Re: [boost] [Safe Numerics] floating point conversions

Subject: Re: [boost] [Safe Numerics] floating point conversions
From: Robert Ramey (ramey_at_[hidden])
Date: 2017-03-12 21:05:53

On 3/12/17 4:49 AM, John Maddock via Boost wrote:
>
> It's fairly common to see in floating point code, something like:
>
> int integer_part = my_float;
> double fractional_part = my_float - integer_part;
>
> which is of course pretty seriously unsafe :(
>
> Casting to a safe<int> to ensure there is no overflow would simplify a
> lot of error checking code.

how so?
>
> BTW for the record, Boost.Math uses itrunc/ltrunc/lltrunc for this
> operation which throws when the float is too large for the target type.
> Lots of ways to tackle this, I just wanted you to be aware that there
> are genuine use cases where float-interoperability is useful and needed.

I've never doubt this.  The main thing here is that I wanted to keep the 
scope of the library small enough to actually do.  Treatment of 
safe<float> is likely a whole 'nuther thing - perhaps even more inticate 
than dealing int, unsigned int. I thought integers would be simple, the 
project started out with just an afternoon's work.  I've long forgotten 
what I wanted/needed for.  It metasized to what it is today.

But as we see, I haven't been able to just ignore floating point.  So 
we'll have to address it at least as it interacts with integers.
>
> I guess there are several possible solutions:
>
> Policies: we have 3 levels of checks:
> * Permissive, allow all conversions from float and truncate (as long as
> the truncated value is in range).
> * Strict, only allow conversion from in-range integral values. Values
> with fractional parts throw.
> * Never: static assert on any attempt to convert from float.
>
> An alternative is to use external functions, lets say we have:
>
> template <class I, class F>
> safe<I> safe_trunc(F f);
>
> Which throws only when the truncated value is out of range. Likewise we
> would need safe_round, safe_floor and safe_ceil. Obviously a certain
> amount of "bike shed colouring" may be required on those names ;)
we're experts on that subject here a bike shed central