Re: [boost] [units] Compilation fails on quantity copy-construction

Subject: Re: [boost] [units] Compilation fails on quantity copy-construction
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2017-05-31 23:00:50

AMDG

On 05/31/2017 01:09 PM, Pavel Kretov via Boost wrote:
> <snip>
> 
> The code looks okay to me, but when I try to compile it, I get a scary
> error message:
> 
>     main.cpp:20:19: error: no match for ‘operator/’ (operand types
>     are ‘float’ and ‘<THE_TERROR_THAT_FLAPS_IN_THE_NIGHT>’)
> 
>          dt = 0.5f / (v * bu::sqrt(1.0f/(dx*dx) + 1.0f/(dy*dy)));
> <snip>
> In the above expression [{}, {}, ...] is a pseudo-code for static
> head-tail list of boost::units::dim<Unit, Rat> pairs. Note the
> 'double' type materialized from barely nowhere: the code explicitly
> defines everything as 'float', it's the 'bu::sqrt' to blame for it to
> appear. Note that replacing 'bu::sqrt' with 'bu::pow<1, 2>()' does
> not help.
> 

That's because bu::sqrt uses the same type
deduction at bu::pow.  The correct result
type should use decltype(sqrt(x.value()))

> As I didn't feel happy with the above error message, I mutated the
> code a bit and made it to work with:
> 
>     bu::quantity<si::time, float> dt;
>     dt = dimless(0.5f) / (v * bu::sqrt(1.0f/(dx*dx) + 1.0f/(dy*dy)));
> 
> I had to both (a) add 'dimless',

  The reason for this is that the generic operator/
which we used originally was causing ambiguities
for some user-defined value types.

> and (b) separate variable declaration
> and definition. Doing only (a) didn't help.
> 

  You're using copy-initialization which
requires an implicit conversion.  Try:
bu::quanity<...> dt{...};

> The questions:
>   1. Is it possible to work around the problem without custom 'sqrt'?
>   2. Is the described behavior intentional, -or-
>      Is there an error in Boost.Units' implementation of 'sqrt'?

Both sqrt and pow look wrong.

>   3. If the 'boost::unit::sqrt' is fine, is there something wrong with
>      my implementation?
> 

In Christ,
Steven Watanabe