Re: [boost] Checked Integral Class

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-09-15 04:56:25

----Original Message----
From: Stephen Gross [mailto:sgross_at_[hidden]]
Sent: 14 September 2005 18:19
To: boost_at_[hidden]
Subject: Re: [boost] Checked Integral Class

>> 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. 
>> 
>> I have built a C++ template class that does the same thing:
>> 
>> template<typename T, T min, T max>
>> struct CheckedIntegralValue
>> 
>> To define a type that can hold the same range as the example above:
>> 
>> typedef CheckedIntegralValue<int, -10, 10> SmallIntType;
>> 
>> SmallIntType i   = -10;//OK
>> SmallIntType i2 = -100;//Will throw an exception at run-time for
>> value out-of-range 
>> 
>> I won't include the whole thing here, but I can do so if there is
>> enough interest. I have defined most of the operators one needs to
>> use this type just as one would use a 'normal' integer.
>> 
>> Would anyone be interested in something like this in the Boost
>> libraries? 
> 
> Since the assignment check is done at runtime (as it has to be), why
> make 
> the range restriction at compile time? Isn't it more useful to have a
> class that works more like this:
> 
> === CODE ===
> CheckedIntegralValue x;
> x.setLowerLimit(-5);
> x.setUpperLimit(1000);
> x = 10;
> x = 9000; // Fails at runtime
> ===========
> 
> Does this make sense?

Because:
a) You often have a conceptual type whose limits are known at compile time.
Different conceptual types in your program will often have different limits,
so you get better typesafety almost for free.

b) If you carry the limits in the objects, sizeof(CheckedIntegralValue) >=
3*sizeof(int); if you make them compile time, sizeof(CheckedIntegralValue)
>= sizeof(int).  Structures which fit in an machine word can more easily be
passed in registers.

c) By making the limits compile time constants, if you assign an in-range
constant the optimizer can eliminate the test entirely.

-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]