32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.4 Locks [thread.lock]

1
#
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
:
Locks are intended to ease the burden of unlocking the lockable object under both normal and exceptional circumstances.
— end note
]
2
#
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 { };
}

32.5.4.1 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
  };
}
1
#
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);
2
#
Preconditions: If mutex_­type is not a recursive mutex, the calling thread does not own the mutex m.
3
#
Effects: Initializes pm with m.
Calls m.lock().
🔗
lock_guard(mutex_type& m, adopt_lock_t);
4
#
Preconditions: The calling thread owns the mutex m.
5
#
Effects: Initializes pm with m.
6
#
Throws: Nothing.
🔗
~lock_guard();
7
#
Effects: As if by pm.unlock().

32.5.4.2 Class template scoped_­lock [thread.lock.scoped]

namespace std {
  template<class... MutexTypes>
  class scoped_lock {
  public:
    using mutex_type = Mutex;   // If MutexTypes... consists of the single type Mutex

    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
  };
}
1
#
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.
When sizeof...(MutexTypes) is 1, the supplied Mutex type shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).
Otherwise, each of the mutex types shall meet the Cpp17Lockable requirements ([thread.req.lockable.req]).
🔗
explicit scoped_lock(MutexTypes&... m);
2
#
Preconditions: If a MutexTypes type is not a recursive mutex, the calling thread does not own the corresponding mutex element of m.
3
#
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);
4
#
Preconditions: The calling thread owns all the mutexes in m.
5
#
Effects: Initializes pm with tie(m...).
6
#
Throws: Nothing.
🔗
~scoped_lock();
7
#
Effects: For all i in [0, sizeof...(MutexTypes)), get<i>(pm).unlock().

32.5.4.3 Class template unique_­lock [thread.lock.unique]

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
  };

  template<class Mutex>
    void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
}
1
#
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]).
2
#
[Note
:
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
]

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

🔗
unique_lock() noexcept;
1
#
Postconditions: pm == 0 and owns == false.
🔗
explicit unique_lock(mutex_type& m);
2
#
Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
3
#
Effects: Calls m.lock().
4
#
Postconditions: pm == addressof(m) and owns == true.
🔗
unique_lock(mutex_type& m, defer_lock_t) noexcept;
5
#
Postconditions: pm == addressof(m) and owns == false.
🔗
unique_lock(mutex_type& m, try_to_lock_t);
6
#
Preconditions: The supplied Mutex type meets the Cpp17Lockable requirements ([thread.req.lockable.req]).
If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
7
#
Effects: Calls m.try_­lock().
8
#
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);
9
#
Preconditions: The calling thread owns the mutex.
10
#
Postconditions: pm == addressof(m) and owns == true.
11
#
Throws: Nothing.
🔗
template<class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
12
#
Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
13
#
Effects: Calls m.try_­lock_­until(abs_­time).
14
#
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);
15
#
Preconditions: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
16
#
Effects: Calls m.try_­lock_­for(rel_­time).
17
#
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;
18
#
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);
19
#
Effects: If owns calls pm->unlock().
20
#
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.
21
#
[Note
:
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
]
22
#
Throws: Nothing.
🔗
~unique_lock();
23
#
Effects: If owns calls pm->unlock().

32.5.4.3.2 Locking [thread.lock.unique.locking]

🔗
void lock();
1
#
Effects: As if by pm->lock().
2
#
Postconditions: owns == true.
3
#
Throws: Any exception thrown by pm->lock().
system_­error when an exception is required ([thread.req.exception]).
4
#
Error conditions:
  • (4.1)
    operation_­not_­permitted — if pm is nullptr.
  • (4.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
bool try_lock();
5
#
Preconditions: The supplied Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]).
6
#
Effects: As if by pm->try_­lock().
7
#
Returns: The value returned by the call to try_­lock().
8
#
Postconditions: owns == res, where res is the value returned by the call to try_­lock().
9
#
Throws: Any exception thrown by pm->try_­lock().
system_­error when an exception is required ([thread.req.exception]).
10
#
Error conditions:
  • (10.1)
    operation_­not_­permitted — if pm is nullptr.
  • (10.2)
    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);
11
#
Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
12
#
Effects: As if by pm->try_­lock_­until(abs_­time).
13
#
Returns: The value returned by the call to try_­lock_­until(abs_­time).
14
#
Postconditions: owns == res, where res is the value returned by the call to try_­lock_­until(abs_­time).
15
#
Throws: Any exception thrown by pm->try_­lock_­until().
system_­error when an exception is required ([thread.req.exception]).
16
#
Error conditions:
  • (16.1)
    operation_­not_­permitted — if pm is nullptr.
  • (16.2)
    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);
17
#
Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
18
#
Effects: As if by pm->try_­lock_­for(rel_­time).
19
#
Returns: The value returned by the call to try_­lock_­for(rel_­time).
20
#
Postconditions: owns == res, where res is the value returned by the call to try_­lock_­for(rel_­time).
21
#
Throws: Any exception thrown by pm->try_­lock_­for().
system_­error when an exception is required ([thread.req.exception]).
22
#
Error conditions:
  • (22.1)
    operation_­not_­permitted — if pm is nullptr.
  • (22.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
void unlock();
23
#
Effects: As if by pm->unlock().
24
#
Postconditions: owns == false.
25
#
Throws: system_­error when an exception is required ([thread.req.exception]).
26
#
Error conditions:
  • (26.1)
    operation_­not_­permitted — if on entry owns is false.

32.5.4.3.3 Modifiers [thread.lock.unique.mod]

🔗
void swap(unique_lock& u) noexcept;
1
#
Effects: Swaps the data members of *this and u.
🔗
mutex_type* release() noexcept;
2
#
Returns: The previous value of pm.
3
#
Postconditions: pm == 0 and owns == false.
🔗
template<class Mutex> void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
4
#
Effects: As if by x.swap(y).

32.5.4.3.4 Observers [thread.lock.unique.obs]

🔗
bool owns_lock() const noexcept;
1
#
Returns: owns.
🔗
explicit operator bool() const noexcept;
2
#
Returns: owns.
🔗
mutex_type *mutex() const noexcept;
3
#
Returns: pm.

32.5.4.4 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;
}
1
#
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]).
2
#
[Note
:
shared_­lock<Mutex> meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).
— end note
]

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

🔗
shared_lock() noexcept;
1
#
Postconditions: pm == nullptr and owns == false.
🔗
explicit shared_lock(mutex_type& m);
2
#
Preconditions: The calling thread does not own the mutex for any ownership mode.
3
#
Effects: Calls m.lock_­shared().
4
#
Postconditions: pm == addressof(m) and owns == true.
🔗
shared_lock(mutex_type& m, defer_lock_t) noexcept;
5
#
Postconditions: pm == addressof(m) and owns == false.
🔗
shared_lock(mutex_type& m, try_to_lock_t);
6
#
Preconditions: The calling thread does not own the mutex for any ownership mode.
7
#
Effects: Calls m.try_­lock_­shared().
8
#
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);
9
#
Preconditions: The calling thread has shared ownership of the mutex.
10
#
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);
11
#
Preconditions: The calling thread does not own the mutex for any ownership mode.
12
#
Effects: Calls m.try_­lock_­shared_­until(abs_­time).
13
#
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);
14
#
Preconditions: The calling thread does not own the mutex for any ownership mode.
15
#
Effects: Calls m.try_­lock_­shared_­for(rel_­time).
16
#
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();
17
#
Effects: If owns calls pm->unlock_­shared().
🔗
shared_lock(shared_lock&& sl) noexcept;
18
#
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;
19
#
Effects: If owns calls pm->unlock_­shared().
20
#
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.

32.5.4.4.2 Locking [thread.lock.shared.locking]

🔗
void lock();
1
#
Effects: As if by pm->lock_­shared().
2
#
Postconditions: owns == true.
3
#
Throws: Any exception thrown by pm->lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
4
#
Error conditions:
  • (4.1)
    operation_­not_­permitted — if pm is nullptr.
  • (4.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
bool try_lock();
5
#
Effects: As if by pm->try_­lock_­shared().
6
#
Returns: The value returned by the call to pm->try_­lock_­shared().
7
#
Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared().
8
#
Throws: Any exception thrown by pm->try_­lock_­shared().
system_­error when an exception is required ([thread.req.exception]).
9
#
Error conditions:
  • (9.1)
    operation_­not_­permitted — if pm is nullptr.
  • (9.2)
    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);
10
#
Effects: As if by pm->try_­lock_­shared_­until(abs_­time).
11
#
Returns: The value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
12
#
Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­until(abs_­time).
13
#
Throws: Any exception thrown by pm->try_­lock_­shared_­until(abs_­time).
system_­error when an exception is required ([thread.req.exception]).
14
#
Error conditions:
  • (14.1)
    operation_­not_­permitted — if pm is nullptr.
  • (14.2)
    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);
15
#
Effects: As if by pm->try_­lock_­shared_­for(rel_­time).
16
#
Returns: The value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
17
#
Postconditions: owns == res, where res is the value returned by the call to pm->try_­lock_­shared_­for(rel_­time).
18
#
Throws: Any exception thrown by pm->try_­lock_­shared_­for(rel_­time).
system_­error when an exception is required ([thread.req.exception]).
19
#
Error conditions:
  • (19.1)
    operation_­not_­permitted — if pm is nullptr.
  • (19.2)
    resource_­deadlock_­would_­occur — if on entry owns is true.
🔗
void unlock();
20
#
Effects: As if by pm->unlock_­shared().
21
#
Postconditions: owns == false.
22
#
Throws: system_­error when an exception is required ([thread.req.exception]).
23
#
Error conditions:
  • (23.1)
    operation_­not_­permitted — if on entry owns is false.

32.5.4.4.3 Modifiers [thread.lock.shared.mod]

🔗
void swap(shared_lock& sl) noexcept;
1
#
Effects: Swaps the data members of *this and sl.
🔗
mutex_type* release() noexcept;
2
#
Returns: The previous value of pm.
3
#
Postconditions: pm == nullptr and owns == false.
🔗
template<class Mutex> void swap(shared_lock<Mutex>& x, shared_lock<Mutex>& y) noexcept;
4
#
Effects: As if by x.swap(y).

32.5.4.4.4 Observers [thread.lock.shared.obs]

🔗
bool owns_lock() const noexcept;
1
#
Returns: owns.
🔗
explicit operator bool() const noexcept;
2
#
Returns: owns.
🔗
mutex_type* mutex() const noexcept;
3
#
Returns: pm.