Re: Callbacks Again

From: William Kempf (sirwillard_at_[hidden])
Date: 2000-11-18 11:00:18

--- In boost_at_[hidden], Jesse Jones <jejones_at_a...> wrote:
> >On Fri, 17 Nov 2000 14:49:26 -0800
> >Jesse Jones <jejones_at_a...> wrote:
> >> >> This makes it tougher to include the bool
> >> >>   conversion (how do we know if a functor has a real target or
> >> >>   not?), which I consider essential.
> >> >
> >> >The bool conversion can be handled by inclusion of a default
> >> >constructor that sets the "impl" pointer to null.
> >>
> >> Yep (of course a const void* conversion would be better than 
bool).
> >
> >Why is that? A bool conversion says "valid" or "invalid" to call.
> 
> No, a bool conversion says that I can be implcitly converted to a 
bool
> which can in turn be used used in an arithmetic expression. So, 
nonsense
> like
>    callback<void> c(Foo);
>    int sum = c + 1;
> compiles just fine. This is a big enough problem that I never use 
bool
> conversion operators.

I honestly don't see this particular example as being a problem at 
all, let alone a big one.  Who cares if the above compiles?  It won't 
hurt anything, and it may well be that the programmer intended the 
above to work just in the way that it does.  Given only that example 
it seems like we're spending too much effort on safety over 
useability.  Is there a more compelling example that shows true 
danger?

> >A const void* implies some sort of useful value (which would 
be...?).
> 
> It doesn't neccesarily imply that you can do anything useful with 
the value
> other than compare it to NULL or another void*.

Or compare it to any value that can be implicitly converted to void*, 
which includes any other pointer type.  So you can compare 
the "callback" with a function pointer with (possibly surprising) 
results.

> However the conversion
> rules make const void* much more type safe than bool's.

I've yet to see any arguments that convince me of this.  The opposite 
was decided for boost::any (though they removed the implicit 
conversion entirely, I believe, which may or may not be appropriate 
here).

Bill Kempf