Re: Ada to C++ Translators

[Brian Rogoff <bpr@shell5.ba.best.com>]

On 21 Feb 1998, Esa Pulkkinen wrote:
> [Sorry this is a bit off-topic for comp.lang.ada, so I'll crosspost
and
> put followups to comp.lang.c++.moderated.]

I think it is perfectly appropriate for comp.lang.ada, as long as
language
warriors restrain themselves.

> Brian Rogoff <bpr@shell5.ba.best.com> writes at comp.lang.ada:
> > I'd be interested in hearing how a well designed Ada program that
makes
> > use of constrained genericity gets translated to a well designed C++
> > program. I haven't been following the C++ standard for a while, but
I
> > thought that there was no constrained genericity in C++, and that
the 
> > work-arounds would not really be considered "well-designed".
> 
> See http://www.cs.tut.fi/~esap/instructive/ for one method for
> converting a program which uses constrained genericity to C++ using
> class template partial specialization. It uses class template partial
> specialization to implement different versions of the class for the
case
> where the class is derived from another class, and for the case where
it
> isn't. I think it's as well-designed as it ever will without explicit
> language support, but I'd like to hear your opinions.

Very interesting; the workaround I am familiar with is the one where you

do an assignment to the constraint type in some method. I'll certainly
try your method out next time I have a capable C++ compiler. Still, I
have
some issues, namely 

(1) What if the generic parameters being constrained are not 
    tagged types/classes? In the case of Ada, there are generic
    constraints to match all of the types in the language. This is 
    not such a big deal in C++, since you don't have ranges, but it 
    surely does make such a translation harder. 

(2) Function and procedure parameters. I assume you'd just wrap these up
    in classes, right? That's a bit tedious.

(3) Formal package parameters. This is my favorite "gee-whiz" feature of

    Ada 95. It's related to (1) and (2) in that I can (and do!) bundle 
    up non-tagged generic abstractions (like Stacks and Dictionaries
say)
    and then use instantiations of both with their actual parameters 
    constrained to be the same. Once again, it seems with your
translation
    I have to first make everything a class. 

That said, I think your translation is a very neat trick. In a "feature 
restrained" program, done in a very stylized way, it would work, but I
fear that a straightforward and idiomatic Ada 95 program would not
translate to a similarly straightforward (Draft) ISO C++ program.
Almost all of my Ada code uses constrained genericity somewhere. 

-- Brian


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