32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.5 Locks [thread.lock]

32.5.5.4 Class template unique_­lock [thread.lock.unique]

32.5.5.4.1 General [thread.lock.unique.general]

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; }
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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 Cpp17BasicLockable requirements ([thread.req.lockable.basic]).
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[Note 1:
unique_­lock<Mutex> meets the Cpp17BasicLockable requirements.
If Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]), unique_­lock<Mutex> also meets the Cpp17Lockable requirements; if Mutex meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]), unique_­lock<Mutex> also meets the Cpp17TimedLockable requirements.
— end note]

32.5.5.4.2 Constructors, destructor, and assignment [thread.lock.unique.cons]

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unique_lock() noexcept;
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Postconditions: pm == 0 and owns == false.
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explicit unique_lock(mutex_type& m);
2
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Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
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Effects: Calls m.lock().
4
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Postconditions: pm == addressof(m) and owns == true.
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unique_lock(mutex_type& m, defer_lock_t) noexcept;
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Postconditions: pm == addressof(m) and owns == false.
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unique_lock(mutex_type& m, try_to_lock_t);
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Preconditions: The supplied Mutex type meets the Cpp17Lockable requirements ([thread.req.lockable.req]).
If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
7
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Effects: Calls m.try_­lock().
8
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Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_­lock().
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unique_lock(mutex_type& m, adopt_lock_t);
9
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Preconditions: The calling thread owns the mutex.
10
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Postconditions: pm == addressof(m) and owns == true.
11
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Throws: Nothing.
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template<class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
12
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Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
13
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Effects: Calls m.try_­lock_­until(abs_­time).
14
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Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_­lock_­until(abs_­time).
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template<class Rep, class Period> unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time);
15
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Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
16
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Effects: Calls m.try_­lock_­for(rel_­time).
17
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Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_­lock_­for(rel_­time).
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unique_lock(unique_lock&& u) noexcept;
18
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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.
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unique_lock& operator=(unique_lock&& u);
19
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Effects: If owns calls pm->unlock().
20
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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.
21
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[Note 1:
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]
22
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Throws: Nothing.
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~unique_lock();
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Effects: If owns calls pm->unlock().

32.5.5.4.3 Locking [thread.lock.unique.locking]

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void lock();
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Effects: As if by pm->lock().
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Postconditions: owns == true.
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Throws: Any exception thrown by pm->lock().
system_­error when an exception is required ([thread.req.exception]).
4
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Error conditions:
  • (4.1)
    operation_­not_­permitted — if pm is nullptr.
  • (4.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
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bool try_lock();
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Preconditions: The supplied Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]).
6
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Effects: As if by pm->try_­lock().
7
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Postconditions: owns == res, where res is the value returned by the call to try_­lock().
8
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Returns: The value returned by the call to try_­lock().
9
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Throws: Any exception thrown by pm->try_­lock().
system_­error when an exception is required ([thread.req.exception]).
10
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Error conditions:
  • (10.1)
    operation_­not_­permitted — if pm is nullptr.
  • (10.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
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template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
11
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Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
12
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Effects: As if by pm->try_­lock_­until(abs_­time).
13
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Postconditions: owns == res, where res is the value returned by the call to try_­lock_­until(abs_­time).
14
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Returns: The value returned by the call to try_­lock_­until(abs_­time).
15
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Throws: Any exception thrown by pm->try_­lock_­until().
system_­error when an exception is required ([thread.req.exception]).
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Error conditions:
  • (16.1)
    operation_­not_­permitted — if pm is nullptr.
  • (16.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
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template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
17
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Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
18
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Effects: As if by pm->try_­lock_­for(rel_­time).
19
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Postconditions: owns == res, where res is the value returned by the call to try_­lock_­for(rel_­time).
20
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Returns: The value returned by the call to try_­lock_­for(rel_­time).
21
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Throws: Any exception thrown by pm->try_­lock_­for().
system_­error when an exception is required ([thread.req.exception]).
22
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Error conditions:
  • (22.1)
    operation_­not_­permitted — if pm is nullptr.
  • (22.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
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void unlock();
23
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Effects: As if by pm->unlock().
24
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Postconditions: owns == false.
25
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Throws: system_­error when an exception is required ([thread.req.exception]).
26
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Error conditions:
  • (26.1)
    operation_­not_­permitted — if on entry owns is false.

32.5.5.4.4 Modifiers [thread.lock.unique.mod]

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void swap(unique_lock& u) noexcept;
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Effects: Swaps the data members of *this and u.
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mutex_type* release() noexcept;
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Postconditions: pm == 0 and owns == false.
3
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Returns: The previous value of pm.
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template<class Mutex> void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
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Effects: As if by x.swap(y).

32.5.5.4.5 Observers [thread.lock.unique.obs]

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bool owns_lock() const noexcept;
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Returns: owns.
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explicit operator bool() const noexcept;
2
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Returns: owns.
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mutex_type *mutex() const noexcept;
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Returns: pm.