On Fri, 18 Jul 2025 at 19:18, Tomasz Kaminski <tkami...@redhat.com> wrote: > > > > On Fri, Jul 18, 2025 at 8:03 PM Jonathan Wakely <jwak...@redhat.com> wrote: >> >> This adds a check when incrementing the shared count and weak count and >> will trap if it overflows. This also double the effective range of the >> counts for most 64-bit targets. >> >> The counter type, _Atomic_word, is usually a signed 32-bit int (except >> on Solaris v9 where it is a signed 64-bit long). The return type of >> std::shared_ptr::use_count() is long. For targets where long is wider >> than _Atomic_word (most 64-bit targets) we can treat the _Atomic_word >> reference counts as unsigned and allow them to wrap around from their >> most positive value to their most negative value without any problems. > > I wonder could we return std::numeric_limits<long>::max() from use_count(), > if make_unsigned_t<_Atomic_word> value does not fit in the long? > The value will not increment, but only the usable one is zero.
True. use_count() has no synchronization, so can be a stale value. So if we saturate to LONG_MAX we can just say "sorry, the increments didn't become visible to this thread yet" :-)
