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Subject: Re: [Boost-users] deleting operators to forbid mixed arithmetic? [multiprecision]
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2017-08-12 12:42:24

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



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.


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 A. Bristow
Prizet Farmhouse
Kendal UK LA8 8AB
+44 (0) 1539 561830

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