std::atomic<T>::wait
From cppreference.com
C++
Concurrency support library
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std::atomic
| Member functions | ||||
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atomic::wait (C++20) |
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(C++20)
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(C++20)
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| Constants | ||||
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(C++17)
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| Specialized member functions | ||||
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Specialized for integral, floating-point (C++20) and pointer types |
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| Specialized for integral and pointer types only | ||||
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(C++26)
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(C++26)
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| Specialized for integral types only | ||||
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void wait( T old, std::memory_order order =
std::memory_order_seq_cst ) const noexcept ; |
(1) | (since C++20) (constexpr since C++26) |
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void wait( T old,
std::memory_order order = |
(2) | (since C++20) |
Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:
- Compare the value representation of this->load(order) with that of old.
- If those are equal, then blocks until *this is notified by notify_one() or notify_all()
- Otherwise, returns.
These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.
If order is not std::memory_order_relaxed, std::memory_order_consume, std::memory_order_acquire or std::memory_order_seq_cst
Parameters
| old | - | the value to check the atomic's object no longer contains
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| order | - | memory order constraints to enforce |
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
The comparison is bitwise (similar to std::memcmp
Example
Run this code
#include <atomic> #include <chrono> #include <future> #include <iostream> #include <thread> using namespace std::literals; int main() { std::atomic<bool> all_tasks_completed{false}; std::atomic<unsigned> completion_count{}; std::future<void> task_futures[16]; std::atomic<unsigned> outstanding_task_count{16}; // Spawn several tasks which take different amounts of // time, then decrement the outstanding task count. for (std::future<void>& task_future : task_futures) task_future = std::async([&] { // This sleep represents doing real work... std::this_thread::sleep_for(50ms); ++completion_count; --outstanding_task_count; // When the task count falls to zero, notify // the waiter (main thread in this case). if (outstanding_task_count.load() == 0) { all_tasks_completed = true; all_tasks_completed.notify_one(); } }); all_tasks_completed.wait(false); std::cout << "Tasks completed = " << completion_count.load() << '\n'; }
Output:
Tasks completed = 16
See also
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(C++20)
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notifies at least one thread waiting on the atomic object (public member function) |
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(C++20)
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notifies all threads blocked waiting on the atomic object (public member function) |
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(C++20)
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notifies a thread blocked in atomic_wait (function template) |
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(C++20)
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notifies all threads blocked in atomic_wait (function template) |