30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.2 Locks [thread.lock]

30.4.2.3 Class template shared_lock [thread.lock.shared]

namespace std {
  template <class Mutex>
  class shared_lock {
  public:
    using mutex_type = Mutex;

    // [thread.lock.shared.cons], construct/copy/destroy
    shared_lock() noexcept;
    explicit shared_lock(mutex_type& m);  // blocking
    shared_lock(mutex_type& m, defer_lock_t) noexcept;
    shared_lock(mutex_type& m, try_to_lock_t);
    shared_lock(mutex_type& m, adopt_lock_t);
    template <class Clock, class Duration>
      shared_lock(mutex_type& m,
                  const chrono::time_point<Clock, Duration>& abs_time);
    template <class Rep, class Period>
      shared_lock(mutex_type& m,
                const chrono::duration<Rep, Period>& rel_time);
    ~shared_lock();

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

    shared_lock(shared_lock&& u) noexcept;
    shared_lock& operator=(shared_lock&& u) noexcept;

    // [thread.lock.shared.locking], locking
    void lock();  // blocking
    bool try_lock();
    template <class Rep, class Period>
      bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
    template <class Clock, class Duration>
      bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
    void unlock();

    // [thread.lock.shared.mod], modifiers
    void swap(shared_lock& u) noexcept;
    mutex_type* release() noexcept;

    // [thread.lock.shared.obs], observers
    bool owns_lock() const noexcept;
    explicit operator bool () const noexcept;
    mutex_type* mutex() const noexcept;

  private:
    mutex_type* pm; // exposition only
    bool owns;      // exposition only
  };

  template <class Mutex>
    void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
}

An object of type shared_lock controls the shared ownership of a lockable object within a scope. Shared ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another shared_lock object. Objects of type shared_lock are not copyable but are movable. The behavior of a program is undefined if the contained pointer pm is not null and the lockable object pointed to by pm does not exist for the entire remaining lifetime ([basic.life]) of the shared_lock object. The supplied Mutex type shall meet the shared mutex requirements ([thread.sharedtimedmutex.requirements]).

Note: shared_lock<Mutex> meets the TimedLockable requirements ([thread.req.lockable.timed]).  — end note ]

30.4.2.3.1 shared_lock constructors, destructor, and assignment [thread.lock.shared.cons]

shared_lock() noexcept;

Effects: Constructs an object of type shared_lock.

Postconditions: pm == nullptr and owns == false.

explicit shared_lock(mutex_type& m);

Requires: The calling thread does not own the mutex for any ownership mode.

Effects: Constructs an object of type shared_lock and calls m.lock_shared().

Postconditions: pm == addressof(m) and owns == true.

shared_lock(mutex_type& m, defer_lock_t) noexcept;

Effects: Constructs an object of type shared_lock.

Postconditions: pm == addressof(m) and owns == false.

shared_lock(mutex_type& m, try_to_lock_t);

Requires: The calling thread does not own the mutex for any ownership mode.

Effects: Constructs an object of type shared_lock and calls m.try_lock_shared().

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared().

shared_lock(mutex_type& m, adopt_lock_t);

Requires: The calling thread has shared ownership of the mutex.

Effects: Constructs an object of type shared_lock.

Postconditions: pm == addressof(m) and owns == true.

template <class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);

Requires: The calling thread does not own the mutex for any ownership mode.

Effects: Constructs an object of type shared_lock and calls m.try_lock_shared_until(abs_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_until(abs_time).

template <class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);

Requires: The calling thread does not own the mutex for any ownership mode.

Effects: Constructs an object of type shared_lock and calls m.try_lock_shared_for(rel_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_for(rel_time).

~shared_lock();

Effects: If owns calls pm->unlock_shared().

shared_lock(shared_lock&& sl) noexcept;

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this construction), sl.pm == nullptr and sl.owns == false.

shared_lock& operator=(shared_lock&& sl) noexcept;

Effects: If owns calls pm->unlock_shared().

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this assignment), sl.pm == nullptr and sl.owns == false.

30.4.2.3.2 shared_lock locking [thread.lock.shared.locking]

void lock();

Effects: As if by pm->lock_shared().

Postconditions: owns == true.

Throws: Any exception thrown by pm->lock_shared(). system_error when an exception is required ([thread.req.exception]).

Error conditions:

  • operation_not_permitted — if pm is nullptr.

  • resource_deadlock_would_occur — if on entry owns is true.

bool try_lock();

Effects: As if by pm->try_lock_shared().

Returns: The value returned by the call to pm->try_lock_shared().

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared().

Throws: Any exception thrown by pm->try_lock_shared(). system_error when an exception is required ([thread.req.exception]).

Error conditions:

  • operation_not_permitted — if pm is nullptr.

  • resource_deadlock_would_occur — if on entry owns is true.

template <class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

Effects: As if by pm->try_lock_shared_until(abs_time).

Returns: The value returned by the call to pm->try_lock_shared_until(abs_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_until(abs_time).

Throws: Any exception thrown by pm->try_lock_shared_until(abs_time). system_error when an exception is required ([thread.req.exception]).

Error conditions:

  • operation_not_permitted — if pm is nullptr.

  • resource_deadlock_would_occur — if on entry owns is true.

template <class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

Effects: As if by pm->try_lock_shared_for(rel_time).

Returns: The value returned by the call to pm->try_lock_shared_for(rel_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_for(rel_time).

Throws: Any exception thrown by pm->try_lock_shared_for(rel_time). system_error when an exception is required ([thread.req.exception]).

Error conditions:

  • operation_not_permitted — if pm is nullptr.

  • resource_deadlock_would_occur — if on entry owns is true.

void unlock();

Effects: As if by pm->unlock_shared().

Postconditions: owns == false.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

  • operation_not_permitted — if on entry owns is false.

30.4.2.3.3 shared_lock modifiers [thread.lock.shared.mod]

void swap(shared_lock& sl) noexcept;

Effects: Swaps the data members of *this and sl.

mutex_type* release() noexcept;

Returns: The previous value of pm.

Postconditions: pm == nullptr and owns == false.

template <class Mutex> void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;

Effects: As if by x.swap(y).

30.4.2.3.4 shared_lock observers [thread.lock.shared.obs]

bool owns_lock() const noexcept;

Returns: owns.

explicit operator bool() const noexcept;

Returns: owns.

mutex_type* mutex() const noexcept;

Returns: pm.