# std::atomic_flag::wait

From cppreference.com
< cpp‎ | atomic‎ | atomic flag

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

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.

### Parameters

old | - | the value to check the `atomic_flag`'s object no longer contains
order | - | the memory synchronization ordering for this operation: must not be std::memory_order::release or std::memory_order::acq_rel

### 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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_flag/wait&oldid=116006"