std::shared_mutex

 

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std::shared_mutex

Member functions

shared_mutex::shared_mutex

Exclusive locking

shared_mutex::lock

shared_mutex::try_lock

shared_mutex::unlock

Shared locking

shared_mutex::lock_shared

shared_mutex::try_lock_shared

shared_mutex::unlock_shared

Native handle

shared_mutex::native_handle

 

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	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)

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 mut;
    /* 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);
        std::lock(lhs, rhs);
        /* assign data */
        return *this;
    }
}

See also

shared_timed_mutex (C++14)

provides shared mutual exclusion facility (class)