Re: generics in Ada 83

["REH" <spamjunk@stny.rr.com>]

Robert A Duff wrote:
> "Hyman Rosen" <hyman.rosen@gmail.com> writes:
>
> > Jean-Pierre Rosen wrote:
> > > In Ada, there is no need to check legality on the expansion of the
> > > generic. Correct me if I'm wrong, but my understanding of templates is
> > > that the legality is rechecked for each instantiation.
> >
> > Well, in Ada, instantiations check the parameters for validity
> > against whatever contract the generic specifies for them. In C++,
> > the contract is implied by the usage within the template, so yes,
> > the legality is rechecked that way. But remember that in C++
> > template instatntiation is done strictly at compile-time. As far
> > as the programmer is concerned, either an instantiation validly
> > meets the template's requirements for its parameters, or the code
> > fails to compile, and that is true for both languages (or at least
> > I think that's true for Ada).
>
> That's right.
>
> The Ada way makes it easier to understand what the contract _is_.
> And this implies that if you want to change the generic/template,
> it's easier to understand whether or not you might break any clients.
> (If you're writing a widely-used library, you might not have access to
> all existing clients, so you can't compile them to see if they break.)
> It also implies that the error messages are easier to understand,
> especially with multiple layers of templates.
>
> On the other hand, the C++ way is more powerful.
>
> - Bob

That has got to be the best argument I have ever seem for Ada generics.

Thank you,
REH