32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.5 Locks [thread.lock]

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 }; template<class Mutex> void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept; }
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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 shared mutex requirements ([thread.sharedtimedmutex.requirements]).
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[Note 1:
shared_­lock<Mutex> meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
— end note]

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

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shared_lock() noexcept;
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Postconditions: pm == nullptr and owns == false.
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explicit shared_lock(mutex_type& m);
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Preconditions: The calling thread does not own the mutex for any ownership mode.
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Effects: Calls m.lock_­shared().
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Postconditions: pm == addressof(m) and owns == true.
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shared_lock(mutex_type& m, defer_lock_t) noexcept;
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Postconditions: pm == addressof(m) and owns == false.
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shared_lock(mutex_type& m, try_to_lock_t);
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Preconditions: The calling thread does not own the mutex for any ownership mode.
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Effects: Calls m.try_­lock_­shared().
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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);
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Preconditions: The calling thread has shared ownership of the mutex.
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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);
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Preconditions: The calling thread does not own the mutex for any ownership mode.
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Effects: Calls m.try_­lock_­shared_­until(abs_­time).
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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);
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Preconditions: The calling thread does not own the mutex for any ownership mode.
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Effects: Calls m.try_­lock_­shared_­for(rel_­time).
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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();
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Effects: If owns calls pm->unlock_­shared().
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shared_lock(shared_lock&& sl) noexcept;
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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;
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Effects: If owns calls pm->unlock_­shared().
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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]

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void lock();
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Effects: As if by pm->lock_­shared().
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Postconditions: owns == true.
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Throws: Any exception thrown by pm->lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
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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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Effects: As if by pm->try_­lock_­shared().
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Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared().
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Returns: The value returned by the call to pm->try_­lock_­shared().
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Throws: Any exception thrown by pm->try_­lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
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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);
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Effects: As if by pm->try_­lock_­shared_­until(abs_­time).
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Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
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Returns: The value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
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Throws: Any exception thrown by pm->try_­lock_­shared_­until(abs_­time).
system_­error when an exception is required ([thread.req.exception]).
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Error conditions:
  • (14.1)
    operation_­not_­permitted — if pm is nullptr.
  • (14.2)
    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);
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Effects: As if by pm->try_­lock_­shared_­for(rel_­time).
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Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
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Returns: The value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
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Throws: Any exception thrown by pm->try_­lock_­shared_­for(rel_­time).
system_­error when an exception is required ([thread.req.exception]).
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Error conditions:
  • (19.1)
    operation_­not_­permitted — if pm is nullptr.
  • (19.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
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void unlock();
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Effects: As if by pm->unlock_­shared().
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Postconditions: owns == false.
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Throws: system_­error when an exception is required ([thread.req.exception]).
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Error conditions:
  • (23.1)
    operation_­not_­permitted — if on entry owns is false.

32.5.5.5.4 Modifiers [thread.lock.shared.mod]

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

32.5.5.5.5 Observers [thread.lock.shared.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;
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Returns: owns.
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mutex_type* mutex() const noexcept;
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Returns: pm.