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@lists.boost.org>:



     From: Boost-users <boost-users-bounces@lists.boost.org
     <mailto:boost-users-bounces@lists.boost.org>> On Behalf Of
     A.Dmitrovsky via Boost-users
     Sent: 15 May 2020 11:03
     To: boost-users@lists.boost.org <mailto:boost-users@lists.boost.org>
     Cc: A.Dmitrovsky <tdiff@yandex.ru <mailto:tdiff@yandex.ru>>
     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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