# 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.

If one thread has acquired the exclusive lock (through lock, try_lock), no other threads can acquire the lock (including the shared).

If one thread has acquired the shared lock (through lock_shared, try_lock_shared), no other thread can acquire the exclusive lock, but can acquire the shared lock.

Only when the exclusive lock has not been acquired by any thread, the shared lock can be acquired by multiple threads.

Within one thread, only one lock (shared or exclusive) can be acquired at the same time.

Shared mutexes are especially useful when shared data can be safely read by any number of threads simultaneously, but a thread may only write the same data when no other thread is reading or writing at the same time.

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

### Member types

Member type | Definition
`native_handle_type`(not always present) | 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 native handle object (public member function)

### Example

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 machine, none of the threads is put to sleep and the output is more likely to be in ascending order.

Run this code

    #include <iostream>
    #include <mutex>
    #include <shared_mutex>
    #include <syncstream>
    #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 lock(mutex_);
            return value_;
        }

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

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

    private:
        mutable std::shared_mutex mutex_;
        unsigned int value_{};
    };

    int main()
    {
        ThreadSafeCounter counter;

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

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

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

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 and implements locking with a timeout (class)
shared_lock (C++14) | implements movable shared mutex ownership wrapper (class template)
unique_lock (C++11) | implements movable mutex ownership wrapper (class template)

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