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## Class template sharable_lock

boost::interprocess::sharable_lock

## Synopsis

    template<typename SharableMutex>
    class sharable_lock {
    public:
      // types
      typedef SharableMutex mutex_type;

      // construct/copy/destruct
      sharable_lock();
      sharable_lock(mutex_type &);
      sharable_lock(mutex_type &, unspecified);
      sharable_lock(mutex_type &, unspecified);
      sharable_lock(mutex_type &, unspecified);
      sharable_lock(mutex_type &, const boost::posix_time::ptime &);
      sharable_lock(unspecified);
      sharable_lock(unspecified);
      sharable_lock(unspecified);
      sharable_lock& operator=(unspecified);
      ~sharable_lock();

      // public member functions
      void lock() ;
      bool try_lock() ;
      bool timed_lock(const boost::posix_time::ptime &) ;
      void unlock() ;
      bool owns() const;
      operator unspecified_bool_type() const;
      mutex_type * mutex() const;
      mutex_type * release() ;
      void swap(unspecified) ;
    };

## Description

sharable_lock is meant to carry out the tasks for sharable-locking (such as read-locking), unlocking, try-sharable-locking and timed-sharable-locking (recursive or not) for the Mutex. The Mutex need not supply all of this functionality. If the client of sharable_lock<Mutex> does not use functionality which the Mutex does not supply, no harm is done. Mutex ownership can be shared among sharable_locks, and a single upgradable_lock. sharable_lock does not support copy semantics. But sharable_lock supports ownership transfer from an sharable_lock, upgradable_lock and scoped_lock via trasfer_lock syntax.

### `sharable_lock` public construct/copy/destruct

1.     sharable_lock();

   Effects: Default constructs a sharable_lock. Postconditions: owns() == false and mutex() == 0.

2.     sharable_lock(mutex_type & m);

   Effects: m.lock_sharable(). Postconditions: owns() == true and mutex() == &m. Notes: The constructor will take sharable-ownership of the mutex. If another thread already owns the mutex with exclusive ownership (scoped_lock), this thread will block until the mutex is released. If another thread owns the mutex with sharable or upgradable ownership, then no blocking will occur. Whether or not this constructor handles recursive locking depends upon the mutex.

3.     sharable_lock(mutex_type & m, unspecified);

   Postconditions: owns() == false, and mutex() == &m. Notes: The constructor will not take ownership of the mutex. There is no effect required on the referenced mutex.

4.     sharable_lock(mutex_type & m, unspecified);

   Postconditions: owns() == true, and mutex() == &m. Notes: The constructor will suppose that the mutex is already sharable locked. There is no effect required on the referenced mutex.

5.     sharable_lock(mutex_type & m, unspecified);

   Effects: m.try_lock_sharable() Postconditions: mutex() == &m. owns() == the return value of the m.try_lock_sharable() executed within the constructor. Notes: The constructor will take sharable-ownership of the mutex if it can do so without waiting. Whether or not this constructor handles recursive locking depends upon the mutex. If the mutex_type does not support try_lock_sharable, this constructor will fail at compile time if instantiated, but otherwise have no effect.

6.     sharable_lock(mutex_type & m, const boost::posix_time::ptime & abs_time);

   Effects: m.timed_lock_sharable(abs_time) Postconditions: mutex() == &m. owns() == the return value of the m.timed_lock_sharable() executed within the constructor. Notes: The constructor will take sharable-ownership of the mutex if it can do so within the time specified. Whether or not this constructor handles recursive locking depends upon the mutex. If the mutex_type does not support timed_lock_sharable, this constructor will fail at compile time if instantiated, but otherwise have no effect.

7.     sharable_lock(unspecified upgr);

   Postconditions: mutex() == upgr.mutex(). owns() == the value of upgr.owns() before the construction. upgr.owns() == false after the construction. Notes: If the upgr sharable_lock owns the mutex, ownership is moved to this sharable_lock with no blocking. If the upgr sharable_lock does not own the mutex, then neither will this sharable_lock. Only a moved sharable_lock's will match this signature. An non-moved sharable_lock can be moved with the expression: "move(lock);". This constructor does not alter the state of the mutex, only potentially who owns it.

8.     sharable_lock(unspecified upgr);

   Effects: If upgr.owns() then calls unlock_upgradable_and_lock_sharable() on the referenced mutex. Postconditions: mutex() == the value upgr.mutex() had before the construction. upgr.mutex() == 0 owns() == the value of upgr.owns() before construction. upgr.owns() == false after the construction. Notes: If upgr is locked, this constructor will lock this sharable_lock while unlocking upgr. Only a moved sharable_lock's will match this signature. An non-moved upgradable_lock can be moved with the expression: "move(lock);".

9.     sharable_lock(unspecified scop);

   Effects: If scop.owns() then calls unlock_and_lock_sharable() on the referenced mutex. Postconditions: mutex() == the value scop.mutex() had before the construction. scop.mutex() == 0 owns() == scop.owns() before the constructor. After the construction, scop.owns() == false. Notes: If scop is locked, this constructor will transfer the exclusive ownership to a sharable-ownership of this sharable_lock. Only a moved scoped_lock's will match this signature. An non-moved scoped_lock can be moved with the expression: "move(lock);".

10.     sharable_lock& operator=(unspecified upgr);

    Effects: If owns() before the call, then unlock_sharable() is called on mutex(). this gets the state of upgr and upgr gets set to a default constructed state. Notes: With a recursive mutex it is possible that both this and upgr own the mutex before the assignment. In this case, this will own the mutex after the assignment (and upgr will not), but the mutex's lock count will be decremented by one.

11.     ~sharable_lock();

    Effects: if (owns()) mp_mutex->unlock_sharable(). Notes: The destructor behavior ensures that the mutex lock is not leaked.

### `sharable_lock` public member functions

1.     void lock() ;

   Effects: If mutex() == 0 or already locked, throws a lock_exception() exception. Calls lock_sharable() on the referenced mutex. Postconditions: owns() == true. Notes: The sharable_lock changes from a state of not owning the mutex, to owning the mutex, blocking if necessary.

2.     bool try_lock() ;

   Effects: If mutex() == 0 or already locked, throws a lock_exception() exception. Calls try_lock_sharable() on the referenced mutex. Postconditions: owns() == the value returned from mutex()->try_lock_sharable(). Notes: The sharable_lock changes from a state of not owning the mutex, to owning the mutex, but only if blocking was not required. If the mutex_type does not support try_lock_sharable(), this function will fail at compile time if instantiated, but otherwise have no effect.

3.     bool timed_lock(const boost::posix_time::ptime & abs_time) ;

   Effects: If mutex() == 0 or already locked, throws a lock_exception() exception. Calls timed_lock_sharable(abs_time) on the referenced mutex. Postconditions: owns() == the value returned from mutex()->timed_lock_sharable(elps_time). Notes: The sharable_lock changes from a state of not owning the mutex, to owning the mutex, but only if it can obtain ownership within the specified time interval. If the mutex_type does not support timed_lock_sharable(), this function will fail at compile time if instantiated, but otherwise have no effect.

4.     void unlock() ;

   Effects: If mutex() == 0 or not locked, throws a lock_exception() exception. Calls unlock_sharable() on the referenced mutex. Postconditions: owns() == false. Notes: The sharable_lock changes from a state of owning the mutex, to not owning the mutex.

5.     bool owns() const;

   Effects: Returns true if this scoped_lock has acquired the referenced mutex.

6.     operator unspecified_bool_type() const;

   Conversion to bool. Returns owns().

7.     mutex_type * mutex() const;

   Effects: Returns a pointer to the referenced mutex, or 0 if there is no mutex to reference.

8.     mutex_type * release() ;

   Effects: Returns a pointer to the referenced mutex, or 0 if there is no mutex to reference. Postconditions: mutex() == 0 and owns() == false.

9.     void swap(unspecified other) ;

   Effects: Swaps state with moved lock. Throws: Nothing.