Re: [PATCH] libstdc++: Implement LWG 4301 changes to condition_variable{_any}

On Fri, 31 Jul 2026 at 14:14, Patrick Palka <[email protected]> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK for trunk

> And perhaps backports?

Yeah, I guess that makes sense, but please wait until after 16.2 is released.

>
> -- >8 --
>
>         * include/std/condition_variable (condition_variable::wait_until):
>         Take timeout parameter by value as per LWG 4301.
>         (condition_variable::wait_for): Likewise.
>         (condition_variable::__wait_until_impl): Likewise.
>         (condition_variable_any::wait_until): Likewise.
>         (condition_variable_any::wait_for): Likewise.
> ---
>  libstdc++-v3/include/std/condition_variable | 34 ++++++++++++---------
>  1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
> index ce995ed57779..b5831cdd8714 100644
> --- a/libstdc++-v3/include/std/condition_variable
> +++ b/libstdc++-v3/include/std/condition_variable
> @@ -107,24 +107,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           wait(__lock);
>        }
>
> +    // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +    // 4301. condition_variable{_any}::wait_{for, until} should take timeout by value
> +
>  #ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
>      template<typename _Duration>
>        cv_status
>        wait_until(unique_lock<mutex>& __lock,
> -                const chrono::time_point<steady_clock, _Duration>& __atime)
> +                chrono::time_point<steady_clock, _Duration> __atime)
>        { return __wait_until_impl(__lock, __atime); }
>  #endif
>
>      template<typename _Duration>
>        cv_status
>        wait_until(unique_lock<mutex>& __lock,
> -                const chrono::time_point<system_clock, _Duration>& __atime)
> +                chrono::time_point<system_clock, _Duration> __atime)
>        { return __wait_until_impl(__lock, __atime); }
>
>      template<typename _Clock, typename _Duration>
>        cv_status
>        wait_until(unique_lock<mutex>& __lock,
> -                const chrono::time_point<_Clock, _Duration>& __atime)
> +                chrono::time_point<_Clock, _Duration> __atime)
>        {
>  #if __cplusplus > 201703L
>         static_assert(chrono::is_clock_v<_Clock>);
> @@ -149,7 +152,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      template<typename _Clock, typename _Duration, typename _Predicate>
>        bool
>        wait_until(unique_lock<mutex>& __lock,
> -                const chrono::time_point<_Clock, _Duration>& __atime,
> +                chrono::time_point<_Clock, _Duration> __atime,
>                  _Predicate __p)
>        {
>         while (!__p())
> @@ -161,7 +164,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      template<typename _Rep, typename _Period>
>        cv_status
>        wait_for(unique_lock<mutex>& __lock,
> -              const chrono::duration<_Rep, _Period>& __rtime)
> +              chrono::duration<_Rep, _Period> __rtime)
>        {
>         // _GLIBCXX_RESOLVE_LIB_DEFECTS
>         // 3504. condition_variable::wait_for is overspecified
> @@ -174,7 +177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      template<typename _Rep, typename _Period, typename _Predicate>
>        bool
>        wait_for(unique_lock<mutex>& __lock,
> -              const chrono::duration<_Rep, _Period>& __rtime,
> +              chrono::duration<_Rep, _Period> __rtime,
>                _Predicate __p)
>        {
>         using __dur = typename steady_clock::duration;
> @@ -193,7 +196,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      template<typename _Dur>
>        cv_status
>        __wait_until_impl(unique_lock<mutex>& __lock,
> -                       const chrono::time_point<steady_clock, _Dur>& __atime)
> +                       chrono::time_point<steady_clock, _Dur> __atime)
>        {
>         __gthread_time_t __ts = chrono::__to_timeout_gthread_time_t(__atime);
>         _M_cond.wait_until(*__lock.mutex(), CLOCK_MONOTONIC, __ts);
> @@ -206,7 +209,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      template<typename _Dur>
>        cv_status
>        __wait_until_impl(unique_lock<mutex>& __lock,
> -                       const chrono::time_point<system_clock, _Dur>& __atime)
> +                       chrono::time_point<system_clock, _Dur> __atime)
>        {
>         __gthread_time_t __ts = chrono::__to_timeout_gthread_time_t(__atime);
>         _M_cond.wait_until(*__lock.mutex(), __ts);
> @@ -312,10 +315,13 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
>           wait(__lock);
>        }
>
> +    // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +    // 4301. condition_variable{_any}::wait_{for, until} should take timeout by value
> +
>      template<typename _Lock, typename _Clock, typename _Duration>
>        cv_status
>        wait_until(_Lock& __lock,
> -                const chrono::time_point<_Clock, _Duration>& __atime)
> +                chrono::time_point<_Clock, _Duration> __atime)
>        {
>         shared_ptr<mutex> __mutex = _M_mutex;
>         unique_lock<mutex> __my_lock(*__mutex);
> @@ -330,7 +336,7 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
>              typename _Duration, typename _Predicate>
>        bool
>        wait_until(_Lock& __lock,
> -                const chrono::time_point<_Clock, _Duration>& __atime,
> +                chrono::time_point<_Clock, _Duration> __atime,
>                  _Predicate __p)
>        {
>         while (!__p())
> @@ -341,14 +347,14 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
>
>      template<typename _Lock, typename _Rep, typename _Period>
>        cv_status
> -      wait_for(_Lock& __lock, const chrono::duration<_Rep, _Period>& __rtime)
> +      wait_for(_Lock& __lock, chrono::duration<_Rep, _Period> __rtime)
>        { return wait_until(__lock, __clock_t::now() + __rtime); }
>
>      template<typename _Lock, typename _Rep,
>              typename _Period, typename _Predicate>
>        bool
>        wait_for(_Lock& __lock,
> -              const chrono::duration<_Rep, _Period>& __rtime, _Predicate __p)
> +              chrono::duration<_Rep, _Period> __rtime, _Predicate __p)
>        { return wait_until(__lock, __clock_t::now() + __rtime, std::move(__p)); }
>
>  #ifdef __glibcxx_jthread
> @@ -383,7 +389,7 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
>      template <class _Lock, class _Clock, class _Duration, class _Predicate>
>      bool wait_until(_Lock& __lock,
>                      stop_token __stoken,
> -                    const chrono::time_point<_Clock, _Duration>& __abs_time,
> +                    chrono::time_point<_Clock, _Duration> __abs_time,
>                      _Predicate __p)
>      {
>        if (__stoken.stop_requested())
> @@ -418,7 +424,7 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
>      template <class _Lock, class _Rep, class _Period, class _Predicate>
>      bool wait_for(_Lock& __lock,
>                    stop_token __stoken,
> -                  const chrono::duration<_Rep, _Period>& __rel_time,
> +                  chrono::duration<_Rep, _Period> __rel_time,
>                    _Predicate __p)
>      {
>        auto __abst = std::chrono::steady_clock::now() + __rel_time;
> --
> 2.55.0.481.ga97fcc37c2
>