30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.2 Locks [thread.lock]

A lock is an object that holds a reference to a lockable object and may unlock the lockable object during the lock's destruction (such as when leaving block scope). An execution agent may use a lock to aid in managing ownership of a lockable object in an exception safe manner. A lock is said to own a lockable object if it is currently managing the ownership of that lockable object for an execution agent. A lock does not manage the lifetime of the lockable object it references. [ Note: Locks are intended to ease the burden of unlocking the lockable object under both normal and exceptional circumstances.  — end note ]

Some lock constructors take tag types which describe what should be done with the lockable object during the lock's construction.

namespace std {
  struct defer_lock_t  { };     // do not acquire ownership of the mutex
  struct try_to_lock_t { };     // try to acquire ownership of the mutex
                                // without blocking
  struct adopt_lock_t  { };     // assume the calling thread has already
                                // obtained mutex ownership and manage it

  constexpr defer_lock_t   defer_lock { };
  constexpr try_to_lock_t  try_to_lock { };
  constexpr adopt_lock_t   adopt_lock { };
}

30.4.2.1 Class template lock_guard [thread.lock.guard]

namespace std {
  template <class Mutex>
  class lock_guard {
  public:
    typedef Mutex mutex_type;

    explicit lock_guard(mutex_type& m);
    lock_guard(mutex_type& m, adopt_lock_t);
    ~lock_guard();

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

  private:
    mutex_type& pm; // exposition only
  };
}

An object of type lock_guard controls the ownership of a lockable object within a scope. A lock_guard object maintains ownership of a lockable object throughout the lock_guard object's lifetime ([basic.life]). The behavior of a program is undefined if the lockable object referenced by pm does not exist for the entire lifetime of the lock_guard object. The supplied Mutex type shall meet the BasicLockable requirements ([thread.req.lockable.basic]).

explicit lock_guard(mutex_type& m);

Requires: If mutex_type is not a recursive mutex, the calling thread does not own the mutex m.

Effects: m.lock()

Postcondition: &pm == &m

lock_guard(mutex_type& m, adopt_lock_t);

Requires: The calling thread owns the mutex m.

Postcondition: &pm == &m

Throws: Nothing.

~lock_guard();

Effects: pm.unlock()

30.4.2.2 Class template unique_lock [thread.lock.unique]

namespace std {
  template <class Mutex>
  class unique_lock {
  public:
    typedef Mutex mutex_type;

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

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

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

    // [thread.lock.unique.locking], locking:
    void lock();
    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.unique.mod], modifiers:
    void swap(unique_lock& u) noexcept;
    mutex_type* release() noexcept;

    // [thread.lock.unique.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(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
}

An object of type unique_lock controls the ownership of a lockable object within a scope. Ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another unique_lock object. Objects of type unique_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 unique_lock object. The supplied Mutex type shall meet the BasicLockable requirements ([thread.req.lockable.basic]).

Note: unique_lock<Mutex> meets the BasicLockable requirements. If Mutex meets the Lockable requirements ([thread.req.lockable.req]), unique_lock<Mutex> also meets the Lockable requirements; if Mutex meets the TimedLockable requirements ([thread.req.lockable.timed]), unique_lock<Mutex> also meets the TimedLockable requirements.  — end note ]

30.4.2.2.1 unique_lock constructors, destructor, and assignment [thread.lock.unique.cons]

unique_lock() noexcept;

Effects: Constructs an object of type unique_lock.

Postconditions: pm == 0 and owns == false.

explicit unique_lock(mutex_type& m);

Requires: If mutex_type is not a recursive mutex the calling thread does not own the mutex.

Effects: Constructs an object of type unique_lock and calls m.lock().

Postconditions: pm == &m and owns == true.

unique_lock(mutex_type& m, defer_lock_t) noexcept;

Effects: Constructs an object of type unique_lock.

Postconditions: pm == &m and owns == false.

unique_lock(mutex_type& m, try_to_lock_t);

Requires: The supplied Mutex type shall meet the Lockable requirements ([thread.req.lockable.req]). If mutex_type is not a recursive mutex the calling thread does not own the mutex.

Effects: Constructs an object of type unique_lock and calls m.try_lock().

Postconditions: pm == &m and owns == res, where res is the value returned by the call to m.try_lock().

unique_lock(mutex_type& m, adopt_lock_t);

Requires: The calling thread own the mutex.

Effects: Constructs an object of type unique_lock.

Postconditions: pm == &m and owns == true.

Throws: Nothing.

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

Requires: If mutex_type is not a recursive mutex the calling thread does not own the mutex. The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).

Effects: Constructs an object of type unique_lock and calls m.try_lock_until(abs_time).

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

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

Requires: If mutex_type is not a recursive mutex the calling thread does not own the mutex. The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).

Effects: Constructs an object of type unique_lock and calls m.try_lock_for(rel_time).

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

unique_lock(unique_lock&& u) noexcept;

Postconditions: pm == u_p.pm and owns == u_p.owns (where u_p is the state of u just prior to this construction), u.pm == 0 and u.owns == false.

unique_lock& operator=(unique_lock&& u);

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

Postconditions: pm == u_p.pm and owns == u_p.owns (where u_p is the state of u just prior to this construction), u.pm == 0 and u.owns == false.

Note: With a recursive mutex it is possible for both *this and u to own the same mutex before the assignment. In this case, *this will own the mutex after the assignment and u will not.  — end note ]

Throws: Nothing.

~unique_lock();

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

30.4.2.2.2 unique_lock locking [thread.lock.unique.locking]

void lock();

Effects: pm->lock()

Postcondition: owns == true

Throws: Any exception thrown by pm->lock(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.

bool try_lock();

Requires: The supplied Mutex shall meet the Lockable requirements ([thread.req.lockable.req]).

Effects: pm->try_lock()

Returns: The value returned by the call to try_lock().

Postcondition: owns == res, where res is the value returned by the call to try_lock().

Throws: Any exception thrown by pm->try_lock(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of 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);

Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).

Effects: pm->try_lock_until(abs_time)

Returns: The value returned by the call to try_lock_until(abs_time).

Postcondition: owns == res, where res is the value returned by the call to try_lock_until(abs_time).

Throws: Any exception thrown by pm->try_lock_until(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of 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);

Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).

Effects: pm->try_lock_for(rel_time).

Returns: The value returned by the call to try_lock_until(rel_time).

Postcondition: owns == res, where res is the value returned by the call to try_lock_for(rel_time).

Throws: Any exception thrown by pm->try_lock_for(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.

void unlock();

Effects: pm->unlock()

Postcondition: owns == false

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

Error conditions:

  • operation_not_permitted — if on entry owns is false.

30.4.2.2.3 unique_lock modifiers [thread.lock.unique.mod]

void swap(unique_lock& u) noexcept;

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

mutex_type* release() noexcept;

Returns: The previous value of pm.

Postconditions: pm == 0 and owns == false.

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

Effects: x.swap(y)

30.4.2.2.4 unique_lock observers [thread.lock.unique.obs]

bool owns_lock() const noexcept;

Returns: owns

explicit operator bool() const noexcept;

Returns: owns

mutex_type *mutex() const noexcept;

Returns: pm

30.4.2.3 Class template shared_lock [thread.lock.shared]

namespace std {

template <class Mutex>
class shared_lock {
public:
  typedef Mutex mutex_type;

  // Shared locking
  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(shared_lock const&) = delete;
  shared_lock& operator=(shared_lock const&) = delete;

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

  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();

  // Setters
  void swap(shared_lock& u) noexcept;
  mutex_type* release() noexcept;

  // Getters
  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;

}  // std

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 == &m and owns == true.

shared_lock(mutex_type& m, defer_lock_t) noexcept;

Effects: Constructs an object of type shared_lock.

Postconditions: pm == &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 == &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 == &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 == &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 == &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: pm->lock_shared().

Postconditions: owns == true.

Throws: Any exception thrown by pm->lock_shared(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is nullptr. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.

bool try_lock();

Effects: 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 if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is nullptr. system_error with an error condition of 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: 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 if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is nullptr. system_error with an error condition of 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: 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 if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is nullptr. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.

void unlock();

Effects: 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: 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.