C++ Logo


Advanced search

Re: [std-proposals] 128-bit integers

From: Nikolay Mihaylov <nmmm_at_[hidden]>
Date: Sun, 11 Feb 2024 14:43:55 +0200
The correct form is std::to_string(std::size_t).
on 32 bit machines, it is uint32_t:

std::to_string(std::int128_t) - this can not be done, because uint128_t
may happen to "hold" a number bigger than size_t.
such big number is not "addressable", e.g. you can not make an array with
more than size_t elements - this sounds strange, but on 32 bit machine, if
you do array of UINT32_MAX (not sure of correct definition in C),
then if the array is from 1 byte elements, it will "eat" all your memory.

same in apply for 64 bit machines - they can address up to UINT64_MAX bytes
of memory and such an array with just one byte elements will "eat" all its


some other thoughts - I would love to be able to print 128 bit numbers on
the screen using standard ways (std::cout, printf etc) . also to do bit
operations faster

For example I have a class that converts a 8 characters string into a 64
bit number, then comparing it as numbers instead of memcmp() derivatives
like operator <=>.
Year ago I tried to do the same with gcc uint128_t and the result was
really slow.

On Sun, Feb 11, 2024 at 2:28 PM Jan Schultke via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> > 1) Is that talking about a possible alternative of trying to add such
> extended as pure library types ...
> No, as fundamental types. The core language already permits the
> implementation to provide additional extended integers such as
> std::int128_t. However, the standard library does not allow an
> implementation to define std::to_string(std::int128_t) in that event.
> This makes no sense; it's an artificial restriction. The problem is
> that the current wording defines these overloads exclusively for int,
> long, long long, etc.
> Basically, the implementation can provide as many fundamental types it
> wants, but it would be illegal to give them library support. That's
> dumb.
> Similarly to std::to_string, the standard defines std::bitset to have
> a constructor to have a constructor which takes only unsigned long
> long. Even if the implementation provides a 128-bit type, it would not
> comply with the standard to add a bitset(std::uint128_t) constructor.
> > 2) ...
> Yes, I mean that the wording in the core part of the C++ standard
> would not be affected. The existing overload resolution, type
> conversion, etc. rules have to work for extended integers already.
> This is because the implementation could provide those if it wanted
> to, and the core wording must be robust against that.
> > In the same section you list some "oddities" ...
> I cover the impact on existing code in a separate section as well.
> Firstly, the introduction of a new type doesn't break any existing
> code unless the user uses that type. New features can have breaking
> changes when you opt into them explicitly; this is okay.
> Secondly, the same issue happened when 8-bit developers had to
> suddenly account for 16-bit types, etc. etc. It's ultimately the
> developer's fault for not writing a generic implementation. C++ has
> provided the means for 30 years.
> If a user has written an overload set that covers only the standard
> integers, then their code was already not covering all types because
> implementations could have provided any amount of extended integers,
> and __int128 and_BitInt(N) already exist in some compilers. I guess
> this proposal throws such code under the bus and I don't care.
> You can't make an omelette without breaking some eggs, and we're
> cooking the mother of all omelettes here.
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

Received on 2024-02-11 12:45:52