[thread.lock.shared]

# 32 Concurrency support library [[thread]](./#thread)

## 32.6 Mutual exclusion [[thread.mutex]](thread.mutex#thread.lock.shared)

### 32.6.5 Locks [[thread.lock]](thread.lock#shared)

#### 32.6.5.5 Class template shared_lock [thread.lock.shared]

#### [32.6.5.5.1](#general) General [[thread.lock.shared.general]](thread.lock.shared.general)

[ð](#lib:shared_lock)

namespace std {template<class Mutex>class shared_lock {public:using mutex_type = Mutex; // [[thread.lock.shared.cons]](#cons "32.6.5.5.2 Constructors, destructor, and assignment"), 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 "32.6.5.5.3 Locking"), lockingvoid lock(); // blockingbool 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]](#mod "32.6.5.5.4 Modifiers"), modifiersvoid swap(shared_lock& u) noexcept;
 mutex_type* release() noexcept; // [[thread.lock.shared.obs]](#obs "32.6.5.5.5 Observers"), observersbool owns_lock() const noexcept; explicit operator bool() const noexcept;
 mutex_type* mutex() const noexcept; private: mutex_type* pm; // *exposition only*bool owns; // *exposition only*};}

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8881)

An object of type shared_lock controls the shared ownership of a
lockable object within a scope[.](#general-1.sentence-1)

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[.](#general-1.sentence-2)

Objects of typeshared_lock are not copyable but are movable[.](#general-1.sentence-3)

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]](basic.life "6.8.4 Lifetime")) of the shared_lock object[.](#general-1.sentence-4)

The suppliedMutex type shall meet the [*Cpp17SharedLockable*](thread.req.lockable.shared#:Cpp17SharedLockable "32.2.5.5 Cpp17SharedLockable requirements [thread.req.lockable.shared]") requirements ([[thread.req.lockable.shared]](thread.req.lockable.shared "32.2.5.5 Cpp17SharedLockable requirements"))[.](#general-1.sentence-5)

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8893)

[*Note [1](#general-note-1)*:

shared_lock<Mutex> meets the [*Cpp17Lockable*](thread.req.lockable.req#:Cpp17Lockable "32.2.5.3 Cpp17Lockable requirements [thread.req.lockable.req]") requirements ([[thread.req.lockable.req]](thread.req.lockable.req "32.2.5.3 Cpp17Lockable requirements"))[.](#general-2.sentence-1)

If Mutex meets the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements")),shared_lock<Mutex> also meets the [*Cpp17TimedLockable*](thread.req.lockable.timed#:Cpp17TimedLockable "32.2.5.4 Cpp17TimedLockable requirements [thread.req.lockable.timed]") requirements ([[thread.req.lockable.timed]](thread.req.lockable.timed "32.2.5.4 Cpp17TimedLockable requirements"))[.](#general-2.sentence-2)

â *end note*]

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

[ð](#lib:shared_lock,constructor)

`shared_lock() noexcept;
`

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8911)

*Postconditions*: pm == nullptr and owns == false[.](#cons-1.sentence-1)

[ð](#lib:shared_lock,constructor_)

`explicit shared_lock(mutex_type& m);
`

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8922)

*Effects*: Calls m.lock_shared()[.](#cons-2.sentence-1)

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8926)

*Postconditions*: pm == addressof(m) and owns == true[.](#cons-3.sentence-1)

[ð](#lib:shared_lock,constructor__)

`shared_lock(mutex_type& m, defer_lock_t) noexcept;
`

[4](#cons-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8937)

*Postconditions*: pm == addressof(m) and owns == false[.](#cons-4.sentence-1)

[ð](#lib:shared_lock,constructor___)

`shared_lock(mutex_type& m, try_to_lock_t);
`

[5](#cons-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8948)

*Effects*: Calls m.try_lock_shared()[.](#cons-5.sentence-1)

[6](#cons-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8952)

*Postconditions*: pm == addressof(m) and owns == res where res is the
value returned by the call to m.try_lock_shared()[.](#cons-6.sentence-1)

[ð](#lib:shared_lock,constructor____)

`shared_lock(mutex_type& m, adopt_lock_t);
`

[7](#cons-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8965)

*Preconditions*: The calling thread holds a shared lock on m[.](#cons-7.sentence-1)

[8](#cons-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8969)

*Postconditions*: pm == addressof(m) and owns == true[.](#cons-8.sentence-1)

[ð](#lib:shared_lock,constructor_____)

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

[9](#cons-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8982)

*Preconditions*: Mutex meets the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements"))[.](#cons-9.sentence-1)

[10](#cons-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8987)

*Effects*: Calls m.try_lock_shared_until(abs_time)[.](#cons-10.sentence-1)

[11](#cons-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L8991)

*Postconditions*: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_until(abs_time)[.](#cons-11.sentence-1)

[ð](#lib:shared_lock,constructor______)

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

[12](#cons-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9006)

*Preconditions*: Mutex meets the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements"))[.](#cons-12.sentence-1)

[13](#cons-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9011)

*Effects*: Calls m.try_lock_shared_for(rel_time)[.](#cons-13.sentence-1)

[14](#cons-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9015)

*Postconditions*: pm == addressof(m) and owns == res where res is
the value returned by the call to m.try_lock_shared_for(rel_time)[.](#cons-14.sentence-1)

[ð](#lib:shared_lock,destructor)

`~shared_lock();
`

[15](#cons-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9028)

*Effects*: If owns calls pm->unlock_shared()[.](#cons-15.sentence-1)

[ð](#lib:shared_lock,constructor_______)

`shared_lock(shared_lock&& sl) noexcept;
`

[16](#cons-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9039)

*Postconditions*: pm == sl_p.pm and owns == sl_p.owns (wheresl_p is the state of sl just prior to this construction),sl.pm == nullptr and sl.owns == false[.](#cons-16.sentence-1)

[ð](#lib:operator=,shared_lock)

`shared_lock& operator=(shared_lock&& sl) noexcept;
`

[17](#cons-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9052)

*Effects*: Equivalent to: shared_lock(std​::​move(sl)).swap(*this)

[18](#cons-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9056)

*Returns*: *this[.](#cons-18.sentence-1)

#### [32.6.5.5.3](#locking) Locking [[thread.lock.shared.locking]](thread.lock.shared.locking)

[ð](#lib:lock,shared_lock)

`void lock();
`

[1](#locking-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9069)

*Effects*: As if by pm->lock_shared()[.](#locking-1.sentence-1)

[2](#locking-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9073)

*Postconditions*: owns == true[.](#locking-2.sentence-1)

[3](#locking-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9077)

*Throws*: Any exception thrown by pm->lock_shared()[.](#locking-3.sentence-1)

system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#locking-3.sentence-2)

[4](#locking-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9082)

*Error conditions*: 

- [(4.1)](#locking-4.1)

  operation_not_permitted â if pm is nullptr[.](#locking-4.1.sentence-1)

- [(4.2)](#locking-4.2)

  resource_deadlock_would_occur â if on entry owns istrue[.](#locking-4.2.sentence-1)

[ð](#lib:try_lock,shared_lock)

`bool try_lock();
`

[5](#locking-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9097)

*Effects*: As if by pm->try_lock_shared()[.](#locking-5.sentence-1)

[6](#locking-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9101)

*Postconditions*: owns == res, where res is the value returned by
the call to pm->try_lock_shared()[.](#locking-6.sentence-1)

[7](#locking-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9106)

*Returns*: The value returned by the call to pm->try_lock_shared()[.](#locking-7.sentence-1)

[8](#locking-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9110)

*Throws*: Any exception thrown by pm->try_lock_shared()[.](#locking-8.sentence-1)

system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#locking-8.sentence-2)

[9](#locking-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9115)

*Error conditions*: 

- [(9.1)](#locking-9.1)

  operation_not_permitted â if pm is nullptr[.](#locking-9.1.sentence-1)

- [(9.2)](#locking-9.2)

  resource_deadlock_would_occur â if on entry owns istrue[.](#locking-9.2.sentence-1)

[ð](#lib:try_lock_until,shared_lock)

`template<class Clock, class Duration>
  bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
`

[10](#locking-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9131)

*Preconditions*: Mutex meets the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements"))[.](#locking-10.sentence-1)

[11](#locking-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9136)

*Effects*: As if by pm->try_lock_shared_until(abs_time)[.](#locking-11.sentence-1)

[12](#locking-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9140)

*Postconditions*: owns == res, where res is the value returned by
the call to pm->try_lock_shared_until(abs_time)[.](#locking-12.sentence-1)

[13](#locking-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9145)

*Returns*: The value returned by the call topm->try_lock_shared_until(abs_time)[.](#locking-13.sentence-1)

[14](#locking-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9150)

*Throws*: Any exception thrown by pm->try_lock_shared_until(abs_time)[.](#locking-14.sentence-1)

system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#locking-14.sentence-2)

[15](#locking-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9155)

*Error conditions*: 

- [(15.1)](#locking-15.1)

  operation_not_permitted â if pm is nullptr[.](#locking-15.1.sentence-1)

- [(15.2)](#locking-15.2)

  resource_deadlock_would_occur â if on entry owns istrue[.](#locking-15.2.sentence-1)

[ð](#lib:try_lock_for,shared_lock)

`template<class Rep, class Period>
  bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
`

[16](#locking-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9171)

*Preconditions*: Mutex meets the [*Cpp17SharedTimedLockable*](thread.req.lockable.shared.timed#:Cpp17SharedTimedLockable "32.2.5.6 Cpp17SharedTimedLockable requirements [thread.req.lockable.shared.timed]") requirements ([[thread.req.lockable.shared.timed]](thread.req.lockable.shared.timed "32.2.5.6 Cpp17SharedTimedLockable requirements"))[.](#locking-16.sentence-1)

[17](#locking-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9176)

*Effects*: As if by pm->try_lock_shared_for(rel_time)[.](#locking-17.sentence-1)

[18](#locking-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9180)

*Postconditions*: owns == res, where res is the value returned by the call to pm->try_lock_shared_for(rel_time)[.](#locking-18.sentence-1)

[19](#locking-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9184)

*Returns*: The value returned by the call to pm->try_lock_shared_for(rel_time)[.](#locking-19.sentence-1)

[20](#locking-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9188)

*Throws*: Any exception thrown by pm->try_lock_shared_for(rel_time)[.](#locking-20.sentence-1)

system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#locking-20.sentence-2)

[21](#locking-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9192)

*Error conditions*: 

- [(21.1)](#locking-21.1)

  operation_not_permitted â if pm is nullptr[.](#locking-21.1.sentence-1)

- [(21.2)](#locking-21.2)

  resource_deadlock_would_occur â if on entry owns istrue[.](#locking-21.2.sentence-1)

[ð](#lib:unlock,shared_lock)

`void unlock();
`

[22](#locking-22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9207)

*Effects*: As if by pm->unlock_shared()[.](#locking-22.sentence-1)

[23](#locking-23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9211)

*Postconditions*: owns == false[.](#locking-23.sentence-1)

[24](#locking-24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9215)

*Throws*: system_error when an exception is required ([[thread.req.exception]](thread.req.exception "32.2.2 Exceptions"))[.](#locking-24.sentence-1)

[25](#locking-25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9219)

*Error conditions*: 

- [(25.1)](#locking-25.1)

operation_not_permitted â if on entry owns isfalse[.](#locking-25.sentence-1)

#### [32.6.5.5.4](#mod) Modifiers [[thread.lock.shared.mod]](thread.lock.shared.mod)

[ð](#lib:swap,shared_lock)

`void swap(shared_lock& sl) noexcept;
`

[1](#mod-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9235)

*Effects*: Swaps the data members of *this and sl[.](#mod-1.sentence-1)

[ð](#lib:release,shared_lock)

`mutex_type* release() noexcept;
`

[2](#mod-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9246)

*Postconditions*: pm == nullptr and owns == false[.](#mod-2.sentence-1)

[3](#mod-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9250)

*Returns*: The previous value of pm[.](#mod-3.sentence-1)

[ð](#lib:swap,shared_lock_)

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

[4](#mod-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9262)

*Effects*: As if by x.swap(y)[.](#mod-4.sentence-1)

#### [32.6.5.5.5](#obs) Observers [[thread.lock.shared.obs]](thread.lock.shared.obs)

[ð](#lib:owns_lock,shared_lock)

`bool owns_lock() const noexcept;
`

[1](#obs-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9275)

*Returns*: owns[.](#obs-1.sentence-1)

[ð](#lib:operator_bool,shared_lock)

`explicit operator bool() const noexcept;
`

[2](#obs-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9286)

*Returns*: owns[.](#obs-2.sentence-1)

[ð](#lib:mutex,shared_lock)

`mutex_type* mutex() const noexcept;
`

[3](#obs-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L9297)

*Returns*: pm[.](#obs-3.sentence-1)
