std::atomic_flag:: wait

void wait ( bool old, std:: memory_order order = std :: memory_order :: seq_cst ) const noexcept ;	(1)	(since C++20)
void wait ( bool old, std:: memory_order order = std :: memory_order :: seq_cst ) const volatile noexcept ;	(2)	(since C++20)

Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:

• Compare this - > test ( order ) with that of old .
  • If those are equal, then blocks until * this is notified by notify_one() or notify_all() , or the thread is unblocked spuriously.
  • Otherwise, returns.

These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.

If order is one of std :: memory_order :: release and std :: memory_order :: acq_rel , the behavior is undefined.

Parameters

old	-	the value to check the atomic_flag 's object no longer contains
order	-	memory order constraints to enforce

Return value

(none)

Notes

This form of change-detection is often more efficient than simple polling or pure spinlocks.

Due to the ABA problem , transient changes from old to another value and back to old might be missed, and not unblock.

Example

This section is incomplete Reason: no example

See also

notify_one (C++20)	notifies at least one thread waiting on the atomic object (public member function)
notify_all (C++20)	notifies all threads blocked waiting on the atomic object (public member function)
atomic_flag_notify_one (C++20)	notifies a thread blocked in atomic_flag_wait (function)
atomic_flag_notify_all (C++20)	notifies all threads blocked in atomic_flag_wait (function)