Re: XML DOM Binding for Ada 95 - matter of style

["Matthew Heaney" <matthewjheaney@earthlink.net>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:egd1jvc6pe888e4ug683tis6a5q5ua12ml@4ax.com...
> On Tue, 5 Aug 2003 13:34:09 +0000 (UTC), Georg Bauhaus
> <sb463ba@d2-hrz.uni-duisburg.de> wrote:
>
> >Actually, you can pass operations of a tagged type as actuals.
>
> Yes, this is a great disadvantage of generics. You have to explicitly
> specify everything you need.

This is a feature, not a flaw.  The Ada generics were deliberately designed
this way, because you get better error checking.


> Compare it with inheritance, you just
> specify a base type.  This is because Ada's generics try to pretend to
> have some contract.

I don't know what you mean by "pretend" to have a contract: Ada generics do
have a contract.  In fact it's called the "generic contract model."

> C++ does not, it openly says that what you become
> is your own business.

Well, yes and no.  You don't just design ex nihilo, but with the expection
that the generic actual type will have a specific set of operations, even if
those operations are only loosely specified.

> template <class Element>void Sort ...;
>    // Guess, what I have used here this week, "<=" or ">="?

No.  Sort will only use operator<.  It will never use operator<= or
operator>=, because those can be derived from the canonical operator<.

You should always define relational operators in terms of the less-than
operatior:

function "<" (L, R : T) return Boolean is ...;

function ">" (L, R : T) return Boolean is
begin
   return R < L;
end;

function ">=" (L, R : T) return Boolean is
begin
   return not (L < R);
end;

function ">=" (L, R : T) return Boolean is
begin
   return R <= L;
end;

In the STL and in Charles, you even use the less-then operator for computing
"equivalence" of keys:

function Is_Equivalent (L, R : T) return Boolean is
begin
   return not (L < R) and not (R < L);
end;

The equality operator "=" for elements is only used to compute the value of
container equality.


> >Can this be done with virtual member functions in C++?
>
> You mean to pass a dispatching operation to a template?

I showed how to do this in Ada in a previous post.