previously serialized is not going 
to be enough info to persist the containing object any way.  In a 
case like this you'll have to maintain more info in  
seperate "database" any way, so I'd rely on it for everything.  I 
don't find the example a compelling example for the need to expose pn.

Bill Kempf