# std::shared_mutex

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< cpp‎ | thread

Defined in header `<shared_mutex>` |  |
class shared_mutex; |  | (since C++17)

The `shared_mutex` class is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads. In contrast to other mutex types which facilitate exclusive access, a shared_mutex has two levels of access:

- shared - several threads can share ownership of the same mutex.

- exclusive - only one thread can own the mutex.

Shared mutexes are usually used in situations when multiple readers can access the same resource at the same time without causing data races, but only one writer can do so.

The `shared_mutex` class satisfies all requirements of `SharedMutex` and `StandardLayoutType`.

### Member types

Member type | Definition
`native_handle_type`(optional) | implementation-defined

### Member functions

(constructor) | constructs the mutex (public member function)
(destructor) | destroys the mutex (public member function)
operator= [deleted] | not copy-assignable (public member function)

##### Exclusive locking

lock | locks the mutex, blocks if the mutex is not available (public member function)
try_lock | tries to lock the mutex, returns if the mutex is not available (public member function)
unlock | unlocks the mutex (public member function)

##### Shared locking

lock_shared | locks the mutex for shared ownership, blocks if the mutex is not available (public member function)
try_lock_shared | tries to lock the mutex for shared ownership, returns if the mutex is not available (public member function)
unlock_shared | unlocks the mutex (shared ownership) (public member function)

##### Native handle

native_handle | returns the underlying implementation-defined thread handle (public member function)

### Example

Run this code

    #include <iostream>
    #include <mutex>  // For std::unique_lock
    #include <shared_mutex>
    #include <thread>

    class ThreadSafeCounter {
     public:
      ThreadSafeCounter() = default;

      // Multiple threads/readers can read the counter's value at the same time.
      unsigned int get() const {
        std::shared_lock<std::shared_mutex> lock(mutex_);
        return value_;
      }

      // Only one thread/writer can increment/write the counter's value.
      void increment() {
        std::unique_lock<std::shared_mutex> lock(mutex_);
        value_++;
      }

      // Only one thread/writer can reset/write the counter's value.
      void reset() {
        std::unique_lock<std::shared_mutex> lock(mutex_);
        value_ = 0;
      }

     private:
      mutable std::shared_mutex mutex_;
      unsigned int value_ = 0;
    };

    int main() {
      ThreadSafeCounter counter;

      auto increment_and_print = [&counter]() {
        for (int i = 0; i < 3; i++) {
          counter.increment();
          std::cout << std::this_thread::get_id() << ' ' << counter.get() << '\n';

          // Note: Writing to std::cout actually needs to be synchronized as well
          // by another std::mutex. This has been omitted to keep the example small.
        }
      };

      std::thread thread1(increment_and_print);
      std::thread thread2(increment_and_print);

      thread1.join();
      thread2.join();
    }

    // Explanation: The output below was generated on a single-core machine. When
    // thread1 starts, it enters the loop for the first time and calls increment()
    // followed by get(). However, before it can print the returned value to
    // std::cout, the scheduler puts thread1 to sleep and wakes up thread2, which
    // obviously has time enough to run all three loop iterations at once. Back to
    // thread1, still in the first loop iteration, it finally prints its local copy
    // of the counter's value, which is 1, to std::cout and then runs the remaining
    // two loop iterations. On a multi-core maschine, none of the threads is put to
    // sleep and the output is more likely to be in ascending order.

Possible output:

    123084176803584 2
    123084176803584 3
    123084176803584 4
    123084185655040 1
    123084185655040 5
    123084185655040 6

### See also

shared_timed_mutex (C++14) | provides shared mutual exclusion facility (class)

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