Re: [Multiprecision] number conversion to __int128

From: A.Dmitrovsky (tdiff_at_[hidden])
Date: 2020-05-18 16:32:03

Hi,

Strictly speaking, if I get it correct, that passage is about compatibility between different boost::multiprecision types (e.g. mp::int128 to mp::int256 conversion) and tells nothing about being able to convert to some other arithmetic type.
 
So that's why I am here :)
 
Regards,
Anton
 
18.05.2020, 16:51, "Paul A. Bristow via Boost-users" <[e-mail]>:
> From: Boost-users <[e-mail]> On Behalf Of A.Dmitrovsky via Boost-users
> Sent: 15 May 2020 11:03
> To: [e-mail]
> Cc: A.Dmitrovsky <[e-mail]>
> Subject: [Boost-users] [Multiprecision] number conversion to __int128
>
> Hi,
>
> Boost::multiprecision docs [1] mentions that the type parameter of number::convert_to method should be "fundamental", pointing to the list of standard types ('A number can be converted to any https://en.cppreference.com/w/cpp/language/types type)'.
> This is different to number::convert_to reference [2], which states that "Type T may be any arithmetic type".
>
> Can [unsigned] __int128 be treated as "fundamental" on compilers/platforms which support it?
>
> [1] https://www.boost.org/doc/libs/1_73_0/libs/multiprecision/doc/html/boost_multiprecision/tut/conversions.html
> [2] https://www.boost.org/doc/libs/1_73_0/libs/multiprecision/doc/html/boost_multiprecision/ref/number.html
>
> Does
>
> “
>   *Interconversions between number types of the same family are allowed and are implicit conversions if no loss of precision is involved, and explicit if it is:
>   …
>
> "
>
> In https://www.boost.org/doc/libs/1_73_0/libs/multiprecision/doc/html/boost_multiprecision/tut/conversions.html
>
> Not cover your case _uint128 ? and thus allowed for conversion.
>
> Even if the _uint* and _int* types are not fundamental builtin types (regrettably - blame the glacial process of C and C++ standardization and the millstone of C (don't ask) - but they might as well be for most purposes, including this one).
>
> HTH
>
> Paul
>
>
>
>
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