From: Boost-users [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Dani Brake via Boost-users
Sent: 12 August 2017 00:54
To: boost-users@lists.boost.org
Cc: Dani Brake
Subject: [Boost-users] deleting operators to forbid mixed arithmetic? [multiprecision]

Hi,

I'd like to forbid multiplication of Boost.Multiprecision floating point numbers by doubles.  Any tips?  I think I want to delete a family of operator*s, but am having a hard time coming up with which ones.

Thanks,

Dani Brake

University of Wisconsin Eau Claire

Been there, done that – fallen into the big pit awaiting the unwary mixing double and multiprecision – again and again… L

I think that you don’t really want to delete a family of operator*s – what you DO want to get a warning when there is loss of significance, similar to the warning when you fit a double into a float.

It’s OK to multiply (or other ops) with a double as long as this doesn’t lose any bits or digits of precision.

So multiplying by 0.5 is OK, but not by multiplying 0.1 (the multiprecision type needs FP_type(1)/10 to represent 0.1 as accurately as possible for the FP type).

If only the compiler provided a trait bool is_exactly_representable(double) so that a warning could be issued…

The compiler knows …

Paul

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Paul A. Bristow

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