# null_rw_mutex

[mutex.null_rw_mutex]

A `null_rw_mutex` is a class that models the ReaderWriterMutex requirement syntactically, but does nothing. The `null_rw_mutex` class also satisfies all syntactic requirements of shared mutex type from the [thread.sharedmutex.requirements] ISO C++ section, but does nothing. It is useful for instantiating a template that expects a ReaderWriterMutex, but no mutual exclusion is actually needed for that instance.

    // Defined in header <oneapi/tbb/null_rw_mutex.h>

    namespace oneapi {
    namespace tbb {
        class null_rw_mutex {
        public:
            constexpr null_rw_mutex() noexcept;
            ~null_rw_mutex();

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

            class scoped_lock;

            void lock();
            bool try_lock();
            void unlock();

            void lock_shared();
            bool try_lock_shared();
            void unlock_shared();

            static constexpr bool is_rw_mutex = true;
            static constexpr bool is_recursive_mutex = true;
            static constexpr bool is_fair_mutex = true;
        };
    } // namespace tbb
    } // namespace oneapi

## Member classes

class scoped_lock

  Corresponding `scoped_lock` class. See the ReaderWriterMutex requirement.

## Member functions

null_rw_mutex()

  Constructs unlocked mutex.

~null_rw_mutex()

  Destroys unlocked mutex.

void lock()

  Acquires a lock.

bool try_lock()

  Attempts to acquire a lock (non-blocking) on write. Returns true.

void unlock()

  Releases a write lock held by the current thread.

void lock_shared()

  Acquires a lock on read.

bool try_lock_shared()

  Attempts to acquire the lock (non-blocking) on read. Returns true.

void unlock_shared()

  Releases a read lock held by the current thread.