From: Maciej Sobczak <no.spam@no.spam.com>
Subject: Re: Type safety, C++ and code generation
Date: Fri, 28 Apr 2006 08:17:00 +0200
Date: 2006-04-28T08:17:00+02:00 [thread overview]
Message-ID: <e2sc0r$8eq$1@sunnews.cern.ch> (raw)
In-Reply-To: <1146153904.898288.238600@y43g2000cwc.googlegroups.com>
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.
As I've said, I dropped this idea (using templates and metaprogramming
techniques as a basis for building safer type system - this is what I
mean by "done that"), because for me it doesn't scale.
>>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;
Of course, but this requires increased involvement of the user. Above,
it is necessary to define two things to achieve what is conceptually
only one goal. This is one of the limiting factors of this approach - it
quickly "saturates" and becomes a maintenance nighmare for both the
library writer and its users.
I akcnowledge that either the language has to inherently support this
kind of stuff (like Ada does), or it's better to step *outside* of the
language and use metamodels and some generation techniques.
>>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.
Which is the "negative logic" (see my answer to Georg Bauhaus) and it
also creates additional entity (the derived class) for reasons that have
nothing to do with the original design. What about the base class, which
still supports the unwanted operations?
What about this:
Velocity v;
Duration t;
Distance d = v * t; // OK
Distance d = v + t; // not OK
Now, the operation involves three types. Derivation and messing with
private specifier is not a very scalable solution.
>>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.
Fine (and I've "done that").
And now, with all this traits-and-derivation-and-tagging-and-what-not,
is it easy for the user to understand the typical compiler error message?
--
Maciej Sobczak : http://www.msobczak.com/
Programming : http://www.msobczak.com/prog/
next prev parent reply other threads:[~2006-04-28 6:17 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-27 10:35 Type safety, C++ and code generation Maciej Sobczak
2006-04-27 13:19 ` REH
2006-04-27 14:08 ` Maciej Sobczak
2006-04-27 14:33 ` REH
2006-04-27 15:16 ` Maciej Sobczak
2006-04-27 15:39 ` Georg Bauhaus
2006-04-28 5:57 ` Maciej Sobczak
2006-04-28 12:16 ` Georg Bauhaus
2006-04-28 12:37 ` brian.b.mcguinness
2006-04-28 17:04 ` Georg Bauhaus
2006-04-28 18:50 ` Jeffrey R. Carter
2006-04-28 19:07 ` Randy Brukardt
2006-04-28 19:46 ` Robert A Duff
2006-04-28 19:55 ` Dmitry A. Kazakov
2006-04-29 21:28 ` Florian Weimer
2006-04-27 16:05 ` REH
2006-04-28 6:17 ` Maciej Sobczak [this message]
2006-04-30 14:40 ` adaworks
2006-04-30 15:23 ` REH
2006-04-30 18:14 ` Martin Krischik
2006-05-02 6:27 ` Maciej Sobczak
2006-05-02 13:33 ` REH
2006-05-02 15:05 ` Maciej Sobczak
2006-05-02 15:54 ` REH
2006-05-02 16:50 ` Martin Krischik
2006-05-02 19:50 ` REH
2006-04-27 14:47 ` REH
2006-04-27 15:10 ` REH
2006-04-28 6:22 ` Maciej Sobczak
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