Re: Does libstdc++-16 stabilize the C++20 ABI?

On Mon, 27 Apr 2026 at 16:21, Jonathan Wakely <[email protected]> wrote:
>
> On Mon, 27 Apr 2026 at 15:02, Daniel Grunwald via Gcc-help
> <[email protected]> wrote:
> >
> > I found some contradictory information:
> >
> > <https://gcc.gnu.org/projects/cxx-status.html#cxx20> claims yes:
> >  > Some C++20 features are available since GCC 8, but support was
> >  > experimental and the ABI of C++20 features was not stable until
> >  > GCC 16. C++20 modules support is still experimental.
> >
> > But <https://gcc.gnu.org/gcc-16/changes.html#libstdcxx> claims the opposite:
> >  > As with all experimental support, programs using C++20 components
> >  > should assume that those components are not compatible between
> >  > different major releases.
> >
> > Can you clarify?
>
> C++20 is stable in GCC 16, I sent a patch to update it which has not
> been pushed yetL
> https://gcc.gnu.org/pipermail/gcc-patches/2026-April/713215.html

I've pushed the patch now, please refresh :-)

>
>
>
> >
> >
> > Additionally, let me ask about a concrete scenario:
> >
> > 1. We build a shared library with `g++-16 -std=c++20 -fvisibility=hidden`
> > 2. The public headers for our library only use C++17 features (and we
> > test that C++17 code can consume our library)
> > 3. This library ends up being used in an application compiled with
> > g++-17 and running with libstdc++-17 (or maybe even with a version
> > further in the future).
> >
> > In this scenario, it seems to me that the interface between our shared
> > library and the application does not care about the ABI stability of
> > C++20 types, since those are not involved in the interface. C++20 types
> > only matter for the stability of the interface between our shared
> > library and libstdc++.so.6.
>
> Correct.
>
> The objects in your library that are built with GCC 16 mean that the
> application must use libstdc++.so.6.0.35 (or later) which is true
> whatever -std option you use.
>
> Because you used -std=c++20, any objects in the entire application
> that use -std=c++20 must be built with GCC 16 (or later) because e.g.
> std::format in GCC 16 is not compatible with std::format in GCC 15.
>
> See https://stackoverflow.com/a/49119902/981959 for more info.
>
>
> > Does this mean our library could use experimental language versions
> > (e.g. C++23) right now? Or is the libstdc++.so.6 ABI stability also
> > restricted to stabilized language versions? I was not able to find any
> > documentation on this. In particular, I checked
> > <https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html> but that
> > makes no mention of experimental language versions.
>
> There are no C++23 symbols in libstdc++.so, precisely so that the
> question of their stability doesn't matter. Any non-inline C++23
> symbols are in the libstdc++exp.a static lib, not in libstdc++.so.
>
> If you use -std=c++23 for any objects in your library, you need to use
> the same version of GCC to compile all of those objects. Because the
> C++23 components in GCC 16 might not be compatible with the C++23
> components in GCC 15 or GCC 17.