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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: "Must instantiate controlled types at library level." Why?
Date: Mon, 17 May 2004 16:31:10 +0000 (UTC)
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Date: 2004-05-17T16:31:10+00:00
List-Id: <comp.lang.ada>

Hyman Rosen <hyrosen@mail.com> wrote:
 
: template <typename T, unsigned N> struct array {
:     T a[N];
:     T &operator[](unsigned i) { return i < N ? a[i] : throw i; }
: };
: 
: Now I can declare my array, for example, as array<double, 100>,
: and pass it around by value. I haven't used #include.

Right. And now, to the Ada fans, from a practical point of view,
how does this compare to Ada arrays?

At the C++ source level: you don't see any difference at the point of use
because operator[] looks like always.

In the compiler: I'm not sure, maybe some C++ compilers have switches
for std  [] vs .at?

But, as Dmitry says, a useful template has been built by hand from
the small things. (Lisp even gives you the lools for new syntax
(re)definitions...)  Is an approach based on discipline and
conventinalism on a par with language definitions?

Somewhat related to this, you can have template parameter constraints
like Can_This or Can_That,  as explained in
http://www.research.att.com/~bs/bs_faq2.html#constraints
But is it the same as saying

generic
  type G is new C with private;
...