Re: c++ "with" keyword

[Gianni Mariani <gmariani at chaincast dot com>]

Robert Dewar wrote:

> > Any language construct that fails in this regard (like with) is very bad 
> > - period.
> >    
> But look carefully at what you are saying. We are talking about a structure
> or class whose members are visible to the client of the library. The change
> we are talking about is adding a new member to the structure. You cannot
> possibly think that such a change can be done in a guaranteed 100%
> upwards compatible manner. That's absurd.
>
> (for example, at the very least the size of an instance will change).
>
> Furthermore, the situations under which adding this member will cause
> specifically a difficulty with WITH are very marginal. It is indeed
> far more likely that the addition of a member will cause other compatibility
> differences.
>
> If you disagree, give a specific realistic example of what you are talking
> about. I certainly can't think of one!
You talk about binary compatability.  I'm more concerned about source 
compatability however there are issues with binary compatability as well 
- in the case, use of enums or typedefs in a class may cause binary 
issues.  Think a little harder, they are there.

>  
> > Again, our perspectives on what is important is different, as a library 
> > maintainer, I see this as much more important an issue than you do.
> >
> >
> >    
> Again, remember we are talking not about the writer of the library but the
> user. 
Hey - I'm talking about both - in any case, almost moderately sized 
project will have libraries and in a software development house, you 
want source level compatability.  The distinction of library producer 
and consumer is mute.

> Suppose the user obtains an offset of a member of the structure, and
> you add a new member. Well then the offset changes, and that could cause
> arbitrary chaos of a really difficult nature. Would you use this to argue
> that it should be impossible to obtain the offset of a structure member.
> Of course not. 
> If a user uses WITH on one of your structs, then it is still extremely
> unlikely that your change will cause trouble.
>
> I really think that you need to give an example. Talking in generalities
> is always misleading. If you think something is dangerous don't just go
> around sounding red alert. Instead give a specific technical example :-)
>  
Robert, we have different perspectives on the same problem.  Obviously 
issues I see as critical, you see as mute.

I've given examples.  They still stand.

G