33 Thread support library [thread]

33.4 Mutual exclusion [thread.mutex]

33.4.4 Locks [thread.lock]

33.4.4.3 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> unique_lock(unique_lock<Mutex>) -> unique_lock<Mutex>;

  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 of the unique_­lock object.
The supplied Mutex type shall meet the BasicLockable requirements.
[Note
:
unique_­lock<Mutex> meets the BasicLockable requirements.
If Mutex meets the Lockable requirements, unique_­lock<Mutex> also meets the Lockable requirements; if Mutex meets the TimedLockable requirements, unique_­lock<Mutex> also meets the TimedLockable requirements.
end note
]

33.4.4.3.1 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.
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.
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.
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().

33.4.4.3.2 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.
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.
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.
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.

33.4.4.3.3 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).

33.4.4.3.4 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.