Re: generics in Ada 83

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"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