Re: Seeking Endorsement: Boost.int128

Matt Borland via Boost <[email protected]> Tue, 28 Apr 2026 18:59:17 +0000

Newsgroups	gmane.comp.lib.boost.devel
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On Tuesday, April 28th, 2026 at 2:04 PM, Peter Dimov <[email protected]> wrote:

> Matt Borland wrote:
> >
> > > How does this relate to / interact with __(u)int128_t?
> >
> >
> > If __(u)int128_t exists, then all operators between those types and the
> > boost.int128 types are defined: construction, conversion, add, sub, mul, div,
> > etc. Same case if you have MSVC's std::_Unsigned128 or std::_Signed128 as
> > the "builtin" 128-bit type. The alignment of the boost.int128 types is also set
> > to match the builtin types when they exist. Internally, the boost.int128 types
> > are a struct of two 64-bit integers regardless of the platform.
> 

> What do the operations actually do, though? Does adding boost::uint128,
> when __uint128_t exists, use __uint128_t addition, or does it continue to
> use the library implementation?
> 

> Similarly, for mixed mode; what does adding boost::uint128 and __uint128_t
> do under the hood? (And what is the return type? I assume boost::uint128?)
> 


In all cases the return type is a boost::uint128 for consistency. Yes, this is typically implemented by casting both types to builtin and then casting the result back to uint128.

Matt

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