C++ named requirements: SharedMutex (since C++17)

The SharedMutex requirements extend the Mutex

Requirements

• Mutex

Additionally, an object m of SharedMutex

• The expression m.lock_shared() has the following properties:

• Behaves as an atomic operation.
• Blocks the calling thread until shared ownership of the mutex can be obtained.
• Prior m.unlock() operations on the same mutex synchronize-with this lock operation (equivalent to release-acquire std::memory_order
• The behavior is undefined if the calling thread already owns the mutex in any mode.
• If an exception is thrown, the shared lock is not acquired.

• The expression m.try_lock_shared() has the following properties:

• Behaves as an atomic operation.
• Attempts to obtain shared ownership of the mutex for the calling thread without blocking. If ownership is not obtained, returns immediately. The function is allowed to spuriously fail and return even if the mutex is not currently owned by any threads in any mode.
• If try_lock_shared() succeeds, prior unlock() operations on the same object synchronize-with this operation (equivalent to release-acquire std::memory_order
• The behavior is undefined if the calling thread already owns the mutex in any mode.

• The expression m.unlock_shared() has the following properties:

• Behaves as an atomic operation.
• Releases the calling thread's ownership of the mutex and synchronizes-with the subsequent successful lock operations on the same object.
• The behavior is undefined if the calling thread does not own the mutex.

• All lock and unlock operations on a single mutex occur in a single total order.

Standard library

The following standard library types satisfy SharedMutex requirements:

shared_mutex (C++17)	provides shared mutual exclusion facility (class)
shared_timed_mutex (C++14)	provides shared mutual exclusion facility and implements locking with a timeout (class)

See also

• Thread support library
• Mutex
• TimedMutex
• SharedTimedMutex