Re: [Boost-users] [multiprecision] static_cast<cpp_bin_float_100> gives strange values.

Subject: Re: [Boost-users] [multiprecision] static_cast<cpp_bin_float_100> gives strange values.
From: John Maddock (jz.maddock_at_[hidden])
Date: 2018-04-20 17:02:17

On 20/04/2018 16:44, Vögl, Christian via Boost-users wrote:
> Hi everyone.
> The following Situation:
>
> I tried to improve the accuracy of a numerical function by replacing 
> all double variables with cpp_bin_float_100.
> The parameters of this function contain two 3-element double arrays, 
> which I convert to cpp_bin_float_100 by the following function:
>
> static void equal(cpp_bin_float_100* a, double* b)
> {
>
> Â Â Â Â for(int i=0; i<3;i++)
> Â Â Â Â {
> Â Â Â Â Â Â Â  a[i]=static_cast<cpp_bin_float_100>(b[i]);
> Â Â Â Â }
> }
>
> the third element seems to be converted correctly, the first and 
> second are totally off. (e. g. 100 -> 1.5625)

Well that's strange, I suspect that something else is going on here, and 
I would need a self-contained test case to investigate.

BTW, the static_cast is unnecessary in this situation, and creates a 
not-needed cpp_bin_float_100 temporary (narrow-to-wider conversions are 
always implicit in the library).

Best, John.

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