32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.5 Locks [thread.lock]

32.5.5.1 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.
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 { }; }

32.5.5.2 Class template lock_­guard [thread.lock.guard]

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]).
explicit lock_guard(mutex_type& m);
Effects: Initializes pm with m.
Calls m.lock().
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.
~lock_guard();
Effects: Equivalent to: pm.unlock()

32.5.5.3 Class template scoped_­lock [thread.lock.scoped]

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

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 }; }
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]

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

unique_lock() noexcept;
Postconditions: pm == 0 and owns == false.
explicit unique_lock(mutex_type& m);
Effects: Calls m.lock().
Postconditions: pm == addressof(m) and owns == true.
unique_lock(mutex_type& m, defer_lock_t) noexcept;
Postconditions: pm == addressof(m) and owns == false.
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().
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.
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).
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).
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 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.
~unique_lock();
Effects: If owns calls pm->unlock().

32.5.5.4.3 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();
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 the call to try_­lock().
Returns: 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);
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 the call to try_­lock_­until(abs_­time).
Returns: 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);
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 the call to try_­lock_­for(rel_­time).
Returns: 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.

32.5.5.4.4 Modifiers [thread.lock.unique.mod]

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

32.5.5.4.5 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.

32.5.5.5 Class template shared_­lock [thread.lock.shared]

32.5.5.5.1 General [thread.lock.shared.general]

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 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 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]

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

shared_lock() noexcept;
Postconditions: pm == nullptr and owns == false.
explicit shared_lock(mutex_type& m);
Effects: Calls m.lock_­shared().
Postconditions: pm == addressof(m) and owns == true.
shared_lock(mutex_type& m, defer_lock_t) noexcept;
Postconditions: pm == addressof(m) and owns == false.
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().
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.
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).
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).
~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.

32.5.5.5.3 Locking [thread.lock.shared.locking]

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:
  • operation_­not_­permitted — if pm is nullptr.
  • resource_­deadlock_­would_­occur — if on entry owns is true.
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:
  • 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);
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:
  • 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);
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:
  • 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_­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.

32.5.5.5.4 Modifiers [thread.lock.shared.mod]

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

32.5.5.5.5 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.