32 Concurrency support library [thread]

32.6 Mutual exclusion [thread.mutex]

32.6.5 Locks [thread.lock]

32.6.5.5 Class template shared_lock [thread.lock.shared]

32.6.5.5.1 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]

32.6.5.5.2 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.