30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.4 Locks [thread.lock]

30.4.4.4 Class template shared_­lock [thread.lock.shared]

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;
}
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 satisfy the shared mutex requirements ([thread.sharedtimedmutex.requirements]).
[Note
:
shared_­lock<Mutex> meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
end note
]

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

shared_lock() noexcept;
Effects: Constructs an object of type shared_­lock.
Ensures: pm == nullptr and owns == false.
explicit shared_lock(mutex_type& m);
Requires: The calling thread does not own the mutex for any ownership mode.
Effects: Constructs an object of type shared_­lock and calls m.lock_­shared().
Ensures: pm == addressof(m) and owns == true.
shared_lock(mutex_type& m, defer_lock_t) noexcept;
Effects: Constructs an object of type shared_­lock.
Ensures: pm == addressof(m) and owns == false.
shared_lock(mutex_type& m, try_to_lock_t);
Requires: The calling thread does not own the mutex for any ownership mode.
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared().
Ensures: 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);
Requires: The calling thread has shared ownership of the mutex.
Effects: Constructs an object of type shared_­lock.
Ensures: pm == addressof(m) and owns == true.
template<class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
Requires: The calling thread does not own the mutex for any ownership mode.
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­until(abs_­time).
Ensures: 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);
Requires: The calling thread does not own the mutex for any ownership mode.
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­for(rel_­time).
Ensures: 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;
Ensures: 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().
Ensures: 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.

30.4.4.4.2 Locking [thread.lock.shared.locking]

void lock();
Effects: As if by pm->lock_­shared().
Ensures: 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().
Returns: The value returned by the call to pm->try_­lock_­shared().
Ensures: owns == res, where res is 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);
Effects: As if by pm->try_­lock_­shared_­until(abs_­time).
Returns: The value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
Ensures: owns == res, where res is 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);
Effects: As if by pm->try_­lock_­shared_­for(rel_­time).
Returns: The value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
Ensures: owns == res, where res is 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().
Ensures: owns == false.
Throws: system_­error when an exception is required ([thread.req.exception]).
Error conditions:
  • operation_­not_­permitted — if on entry owns is false.

30.4.4.4.3 Modifiers [thread.lock.shared.mod]

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

30.4.4.4.4 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.