33 Thread support library [thread]

33.4 Mutual exclusion [thread.mutex]

33.4.4 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 { };
}

33.4.4.1 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
  };

  template<class Mutex> lock_guard(lock_guard<Mutex>) -> lock_guard<Mutex>;
}
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 ([basic.life]).
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 BasicLockable requirements ([thread.req.lockable.basic]).
explicit lock_guard(mutex_type& m);
2
#
Requires: If mutex_­type is not a recursive mutex, the calling thread does not own the mutex m.
3
#
Effects: As if by m.lock().
4
#
Postconditions:&pm == &m
lock_guard(mutex_type& m, adopt_lock_t);
5
#
Requires: The calling thread owns the mutex m.
6
#
Postconditions:&pm == &m
7
#
Throws: Nothing.
~lock_guard();
8
#
Effects: As if by pm.unlock().

33.4.4.2 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(MutexTypes&... m, adopt_lock_t);
    ~scoped_lock();

    scoped_lock(const scoped_lock&) = delete;
    scoped_lock& operator=(const scoped_lock&) = delete;

  private:
    tuple<MutexTypes&...> pm; // exposition only
  };

  template<class... MutexTypes>
    scoped_lock(scoped_lock<MutexTypes...>) -> scoped_lock<MutexTypes...>;
}
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 ([basic.life]).
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 BasicLockable requirements ([thread.req.lockable.basic]).
Otherwise, each of the mutex types shall meet the Lockable requirements ([thread.req.lockable.req]).
explicit scoped_lock(MutexTypes&... m);
2
#
Requires: 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(MutexTypes&... m, adopt_lock_t);
4
#
Requires: 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().

33.4.4.3 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> unique_lock(unique_lock<Mutex>) -> unique_lock<Mutex>;

  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 BasicLockable requirements ([thread.req.lockable.basic]).
2
#
[ Note
:
unique_­lock<Mutex> meets the BasicLockable requirements.
If Mutex meets the Lockable requirements ([thread.req.lockable.req]), unique_­lock<Mutex> also meets the Lockable requirements; if Mutex meets the TimedLockable requirements ([thread.req.lockable.timed]), unique_­lock<Mutex> also meets the TimedLockable requirements.
— end note
 ]

33.4.4.3.1 unique_­lock constructors, destructor, and assignment [thread.lock.unique.cons]

unique_lock() noexcept;
1
#
Effects: Constructs an object of type unique_­lock.
2
#
Postconditions: pm == 0 and owns == false.
explicit unique_lock(mutex_type& m);
3
#
Requires: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
4
#
Effects: Constructs an object of type unique_­lock and calls m.lock().
5
#
Postconditions: pm == addressof(m) and owns == true.
unique_lock(mutex_type& m, defer_lock_t) noexcept;
6
#
Effects: Constructs an object of type unique_­lock.
7
#
Postconditions: pm == addressof(m) and owns == false.
unique_lock(mutex_type& m, try_to_lock_t);
8
#
Requires: The supplied Mutex type shall meet the Lockable requirements ([thread.req.lockable.req]).
If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
9
#
Effects: Constructs an object of type unique_­lock and calls m.try_­lock().
10
#
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);
11
#
Requires: The calling thread owns the mutex.
12
#
Effects: Constructs an object of type unique_­lock.
13
#
Postconditions: pm == addressof(m) and owns == true.
14
#
Throws: Nothing.
template <class Clock, class Duration> unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
15
#
Requires: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
16
#
Effects: Constructs an object of type unique_­lock and calls m.try_­lock_­until(abs_­time).
17
#
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);
18
#
Requires: If mutex_­type is not a recursive mutex the calling thread does not own the mutex.
The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
19
#
Effects: Constructs an object of type unique_­lock and calls m.try_­lock_­for(rel_­time).
20
#
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;
21
#
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);
22
#
Effects: If owns calls pm->unlock().
23
#
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.
24
#
[ 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
 ]
25
#
Throws: Nothing.
~unique_lock();
26
#
Effects: If owns calls pm->unlock().

33.4.4.3.2 unique_­lock 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
#
Requires: The supplied Mutex shall meet the Lockable 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
#
Requires: The supplied Mutex type shall meet the TimedLockable 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
#
Requires: The supplied Mutex type shall meet the TimedLockable 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_­until(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.

33.4.4.3.3 unique_­lock 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).

33.4.4.3.4 unique_­lock 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.

33.4.4.4 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> shared_lock(shared_lock<Mutex>) -> shared_lock<Mutex>;

  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 TimedLockable requirements ([thread.req.lockable.timed]).
— end note
 ]

33.4.4.4.1 shared_­lock constructors, destructor, and assignment [thread.lock.shared.cons]

shared_lock() noexcept;
1
#
Effects: Constructs an object of type shared_­lock.
2
#
Postconditions: pm == nullptr and owns == false.
explicit shared_lock(mutex_type& m);
3
#
Requires: The calling thread does not own the mutex for any ownership mode.
4
#
Effects: Constructs an object of type shared_­lock and calls m.lock_­shared().
5
#
Postconditions: pm == addressof(m) and owns == true.
shared_lock(mutex_type& m, defer_lock_t) noexcept;
6
#
Effects: Constructs an object of type shared_­lock.
7
#
Postconditions: pm == addressof(m) and owns == false.
shared_lock(mutex_type& m, try_to_lock_t);
8
#
Requires: The calling thread does not own the mutex for any ownership mode.
9
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared().
10
#
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);
11
#
Requires: The calling thread has shared ownership of the mutex.
12
#
Effects: Constructs an object of type shared_­lock.
13
#
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);
14
#
Requires: The calling thread does not own the mutex for any ownership mode.
15
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­until(abs_­time).
16
#
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);
17
#
Requires: The calling thread does not own the mutex for any ownership mode.
18
#
Effects: Constructs an object of type shared_­lock and calls m.try_­lock_­shared_­for(rel_­time).
19
#
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();
20
#
Effects: If owns calls pm->unlock_­shared().
shared_lock(shared_lock&& sl) noexcept;
21
#
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;
22
#
Effects: If owns calls pm->unlock_­shared().
23
#
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.

33.4.4.4.2 shared_­lock 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.

33.4.4.4.3 shared_­lock 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).

33.4.4.4.4 shared_­lock 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.