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Showing content from http://en.cppreference.com/w/cpp/string/basic_string/../../../cpp/atomic/atomic/wait.html below:

std::atomic<T>::wait - cppreference.com

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

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, the behavior is undefined.

[edit] Parameters old - the value to check the atomic's object no longer contains order - memory order constraints to enforce [edit] 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.

The comparison is bitwise (similar to std::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.

[edit] Example
#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:

[edit] See also notifies at least one thread waiting on the atomic object
(public member function) [edit] notifies all threads blocked waiting on the atomic object
(public member function) [edit] notifies a thread blocked in atomic_wait
(function template) [edit] notifies all threads blocked in atomic_wait
(function template) [edit]

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