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| Member functions | ||||
atomic::wait (C++20) | ||||
(C++20) | ||||
(C++20) | ||||
| Constants | ||||
(C++17) | ||||
| Specialized member functions | ||||
| Specialized for integral, floating-point(C++20) and pointer types | ||||
| Specialized for integral and pointer types only | ||||
(C++26) | ||||
(C++26) | ||||
| Specialized for integral types only | ||||
void wait( T old,std::memory_order order= std::memory_order_seq_cst)constnoexcept; | (1) | (since C++20) (constexpr since C++26) |
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:
These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.
Iforder is notstd::memory_order_relaxed,std::memory_order_consume,std::memory_order_acquire orstd::memory_order_seq_cst, the behavior is undefined.
Contents |
| old | - | the value to check theatomic's object no longer contains |
| order | - | memory order constraints to enforce |
This form of change-detection is often more efficient than simple polling or pure spinlocks.
Due to theABA problem, transient changes fromold to another value and back toold might be missed, and not unblock.
The comparison is bitwise (similar tostd::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.
#include <atomic>#include <chrono>#include <future>#include <iostream>#include <thread> usingnamespace 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
(C++20) | notifies at least one thread waiting on the atomic object (public member function)[edit] |
(C++20) | notifies all threads blocked waiting on the atomic object (public member function)[edit] |
(C++20) | notifies a thread blocked in atomic_wait (function template)[edit] |
(C++20) | notifies all threads blocked in atomic_wait (function template)[edit] |
