[boost] Re: Singular variants?

From: Rainer Deyke (rainerd_at_[hidden])
Date: 2003-10-11 00:08:54

David Abrahams wrote:
> That may be true, but I don't think so.  The idea of a special
> error-only singular state makes me nervous.  It means that you always
> need to be aware of every bit of code where you might be dealing with
> the singular state, and those conditions can change in ways that break
> encapsulation.  For example, if you write a function which ignores the
> possibility of the singular state, because you know you're only using
> it on "good" objects.  Then one day you find you want to use the
> function in some destructor... but that might get invoked during stack
> unwinding from an exception that throws the very object in question
> into its singular state. insidious, isn't it?

Such a function would need the following properties:

1. Passing in an arbitrary non-singular variant causes no ill side effects.

2. Calling it with a a singluar variant does cause ill side effects.

3. It is somehow useful to call this function with garbage input.

I am not convinced that such a function exists.  I'm particularily skeptical
about requirement 3.

-- 
Rainer Deyke - rainerd_at_[hidden] - http://eldwood.com