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From: "REH" <spamjunk@stny.rr.com>
Newsgroups: comp.lang.ada
Subject: Re: Type safety, C++ and code generation
Date: 27 Apr 2006 09:05:04 -0700
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Date: 2006-04-27T09:05:04-07:00
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Maciej Sobczak wrote:
> REH wrote:
>
> > I don't see where you've "done that."
>
> The template class that implements range checking?

NO, a class that uses template to ELIMINATE unnecessary checks.

> What about making different types really distinct?
>
> typedef ranged_type<int, 0, 100> R1;
> typedef ranged_type<int, 0, 100> R2;
> typedef ranged_type<int, 0, 101> R3;

My code uses this technique if you want truely unqiue types:

class R1_unique{};
class R2_unique{};
typedef ranged_type<int, 0, 100, R1_unique> R1;
typedef ranged_type<int, 0, 100, R2_unique> R2;


>
> Another problem is that the range, as a set of values, is not the only
> thing that constitues a type - you also need to define the set of legal
> operations. The problem is that I usually want them to be different in
> each type.
> Consider this:
>
> type ranged_type<int, 0, 250> Speed;
> Speed s1, s2, s3; // with some values
> s1 = s2 + s3; // OK
> s1 = s2 * s3; // not OK
>
> The addition is fine, but the multiplication should not be provided,
> because speed multiplied by speed is not a speed. Can you extend your
> class so that the compiler will refuse to compile the second operation
> above?

Yes, you derive from the class and put the multiplication operator in
the private scope.

> Another problem is variation of the behaviour in the out-of-range
> condition. What should happen then? Throw an exception? That's only one
> of at least four different options I can imagine, and also not the one I
> would choose most of the time. Does your class allow variations here?

Yes, the template takes a traits class.  If allow modification of
various behaviors, such as what should be done with an out-of-range
value, an overflow condition, a divide-by-zero, etc.

>
> And so on - all this *can* be done with templates, but at some point
> this approach just saturates and the code becomes a nightmare. Not
> mentioning the error messages that you get from the compiler (and the
> whole purpose of this is to actually get *useful* error messages,
> right?). And not mentioning the compilation time.

I've heard all these things.  That may have been true with older
compilers.  They are a lot better today.  

REH