32 Concurrency support library [thread]

32.6 Mutual exclusion [thread.mutex]

32.6.5 Locks [thread.lock]

32.6.5.1 General [thread.lock.general]

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 1:

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.

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

32.6.5.2 Class template lock_guard [thread.lock.guard]

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namespace std { template<class Mutex> class lock_guard { public: using mutex_type = Mutex; explicit lock_guard(mutex_type& m); lock_guard(mutex_type& m, adopt_lock_t); ~lock_guard(); lock_guard(const lock_guard&) = delete; lock_guard& operator=(const lock_guard&) = 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 .

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 Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

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explicit lock_guard(mutex_type& m);

Effects: Initializes pm with m.

Calls m.lock().

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lock_guard(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a non-shared lock on m.

Effects: Initializes pm with m.

Throws: Nothing.

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~lock_guard();

Effects: Equivalent to: pm.unlock()

32.6.5.3 Class template scoped_lock [thread.lock.scoped]

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namespace std { template<class... MutexTypes> class scoped_lock { public: using mutex_type = see below; // Only if sizeof...(MutexTypes) == 1 is true explicit scoped_lock(MutexTypes&... m); explicit scoped_lock(adopt_lock_t, MutexTypes&... m); ~scoped_lock(); scoped_lock(const scoped_lock&) = delete; scoped_lock& operator=(const scoped_lock&) = delete; private: tuple<MutexTypes&...> pm; // exposition only }; }

An object of type scoped_lock controls the ownership of lockable objects within a scope.

A scoped_lock object maintains ownership of lockable objects throughout the scoped_lock object's lifetime .

The behavior of a program is undefined if the lockable objects referenced by pm do not exist for the entire lifetime of the scoped_lock object.

(1.1)
If sizeof...(MutexTypes) is one, let Mutex denote the sole type constituting the pack MutexTypes.

Mutex shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

The member typedef-name mutex_type denotes the same type as Mutex.
(1.2)
Otherwise, all types in the template parameter pack MutexTypes shall meet the Cpp17Lockable requirements ([thread.req.lockable.req]) and there is no member mutex_type.

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explicit scoped_lock(MutexTypes&... m);

Effects: Initializes pm with tie(m...).

Then if sizeof...(MutexTypes) is 0, no effects.

Otherwise if sizeof...(MutexTypes) is 1, then m.lock().

Otherwise, lock(m...).

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explicit scoped_lock(adopt_lock_t, MutexTypes&... m);

Preconditions: The calling thread holds a non-shared lock on each element of m.

Effects: Initializes pm with tie(m...).

Throws: Nothing.

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~scoped_lock();

Effects: For all i in [0, sizeof...(MutexTypes)), get<i>(pm).unlock().

32.6.5.4 Class template unique_lock [thread.lock.unique]

32.6.5.4.1 General [thread.lock.unique.general]

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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 }; }

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]).

[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]

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

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unique_lock() noexcept;

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

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explicit unique_lock(mutex_type& m);

Effects: Calls m.lock().

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

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unique_lock(mutex_type& m, defer_lock_t) noexcept;

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

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unique_lock(mutex_type& m, try_to_lock_t);

Preconditions: The supplied Mutex type meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

Effects: Calls m.try_lock().

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);

Preconditions: The calling thread holds a non-shared lock on m.

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

Throws: Nothing.

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: 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).

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: 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).

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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.

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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 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]

Throws: Nothing.

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~unique_lock();

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

32.6.5.4.3 Locking [thread.lock.unique.locking]

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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:

(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();

Preconditions: The supplied Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

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

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

Returns: The value returned by pm->try_lock().

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

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

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);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

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

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

Returns: The value returned by pm->try_lock_until(abs_time).

Throws: Any exception thrown by pm->try_lock_until(abstime).

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

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);

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

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

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

Returns: The value returned by pm->try_lock_for(rel_time).

Throws: Any exception thrown by pm->try_lock_for(rel_time).

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

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();

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

Postconditions: owns == false.

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

Error conditions:

(26.1)
operation_not_permitted — if on entry owns is false.

32.6.5.4.4 Modifiers [thread.lock.unique.mod]

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void swap(unique_lock& u) noexcept;

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

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mutex_type* release() noexcept;

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

Returns: The previous value of pm.

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

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

32.6.5.4.5 Observers [thread.lock.unique.obs]

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bool owns_lock() const noexcept;

Returns: owns.

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explicit operator bool() const noexcept;

Returns: owns.

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mutex_type *mutex() const noexcept;

Returns: pm.

32.6.5.5 Class template shared_lock [thread.lock.shared]

32.6.5.5.1 General [thread.lock.shared.general]

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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 }; }

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 supplied Mutex type shall meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

[Note 1:

shared_lock<Mutex> meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

If Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]), shared_lock<Mutex> also meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

— end note]

32.6.5.5.2 Constructors, destructor, and assignment [thread.lock.shared.cons]

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shared_lock() noexcept;

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

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explicit shared_lock(mutex_type& m);

Effects: Calls m.lock_shared().

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

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shared_lock(mutex_type& m, defer_lock_t) noexcept;

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

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shared_lock(mutex_type& m, try_to_lock_t);

Effects: 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().

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shared_lock(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a shared lock on m.

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

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: 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).

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: 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).

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~shared_lock();

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

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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.

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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.

32.6.5.5.3 Locking [thread.lock.shared.locking]

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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:

(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();

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

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

Returns: 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:

(9.1)
operation_not_permitted — if pm is nullptr.
(9.2)
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);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by 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).

Returns: 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:

(15.1)
operation_not_permitted — if pm is nullptr.
(15.2)
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);

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by 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).

Returns: 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:

(21.1)
operation_not_permitted — if pm is nullptr.
(21.2)
resource_deadlock_would_occur — if on entry owns is true.

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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:

(25.1)
operation_not_permitted — if on entry owns is false.

32.6.5.5.4 Modifiers [thread.lock.shared.mod]

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void swap(shared_lock& sl) noexcept;

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

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mutex_type* release() noexcept;

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

Returns: The previous value of pm.

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

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

32.6.5.5.5 Observers [thread.lock.shared.obs]

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bool owns_lock() const noexcept;

Returns: owns.

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explicit operator bool() const noexcept;

Returns: owns.

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mutex_type* mutex() const noexcept;

Returns: pm.