ReaderWriterMutex#

[req.rw_mutex]

The ReaderWriterMutex requirement extends the Mutex Requirement to include the notion of reader-writer locks. It introduces a boolean parameter write that specifies whether a writer lock (write = true) or reader lock (write = false) is being requested. Multiple reader locks can be held simultaneously on a ReaderWriterMutex if it does not have a writer lock on it. A writer lock on a ReaderWriterMutex excludes all other threads from holding a lock on the mutex at the same time.

class RWM {
    // Implementation specifics
    // ...

    // Represents acquisition of a mutex.
    class scoped_lock {
    public:
        constexpr scoped_lock() noexcept;
        scoped_lock(RWM& m, bool write = true);
        ~scoped_lock();

        scoped_lock(const scoped_lock&) = delete;
        scoped_lock& operator=(const scoped_lock&) = delete;

        void acquire(RWM& m, bool write = true);
        bool try_acquire(RWM& m, bool write = true);
        void release();

        bool upgrade_to_writer();
        bool downgrade_to_reader();
    };
};

A type RWM satisfies ReaderWriterMutex if it meets the following requirements. They form a superset of the Mutex requirements.

type RWM::scoped_lock#: Corresponding scoped-lock type.

RWM::scoped_lock()#: Constructs scoped_lock without acquiring any mutex.

RWM::scoped_lock(RWM&, bool write = true)#: Constructs scoped_lock and acquires a lock on a given mutex. The lock is a writer lock if write is true; a reader lock otherwise.

RWM::~scoped_lock()#: Releases a lock (if acquired).

void RWM::scoped_lock::acquire(RWM&, bool write = true)#: Acquires a lock on a given mutex. The lock is a writer lock if write is true; it is a reader lock, otherwise.

bool RWM::scoped_lock::try_acquire(RWM&, bool write = true)#: Attempts to acquire a lock on a given mutex. The lock is a writer lock if write is true; it is a reader lock, otherwise. Returns true if the lock is acquired, false otherwise.

RWM::scoped_lock::release()#: Releases a lock. The effect is undefined if no lock is held.

bool RWM::scoped_lock::upgrade_to_writer()#: Changes a reader lock to a writer lock. Returns false if lock was released and reacquired. Otherwise, returns true, including the case when the lock was already a writer lock.

bool RWM::scoped_lock::downgrade_to_reader()#: Changes a writer lock to a reader lock. Returns false if lock was released and reacquired. Otherwise, returns true, including the case when the lock was already a reader lock.

Like the Mutex requirement, ReaderWriterMutex also requires a set of traits to be defined.

static constexpr bool M::is_rw_mutex#: True if mutex is a reader-writer mutex; false, otherwise.

static constexpr bool M::is_recursive_mutex#: True if mutex is a recursive mutex; false, otherwise.

static constexpr bool M::is_fair_mutex#: True if mutex is fair; false, otherwise.

The following table summarizes the library classes that model the ReaderWriterMutex requirement and provided guarantees.

Provided guarantees for Mutexes that model the ReaderWriterMutex requirement#
.	Fair	Reentrant
`rw_mutex`	No	No
`spin_rw_mutex`	No	No
`speculative_spin_rw_mutex`	No	No
`queuing_rw_mutex`	Yes	No
`null_rw_mutex`	Yes	Yes

Note

Implementation is allowed to have an opposite guarantees (positive) in case of negative statements from the table above.

Note

For all currently provided reader-writer mutexes,

is_recursive_mutex is false
scoped_lock::downgrade_to_reader always returns true

However, other implementations of the ReaderWriterMutex requirement are not required to do the same.