Re: [Multiprecision] number conversion to __int128

From: John Maddock (jz.maddock_at_[hidden])
Date: 2020-05-18 17:08:13

On 18/05/2020 17:32, A.Dmitrovsky via Boost-users wrote:
> 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.

Yes that's correct, like I said I thought I had this fixed, but 
apparently not fixed enough :(

Can you please file a defect report on github so we don't forget?

Thanks!  John.

> So that's why I am here :)
> Regards,
> Anton
> 18.05.2020, 16:51, "Paul A. Bristow via Boost-users" 
> <boost-users_at_[hidden]>:
>
>
>
>     From: Boost-users <boost-users-bounces_at_[hidden]
>     <mailto:boost-users-bounces_at_[hidden]>> On Behalf Of
>     A.Dmitrovsky via Boost-users
>     Sent: 15 May 2020 11:03
>     To: boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>
>     Cc: A.Dmitrovsky <tdiff_at_[hidden] <mailto:tdiff_at_[hidden]>>
>     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
>
>     “
>     Â Â *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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