Re: [boost] Checked Integral Class

From: Maciej Sobczak (prog_at_[hidden])
Date: 2005-09-14 16:09:11

Hi,

Dan McLeran wrote:
> I've written a template class to hold a range of valid integral values. My 
> intent was to mimic Ada's ability to define a type like this:
> 
> type SmallInt is range -10 .. 10;
> 
> One can then declare objects of this type and any subsequent assignment that 
> violated this range constraint woud throw an exception.

Careful with that. This approach, in my humble opinion, is very limited. 
Why do you assume that exception is The Right Thing when the out of 
bound condition is detected?

These are things that I would like to have, *depending on situation*:

1. throw some exception (this is what you have)

2. abort immediately without doing *anything* (note that exception 
causes some code to be executed, like destructors of automatic objects - 
I might want to avoid this and just kill the program immediately)

3. wrap the result (kind of modulo arithmetic, but more general because 
allowing to have the lower bound different than 0 - say that if the 
range is 10..20 then 23 should be wrapped to be 13)

4. saturate the result (if you have the range 0..100 and the result of 
some computation is 150, cut it to 100 and continue with it) - actually, 
saturated arithmetic is very convenient, for example in signal processing

5. actually allow the out-of-range value, but log that fact somewhere or 
send me an e-mail, etc.

6. ...

I leave the list open, beacuse it is clear that the library itself 
should be customizeable in this regard.

More to this - the bare checked class is not really emulating what Ada 
supports. What about this (Ada):

type SmallInt is range -10 .. 10;
subtype EvenSmallerInt is SmallInt range -5 .. 5;

Now, you can use EvenSmallerInt wherever SmallInt is expected, because 
one is a subtype of another. Ada does not allow multiple supertypes, 
although that is a logical extension of the idea, quite useful in my 
opinion and worth to have.

You will find a very sketchy and incomplete implementation of all these 
ideas at:

<http://www.msobczak.com/prog/bin/safetypes.tar.gz> (3kB)

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/