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# std::shared_mutex

From cppreference.com
< cpp‎ | thread

Defined in header `<mutex>` |  |
class shared_mutex; |  | (since C++14)

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.

In a manner similar to `timed_mutex`, `shared_mutex` provides the ability to attempt to claim ownership of a `shared_mutex` with a timeout via the `try_lock_for()`, `try_lock_until()`, `try_lock_shared_for()`, `try_lock_shared_until()` methods.

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

### [edit] Member functions

(constructor) | constructs the mutex (public member function) [edit]
(destructor) | destroys the mutex (public member function) [edit]
operator= [deleted] | not copy-assignable (public member function) [edit]
##### Exclusive locking
lock | locks the mutex, blocks if the mutex is not available (public member function) [edit]
try_lock | tries to lock the mutex, returns if the mutex is not available (public member function) [edit]
try_lock_for | tries to lock the mutex, returns if the mutex has been unavailable for the specified timeout duration (public member function) [edit]
try_lock_until | tries to lock the mutex, returns if the mutex has been unavailable until specified time point has been reached (public member function) [edit]
unlock | unlocks the mutex (public member function) [edit]
##### Shared locking
lock_shared | locks the mutex for shared ownership, blocks if the mutex is not available (public member function) [edit]
try_lock_shared | tries to lock the mutex for shared ownership, returns if the mutex is not available (public member function) [edit]
try_lock_shared_for | tries to lock the mutex for shared ownership, returns if the mutex has been unavailable for the specified timeout duration (public member function) [edit]
try_lock_shared_until | tries to lock the mutex for shared ownership, returns if the mutex has been unavailable until specified time point has been reached (public member function) [edit]
unlock_shared | unlocks the mutex (shared ownership) (public member function) [edit]

| This section is incomplete Reason: build a motivating example

A copy assignment operator for a class that holds resources that can handle multiple readers, but only one writer

Run this code

    class R
    {
        mutable std::shared_mutex m;
        /* data */
    public:
        R& operator=(const R& other)
        {
            // requires exclusive ownership to write to *this
            std::unique_lock<std::shared_mutex> lhs(mut, std::defer_lock);
            // requires shared ownership to read from other
            std::shared_lock<std::shared_mutex> rhs(other.mut, std::defer_lock);
            lock(lhs, rhs);
            /* assign data */
            return *this;
        }
    }

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