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 ]
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
constexpr defer_lock_t defer_lock { };
constexpr try_to_lock_t try_to_lock { };
constexpr adopt_lock_t adopt_lock { };
}
namespace std {
template <class... MutexTypes>
class lock_guard {
public:
using mutex_type = Mutex; // If MutexTypes... consists of the single type Mutex
explicit lock_guard(MutexTypes&... m);
lock_guard(MutexTypes&... m, adopt_lock_t);
~lock_guard();
lock_guard(const lock_guard&) = delete;
lock_guard& operator=(const lock_guard&) = delete;
private:
tuple<MutexTypes&...> pm; // exposition only
};
}
An object of type lock_guard controls the ownership of lockable objects within a scope. A lock_guard object maintains ownership of lockable objects throughout the lock_guard 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 lock_guard object. When sizeof...(MutexTypes) is 1, the supplied Mutex type shall meet the BasicLockable requirements. Otherwise, each of the mutex types shall meet the Lockable requirements ([thread.req.lockable.basic]).
Requires: If a MutexTypes type is not a recursive mutex, the calling thread does not own the corresponding mutex element of m.
Effects: Initializes pm with tie(m...). Then if sizeof...(MutexTypes) is 0, no effects. Otherwise if sizeof...(MutexTypes) is 1, then m.lock(). Otherwise, then lock(m...).
lock_guard(MutexTypes&... m, adopt_lock_t);
Requires: The calling thread owns all the mutexes in m.
Effects: Initializes pm with tie(m...).
Throws: Nothing.
Effects: For all i in [0, sizeof...(MutexTypes)), get<i>(pm).unlock().
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;
}
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]).
[ 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 ]
Effects: Constructs an object of type unique_lock.
Postconditions: pm == 0 and owns == false.
explicit unique_lock(mutex_type& m);
Requires: If mutex_type is not a recursive mutex the calling thread does not own the mutex.
Effects: Constructs an object of type unique_lock and calls m.lock().
Postconditions: pm == addressof(m) and owns == true.
unique_lock(mutex_type& m, defer_lock_t) noexcept;
Effects: Constructs an object of type unique_lock.
Postconditions: pm == addressof(m) and owns == false.
unique_lock(mutex_type& m, try_to_lock_t);
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.
Effects: Constructs an object of type unique_lock and calls m.try_lock().
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);
Requires: The calling thread owns the mutex.
Effects: Constructs an object of type unique_lock.
Postconditions: pm == addressof(m) and owns == true.
Throws: Nothing.
template <class Clock, class Duration>
unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);
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]).
Effects: Constructs an object of type unique_lock and calls m.try_lock_until(abs_time).
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);
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]).
Effects: Constructs an object of type unique_lock and calls m.try_lock_for(rel_time).
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;
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);
Effects: If owns calls pm->unlock().
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.
[ 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 ]
Throws: Nothing.
Effects: If owns calls pm->unlock().
Effects: As if by pm->lock().
Postconditions: owns == true
Throws: Any exception thrown by pm->lock(). system_error when an exception is required ([thread.req.exception]).
Requires: The supplied Mutex shall meet the Lockable requirements ([thread.req.lockable.req]).
Effects: As if by pm->try_lock().
Returns: The value returned by the call to try_lock().
Postconditions: owns == res, where res is the value returned by the call to try_lock().
Throws: Any exception thrown by pm->try_lock(). system_error when an exception is required ([thread.req.exception]).
template <class Clock, class Duration>
bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);
Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
Effects: As if by pm->try_lock_until(abs_time).
Returns: The value returned by the call to try_lock_until(abs_time).
Postconditions: owns == res, where res is the value returned by the call to try_lock_until(abs_time).
Throws: Any exception thrown by pm->try_lock_until(). system_error when an exception is required ([thread.req.exception]).
template <class Rep, class Period>
bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);
Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
Effects: As if by pm->try_lock_for(rel_time).
Returns: The value returned by the call to try_lock_until(rel_time).
Postconditions: owns == res, where res is the value returned by the call to try_lock_for(rel_time).
Throws: Any exception thrown by pm->try_lock_for(). system_error when an exception is required ([thread.req.exception]).
Effects: As if by pm->unlock().
Postconditions: owns == false
Throws: system_error when an exception is required ([thread.req.exception]).
void swap(unique_lock& u) noexcept;
Effects: Swaps the data members of *this and u.
mutex_type* release() noexcept;
Returns: The previous value of pm.
Postconditions: pm == 0 and owns == false.
template <class Mutex>
void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
Effects: As if by x.swap(y).
bool owns_lock() const noexcept;
Returns: owns
explicit operator bool() const noexcept;
Returns: owns
mutex_type *mutex() const noexcept;
Returns: pm