30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.2 Locks [thread.lock]

30.4.2.2 Class template unique_lock [thread.lock.unique]

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

    // [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(const unique_lock&) = delete;
    unique_lock& operator=(const unique_lock&) = 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 == addressof(m) and owns == true.

unique_lock(mutex_type& m, defer_lock_t) noexcept;

Effects: Constructs an object of type unique_lock.

Postconditions: pm == addressof(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 == addressof(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 owns the mutex.

Effects: Constructs an object of type unique_lock.

Postconditions: pm == addressof(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 == addressof(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 == addressof(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: As if by pm->lock().

Postconditions: owns == true

Throws: Any exception thrown by pm->lock(). 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();

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

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

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

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

Throws: Any exception thrown by pm->try_lock(). 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);

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

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

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

Postconditions: 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 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);

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

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

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

Postconditions: 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 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().

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.

30.4.2.2.3 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: As if by 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