The following subclauses describe components to create and manage threads ([intro.multithread]), perform mutual exclusion, and communicate conditions and values between threads, as summarized in Table [tab:thread.lib.summary].
| Subclause | Header(s) | |
| [thread.req] | Requirements | |
| [thread.threads] | Threads | <thread> |
| [thread.mutex] | Mutual exclusion | <mutex> |
| [thread.condition] | Condition variables | <condition_variable> |
| [futures] | Futures | <future> |
Throughout this Clause, the names of template parameters are used to express type requirements.
If a parameter is Predicate, operator() applied to the actual template argument shall return a value that is convertible to bool.
Some functions described in this Clause are specified to throw exceptions of type system_error ([syserr.syserr]). Such exceptions shall be thrown if any of the function's error conditions is detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. Failure to allocate storage shall be reported as described in [res.on.exception.handling].
[ Example: Consider a function in this clause that is specified to throw exceptions of type system_error and specifies error conditions that include operation_not_permitted for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, an errno of EPERM is reported by a POSIX API call used by the implementation. Since POSIX specifies an errno of EPERM when “the caller does not have the privilege to perform the operation”, the implementation maps EPERM to an error_condition of operation_not_permitted ([syserr]) and an exception of type system_error is thrown. — end example ]
The error_code reported by such an exception's code() member function shall compare equal to one of the conditions specified in the function's error condition element.
Several classes described in this Clause have members native_handle_type and native_handle. The presence of these members and their semantics is implementation-defined. [ Note: These members allow implementations to provide access to implementation details. Their names are specified to facilitate portable compile-time detection. Actual use of these members is inherently non-portable. — end note ]
Several functions described in this Clause take an argument to specify a timeout. These timeouts are specified as either a duration or a time_point type as specified in [time].
Implementations necessarily have some delay in returning from a timeout. Any overhead in interrupt response, function return, and scheduling induces a “quality of implementation” delay, expressed as duration Di. Ideally, this delay would be zero. Further, any contention for processor and memory resources induces a “quality of management” delay, expressed as duration Dm. The delay durations may vary from timeout to timeout, but in all cases shorter is better.
The member functions whose names end in _for take an argument that specifies a duration. These functions produce relative timeouts. Implementations should use a steady clock to measure time for these functions.337 Given a duration argument Dt, the real-time duration of the timeout is Dt + Di + Dm.
The member functions whose names end in _until take an argument that specifies a time point. These functions produce absolute timeouts. Implementations should use the clock specified in the time point to measure time for these functions. Given a clock time point argument Ct, the clock time point of the return from timeout should be Ct + Di + Dm when the clock is not adjusted during the timeout. If the clock is adjusted to the time Ca during the timeout, the behavior should be as follows:
if Ca > Ct, the waiting function should wake as soon as possible, i.e. Ca + Di + Dm, since the timeout is already satisfied. [ Note: This specification may result in the total duration of the wait decreasing when measured against a steady clock. — end note ]
if Ca <= Ct, the waiting function should not time out until Clock::now() returns a time Cn >= Ct, i.e. waking at Ct + Di + Dm. [ Note: When the clock is adjusted backwards, this specification may result in the total duration of the wait increasing when measured against a steady clock. When the clock is adjusted forwards, this specification may result in the total duration of the wait decreasing when measured against a steady clock. — end note ]
An implementation shall return from such a timeout at any point from the time specified above to the time it would return from a steady-clock relative timeout on the difference between Ct and the time point of the call to the _until function. [ Note: Implementations should decrease the duration of the wait when the clock is adjusted forwards. — end note ]
[ Note: If the clock is not synchronized with a steady clock, e.g., a CPU time clock, these timeouts might not provide useful functionality. — end note ]
The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.
Implementation-provided clocks that are used for these functions shall meet the TrivialClock requirements ([time.clock.req]).
All implementations for which standard time units are meaningful must necessarily have a steady clock within their hardware implementation.
An execution agent is an entity such as a thread that may perform work in parallel with other execution agents. [ Note: Implementations or users may introduce other kinds of agents such as processes or thread-pool tasks. — end note ] The calling agent is determined by context, e.g. the calling thread that contains the call, and so on.
[ Note: Some lockable objects are “agent oblivious” in that they work for any execution agent model because they do not determine or store the agent's ID (e.g., an ordinary spin lock). — end note ]
The standard library templates unique_lock ([thread.lock.unique]), lock_guard ([thread.lock.guard]), lock, try_lock ([thread.lock.algorithm]), and condition_variable_any ([thread.condition.condvarany]) all operate on user-supplied lockable objects. The BasicLockable requirements, the Lockable requirements, and the TimedLockable requirements list the requirements imposed by these library types in order to acquire or release ownership of a lock by a given execution agent. [ Note: The nature of any lock ownership and any synchronization it may entail are not part of these requirements. — end note ]
A type L meets the BasicLockable requirements if the following expressions are well-formed and have the specified semantics (m denotes a value of type L).
m.lock()
Effects: Blocks until a lock can be acquired for the current execution agent. If an exception is thrown then a lock shall not have been acquired for the current execution agent.
m.unlock()
Requires: The current execution agent shall hold a lock on m.
Effects: Releases a lock on m held by the current execution agent.
Throws: Nothing.
A type L meets the Lockable requirements if it meets the BasicLockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L).
m.try_lock()
Effects: attempts to acquire a lock for the current execution agent without blocking. If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.
A type L meets the TimedLockable requirements if it meets the Lockable requirements and the following expressions are well-formed and have the specified semantics (m denotes a value of type L, rel_time denotes a value of an instantiation of duration ([time.duration]), and abs_time denotes a value of an instantiation of time_point ([time.point])).
m.try_lock_for(rel_time)
Effects: attempts to acquire a lock for the current execution agent within the relative timeout ([thread.req.timing]) specified by rel_time. The function shall not return within the timeout specified by rel_time unless it has obtained a lock on m for the current execution agent. If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.
m.try_lock_until(abs_time)
Effects: attempts to acquire a lock for the current execution agent before the absolute timeout ([thread.req.timing]) specified by abs_time. The function shall not return before the timeout specified by abs_time unless it has obtained a lock on m for the current execution agent. If an exception is thrown then a lock shall not have been acquired for the current execution agent.
Return type: bool.
Returns: true if the lock was acquired, false otherwise.
In several places in this Clause the operation DECAY_COPY(x) is used. All such uses mean call the function decay_copy(x) and use the result, where decay_copy is defined as follows:
template <class T> typename decay<T>::type decay_copy(T&& v)
{ return std::forward<T>(v); }
[thread.threads] describes components that can be used to create and manage threads. [ Note: These threads are intended to map one-to-one with operating system threads. — end note ]
namespace std {
class thread;
void swap(thread& x, thread& y) noexcept;
namespace this_thread {
thread::id get_id() noexcept;
void yield() noexcept;
template <class Clock, class Duration>
void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);
template <class Rep, class Period>
void sleep_for(const chrono::duration<Rep, Period>& rel_time);
}
}
The class thread provides a mechanism to create a new thread of execution, to join with a thread (i.e., wait for a thread to complete), and to perform other operations that manage and query the state of a thread. A thread object uniquely represents a particular thread of execution. That representation may be transferred to other thread objects in such a way that no two thread objects simultaneously represent the same thread of execution. A thread of execution is detached when no thread object represents that thread. Objects of class thread can be in a state that does not represent a thread of execution. [ Note: A thread object does not represent a thread of execution after default construction, after being moved from, or after a successful call to detach or join. — end note ]
namespace std {
class thread {
public:
// types:
class id;
typedef implementation-defined native_handle_type; // See [thread.req.native]
// construct/copy/destroy:
thread() noexcept;
template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
~thread();
thread(const thread&) = delete;
thread(thread&&) noexcept;
thread& operator=(const thread&) = delete;
thread& operator=(thread&&) noexcept;
// members:
void swap(thread&) noexcept;
bool joinable() const noexcept;
void join();
void detach();
id get_id() const noexcept;
native_handle_type native_handle(); // See [thread.req.native]
// static members:
static unsigned hardware_concurrency() noexcept;
};
}
namespace std {
class thread::id {
public:
id() noexcept;
};
bool operator==(thread::id x, thread::id y) noexcept;
bool operator!=(thread::id x, thread::id y) noexcept;
bool operator<(thread::id x, thread::id y) noexcept;
bool operator<=(thread::id x, thread::id y) noexcept;
bool operator>(thread::id x, thread::id y) noexcept;
bool operator>=(thread::id x, thread::id y) noexcept;
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<< (basic_ostream<charT, traits>& out, thread::id id);
// Hash support
template <class T> struct hash;
template <> struct hash<thread::id>;
}
An object of type thread::id provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.thread.class]). Each thread of execution has an associated thread::id object that is not equal to the thread::id object of any other thread of execution and that is not equal to the thread::id object of any std::thread object that does not represent threads of execution.
thread::id shall be a trivially copyable class (Clause [class]). The library may reuse the value of a thread::id of a terminated thread that can no longer be joined.
[ Note: Relational operators allow thread::id objects to be used as keys in associative containers. — end note ]
Effects: Constructs an object of type id.
Postconditions: The constructed object does not represent a thread of execution.
bool operator==(thread::id x, thread::id y) noexcept;
Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.
bool operator!=(thread::id x, thread::id y) noexcept;
Returns: !(x == y)
bool operator<(thread::id x, thread::id y) noexcept;
Returns: A value such that operator< is a total ordering as described in [alg.sorting].
bool operator<=(thread::id x, thread::id y) noexcept;
Returns: !(y < x)
bool operator>(thread::id x, thread::id y) noexcept;
Returns: y < x
bool operator>=(thread::id x, thread::id y) noexcept;
Returns: !(x < y)
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<< (basic_ostream<charT, traits>&& out, thread::id id);
Effects: Inserts an unspecified text representation of id into out. For two objects of type thread::id x and y, if x == y the thread::id objects shall have the same text representation and if x != y the thread::id objects shall have distinct text representations.
Returns: out
template <> struct hash<thread::id>;
Requires: the template specialization shall meet the requirements of class template hash ([unord.hash]).
Effects: Constructs a thread object that does not represent a thread of execution.
Postcondition: get_id() == id()
template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
Requires: F and each Ti in Args shall satisfy the MoveConstructible requirements. INVOKE(DECAY_COPY( std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...) ([func.require]) shall be a valid expression.
Effects: Constructs an object of type thread. The new thread of execution executes INVOKE(DECAY_COPY( std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...) with the calls to DECAY_COPY being evaluated in the constructing thread. Any return value from this invocation is ignored. [ Note: This implies that any exceptions not thrown from the invocation of the copy of f will be thrown in the constructing thread, not the new thread. — end note ] If the invocation of INVOKE(DECAY_COPY( std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...) terminates with an uncaught exception, std::terminate shall be called.
Synchronization: The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f.
Postconditions: get_id() != id(). *this represents the newly started thread.
Throws: system_error if unable to start the new thread.
Error conditions:
resource_unavailable_try_again — the system lacked the necessary resources to create another thread, or the system-imposed limit on the number of threads in a process would be exceeded.
Effects: Constructs an object of type thread from x, and sets x to a default constructed state.
Postconditions: x.get_id() == id() and get_id() returns the value of x.get_id() prior to the start of construction.
If joinable(), calls std::terminate(). Otherwise, has no effects. [ Note: Either implicitly detaching or joining a joinable() thread in its destructor could result in difficult to debug correctness (for detach) or performance (for join) bugs encountered only when an exception is raised. Thus the programmer must ensure that the destructor is never executed while the thread is still joinable. — end note ]
thread& operator=(thread&& x) noexcept;
Effects: If joinable(), calls std::terminate(). Otherwise, assigns the state of x to *this and sets x to a default constructed state.
Postconditions: x.get_id() == id() and get_id() returns the value of x.get_id() prior to the assignment.
Returns: *this
void swap(thread& x) noexcept;
Effects: Swaps the state of *this and x.
bool joinable() const noexcept;
Returns: get_id() != id()
Requires: joinable() is true.
Effects: Blocks until the thread represented by *this has completed.
Synchronization: The completion of the thread represented by *this synchronizes with ([intro.multithread]) the corresponding successful join() return. [ Note: Operations on *this are not synchronized. — end note ]
Postconditions: The thread represented by *this has completed. get_id() == id().
Throws: system_error when an exception is required ([thread.req.exception]).
Requires: joinable() is true.
Effects: The thread represented by *this continues execution without the calling thread blocking. When detach() returns, *this no longer represents the possibly continuing thread of execution. When the thread previously represented by *this ends execution, the implementation shall release any owned resources.
Postcondition: get_id() == id().
Throws: system_error when an exception is required ([thread.req.exception]).
Returns: A default constructed id object if *this does not represent a thread, otherwise this_thread::get_id() for the thread of execution represented by *this.
unsigned hardware_concurrency() noexcept;
Returns: The number of hardware thread contexts. [ Note: This value should only be considered to be a hint. — end note ] If this value is not computable or well defined an implementation should return 0.
void swap(thread& x, thread& y) noexcept;
Effects: x.swap(y)
namespace std {
namespace this_thread {
thread::id get_id() noexcept;
void yield() noexcept;
template <class Clock, class Duration>
void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);
template <class Rep, class Period>
void sleep_for(const chrono::duration<Rep, Period>& rel_time);
}
}
thread::id this_thread::get_id() noexcept;
Returns: An object of type thread::id that uniquely identifies the current thread of execution. No other thread of execution shall have this id and this thread of execution shall always have this id. The object returned shall not compare equal to a default constructed thread::id.
void this_thread::yield() noexcept;
Effects: Offers the implementation the opportunity to reschedule.
Synchronization: None.
template <class Clock, class Duration>
void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);
Effects: Blocks the calling thread for the absolute timeout ([thread.req.timing]) specified by abs_time.
Synchronization: None.
Throws: Nothing if Clock satisfies the TrivialClock requirements ([time.clock.req]) and operations of Duration do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in [time.clock] do not throw exceptions. — end note ]
template <class Rep, class Period>
void sleep_for(const chrono::duration<Rep, Period>& rel_time);
Effects: Blocks the calling thread for the relative timeout ([thread.req.timing]) specified by rel_time.
Synchronization: None.
Throws: Nothing if operations of chrono::duration<Rep, Period> do not throw exceptions. [ Note: instantiations of duration types supplied by the implementation as specified in [time.clock] do not throw exceptions. — end note ]
This section provides mechanisms for mutual exclusion: mutexes, locks, and call once. These mechanisms ease the production of race-free programs ([intro.multithread]).
namespace std {
class mutex;
class recursive_mutex;
class timed_mutex;
class recursive_timed_mutex;
struct defer_lock_t { };
struct try_to_lock_t { };
struct adopt_lock_t { };
constexpr defer_lock_t defer_lock { };
constexpr try_to_lock_t try_to_lock { };
constexpr adopt_lock_t adopt_lock { };
template <class Mutex> class lock_guard;
template <class Mutex> class unique_lock;
template <class Mutex>
void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept;
template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);
template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);
struct once_flag {
constexpr once_flag() noexcept;
once_flag(const once_flag&) = delete;
once_flag& operator=(const once_flag&) = delete;
};
template<class Callable, class ...Args>
void call_once(once_flag& flag, Callable func, Args&&... args);
}
A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents ([thread.req.lockable]). An execution agent owns a mutex from the time it successfully calls one of the lock functions until it calls unlock. Mutexes can be either recursive or non-recursive, and can grant simultaneous ownership to one or many execution agents. The mutex types supplied by the standard library provide exclusive ownership semantics: only one thread may own the mutex at a time. Both recursive and non-recursive mutexes are supplied.
The mutex types are the standard library types std::mutex, std::recursive_mutex, std::timed_mutex, and std::recursive_timed_mutex. They shall meet the requirements set out in this section. In this description, m denotes an object of a mutex type.
The mutex types shall meet the Lockable requirements ([thread.req.lockable.req]).
The mutex types shall be DefaultConstructible and Destructible. If initialization of an object of a mutex type fails, an exception of type system_error shall be thrown. The mutex types shall not be copyable or movable.
The error conditions for error codes, if any, reported by member functions of the mutex types shall be:
resource_unavailable_try_again — if any native handle type manipulated is not available.
operation_not_permitted — if the thread does not have the privilege to perform the operation.
device_or_resource_busy — if any native handle type manipulated is already locked.
invalid_argument — if any native handle type manipulated as part of mutex construction is incorrect.
The implementation shall provide lock and unlock operations, as described below. For purposes of determining the existence of a data race, these behave as atomic operations ([intro.multithread]). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [ Note: this can be viewed as the modification order ([intro.multithread]) of the mutex. — end note ] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]
The expression m.lock() shall be well-formed and have the following semantics:
Requires: If m is of type std::mutex or std::timed_mutex, the calling thread does not own the mutex.
Effects: Blocks the calling thread until ownership of the mutex can be obtained for the calling thread.
Postcondition: The calling thread owns the mutex.
Return type: void
Synchronization: Prior unlock() operations on the same object shall synchronize with ([intro.multithread]) this operation.
Throws: system_error when an exception is required ([thread.req.exception]).
Error conditions:
operation_not_permitted — if the thread does not have the privilege to perform the operation.
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.
device_or_resource_busy — if the mutex is already locked and blocking is not possible.
The expression m.try_lock() shall be well-formed and have the following semantics:
Requires: If m is of type std::mutex or std::timed_mutex, the calling thread does not own the mutex.
Effects: Attempts to obtain ownership of the mutex for the calling thread without blocking. If ownership is not obtained, there is no effect and try_lock() immediately returns. An implementation may fail to obtain the lock even if it is not held by any other thread. [ Note: This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange (Clause [atomics]). — end note ] An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.
Return type: bool
Returns: true if ownership of the mutex was obtained for the calling thread, otherwise false.
Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation. [ Note: Since lock() does not synchronize with a failed subsequent try_lock(), the visibility rules are weak enough that little would be known about the state after a failure, even in the absence of spurious failures. — end note ]
Throws: Nothing.
The expression m.unlock() shall be well-formed and have the following semantics:
Requires: The calling thread shall own the mutex.
Effects: Releases the calling thread's ownership of the mutex.
Return type: void
Synchronization: This operation synchronizes with ([intro.multithread]) subsequent lock operations that obtain ownership on the same object.
Throws: Nothing.
namespace std {
class mutex {
public:
constexpr mutex() noexcept;
~mutex();
mutex(const mutex&) = delete;
mutex& operator=(const mutex&) = delete;
void lock();
bool try_lock();
void unlock();
typedef implementation-defined native_handle_type; // See [thread.req.native]
native_handle_type native_handle(); // See [thread.req.native]
};
}
The class mutex provides a non-recursive mutex with exclusive ownership semantics. If one thread owns a mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock()) until the owning thread has released ownership with a call to unlock().
[ Note: After a thread A has called unlock(), releasing a mutex, it is possible for another thread B to lock the same mutex, observe that it is no longer in use, unlock it, and destroy it, before thread A appears to have returned from its unlock call. Implementations are required to handle such scenarios correctly, as long as thread A doesn't access the mutex after the unlock call returns. These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count. — end note ]
The class mutex shall satisfy all the Mutex requirements ([thread.mutex.requirements]). It shall be a standard-layout class (Clause [class]).
[ Note: A program may deadlock if the thread that owns a mutex object calls lock() on that object. If the implementation can detect the deadlock, a resource_deadlock_would_occur error condition may be observed. — end note ]
The behavior of a program is undefined if it destroys a mutex object owned by any thread or a thread terminates while owning a mutex object.
namespace std {
class recursive_mutex {
public:
recursive_mutex();
~recursive_mutex();
recursive_mutex(const recursive_mutex&) = delete;
recursive_mutex& operator=(const recursive_mutex&) = delete;
void lock();
bool try_lock() noexcept;
void unlock();
typedef implementation-defined native_handle_type; // See [thread.req.native]
native_handle_type native_handle(); // See [thread.req.native]
};
}
The class recursive_mutex provides a recursive mutex with exclusive ownership semantics. If one thread owns a recursive_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock()) until the first thread has completely released ownership.
The class recursive_mutex shall satisfy all the Mutex requirements ([thread.mutex.requirements]). It shall be a standard-layout class (Clause [class]).
A thread that owns a recursive_mutex object may acquire additional levels of ownership by calling lock() or try_lock() on that object. It is unspecified how many levels of ownership may be acquired by a single thread. If a thread has already acquired the maximum level of ownership for a recursive_mutex object, additional calls to try_lock() shall fail, and additional calls to lock() shall throw an exception of type system_error. A thread shall call unlock() once for each level of ownership acquired by calls to lock() and try_lock(). Only when all levels of ownership have been released may ownership be acquired by another thread.
The timed mutex types are the standard library types std::timed_mutex and std::recursive_timed_mutex. They shall meet the requirements set out below. In this description, m denotes an object of a mutex type, rel_time denotes an object of an instantiation of duration ([time.duration]), and abs_time denotes an object of an instantiation of time_point ([time.point]).
The timed mutex types shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
The expression m.try_lock_for(rel_time) shall be well-formed and have the following semantics:
Requires: If the tick period of rel_time is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. If m is of type std::timed_mutex, the calling thread does not own the mutex.
Effects: The function attempts to obtain ownership of the mutex within the relative timeout ([thread.req.timing]) specified by rel_time. If the time specified by rel_time is less than or equal to rel_time.zero(), the function attempts to obtain ownership without blocking (as if by calling try_lock()). The function shall return within the timeout specified by rel_time only if it has obtained ownership of the mutex object. [ Note: As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]
Return type: bool
Returns: true if ownership was obtained, otherwise false.
Synchronization: If try_lock_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Throws: Nothing.
The expression m.try_lock_until(abs_time) shall be well-formed and have the following semantics:
Requires: If m is of type std::timed_mutex, the calling thread does not own the mutex.
Effects: The function attempts to obtain ownership of the mutex. If abs_time has already passed, the function attempts to obtain ownership without blocking (as if by calling try_lock()). The function shall return before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained ownership of the mutex object. [ Note: As with try_lock(), there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]
Return type: bool
Returns: true if ownership was obtained, otherwise false.
Synchronization: If try_lock_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.
Throws: Nothing.
namespace std {
class timed_mutex {
public:
timed_mutex();
~timed_mutex();
timed_mutex(const timed_mutex&) = delete;
timed_mutex& operator=(const timed_mutex&) = delete;
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();
typedef implementation-defined native_handle_type; // See [thread.req.native]
native_handle_type native_handle(); // See [thread.req.native]
};
}
The class timed_mutex provides a non-recursive mutex with exclusive ownership semantics. If one thread owns a timed_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock(), try_lock_for(), and try_lock_until()) until the owning thread has released ownership with a call to unlock() or the call to try_lock_for() or try_lock_until() times out (having failed to obtain ownership).
The class timed_mutex shall satisfy all of the TimedMutex requirements ([thread.timedmutex.requirements]). It shall be a standard-layout class (Clause [class]).
namespace std {
class recursive_timed_mutex {
public:
recursive_timed_mutex();
~recursive_timed_mutex();
recursive_timed_mutex(const recursive_timed_mutex&) = delete;
recursive_timed_mutex& operator=(const recursive_timed_mutex&) = delete;
void lock();
bool try_lock() noexcept;
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();
typedef implementation-defined native_handle_type; // See [thread.req.native]
native_handle_type native_handle(); // See [thread.req.native]
};
}
The class recursive_timed_mutex provides a recursive mutex with exclusive ownership semantics. If one thread owns a recursive_timed_mutex object, attempts by another thread to acquire ownership of that object will fail (for try_lock()) or block (for lock(), try_lock_for(), and try_lock_until()) until the owning thread has completely released ownership or the call to try_lock_for() or try_lock_until() times out (having failed to obtain ownership).
The class recursive_timed_mutex shall satisfy all of the TimedMutex requirements ([thread.timedmutex.requirements]). It shall be a standard-layout class (Clause [class]).
A thread that owns a recursive_timed_mutex object may acquire additional levels of ownership by calling lock(), try_lock(), try_lock_for(), or try_lock_until() on that object. It is unspecified how many levels of ownership may be acquired by a single thread. If a thread has already acquired the maximum level of ownership for a recursive_timed_mutex object, additional calls to try_lock(), try_lock_for(), or try_lock_until() shall fail, and additional calls to lock() shall throw an exception of type system_error. A thread shall call unlock() once for each level of ownership acquired by calls to lock(), try_lock(), try_lock_for(), and try_lock_until(). Only when all levels of ownership have been released may ownership of the object be acquired by another thread.
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 Mutex>
class lock_guard {
public:
typedef Mutex mutex_type;
explicit lock_guard(mutex_type& m);
lock_guard(mutex_type& m, adopt_lock_t);
~lock_guard();
lock_guard(lock_guard const&) = delete;
lock_guard& operator=(lock_guard const&) = delete;
private:
mutex_type& pm; // exposition only
};
}
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]).
Requires: If mutex_type is not a recursive mutex, the calling thread does not own the mutex m.
Effects: m.lock()
Postcondition: &pm == &m
lock_guard(mutex_type& m, adopt_lock_t);
Requires: The calling thread owns the mutex m.
Postcondition: &pm == &m
Throws: Nothing.
Effects: pm.unlock()
namespace std {
template <class Mutex>
class unique_lock {
public:
typedef Mutex mutex_type;
// [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(unique_lock const&) = delete;
unique_lock& operator=(unique_lock const&) = delete;
unique_lock(unique_lock&& u) noexcept;
unique_lock& operator=(unique_lock&& u) noexcept;
// [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 == &m and owns == true.
unique_lock(mutex_type& m, defer_lock_t) noexcept;
Effects: Constructs an object of type unique_lock.
Postconditions: pm == &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 == &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 own the mutex.
Effects: Constructs an object of type unique_lock.
Postconditions: pm == &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 == &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 == &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) noexcept;
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 ]
Effects: If owns calls pm->unlock().
Effects: pm->lock()
Postcondition: owns == true
Throws: Any exception thrown by pm->lock(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.
Requires: The supplied Mutex shall meet the Lockable requirements ([thread.req.lockable.req]).
Effects: pm->try_lock()
Returns: The value returned by the call to try_lock().
Postcondition: owns == res, where res is the value returned by the call to try_lock().
Throws: Any exception thrown by pm->try_lock(). system_error if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of 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);
Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
Effects: pm->try_lock_until(abs_time)
Returns: The value returned by the call to try_lock_until(abs_time).
Postcondition: 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 if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of 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);
Requires: The supplied Mutex type shall meet the TimedLockable requirements ([thread.req.lockable.timed]).
Effects: pm->try_lock_for(rel_time).
Returns: The value returned by the call to try_lock_until(rel_time).
Postcondition: 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 if an exception is required ([thread.req.exception]). system_error with an error condition of operation_not_permitted if pm is 0. system_error with an error condition of resource_deadlock_would_occur if on entry owns is true.
Effects: pm->unlock()
Postcondition: 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: x.swap(y)
bool owns_lock() const noexcept;
Returns: owns
explicit operator bool() const noexcept;
Returns: owns
mutex_type *mutex() const noexcept;
Returns: pm
template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);
Requires: Each template parameter type shall meet the Lockable requirements. [ Note: The unique_lock class template meets these requirements when suitably instantiated. — end note ]
Effects: Calls try_lock() for each argument in order beginning with the first until all arguments have been processed or a call to try_lock() fails, either by returning false or by throwing an exception. If a call to try_lock() fails, unlock() shall be called for all prior arguments and there shall be no further calls to try_lock().
Returns: -1 if all calls to try_lock() returned true, otherwise a 0-based index value that indicates the argument for which try_lock() returned false.
template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);
Requires: Each template parameter type shall meet the Lockable requirements, [ Note: The unique_lock class template meets these requirements when suitably instantiated. — end note ]
Effects: All arguments are locked via a sequence of calls to lock(), try_lock(), or unlock() on each argument. The sequence of calls shall not result in deadlock, but is otherwise unspecified. [ Note: A deadlock avoidance algorithm such as try-and-back-off must be used, but the specific algorithm is not specified to avoid over-constraining implementations. — end note ] If a call to lock() or try_lock() throws an exception, unlock() shall be called for any argument that had been locked by a call to lock() or try_lock().
The class once_flag is an opaque data structure that call_once uses to initialize data without causing a data race or deadlock.
constexpr once_flag() noexcept;
Effects: Constructs an object of type once_flag.
Synchronization: The construction of a once_flag object is not synchronized.
Postcondition: The object's internal state is set to indicate to an invocation of call_once with the object as its initial argument that no function has been called.
template<class Callable, class ...Args>
void call_once(once_flag& flag, Callable&& func, Args&&... args);
Requires: Callable and each Ti in Args shall satisfy the MoveConstructible requirements. INVOKE(DECAY_COPY( std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...) ([func.require]) shall be a valid expression.
Effects: An execution of call_once that does not call its func is a passive execution. An execution of call_once that calls its func is an active execution. An active execution shall call INVOKE(DECAY_COPY( std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...). If such a call to func throws an exception the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller of call_once. Among all executions of call_once for any given once_flag: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution. [ Note: passive executions allow other threads to reliably observe the results produced by the earlier returning execution. — end note ]
Synchronization: For any given once_flag: all active executions occur in a total order; completion of an active execution synchronizes with ([intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.
Throws: system_error when an exception is required ([thread.req.exception]), or any exception thrown by func.
[ Example:
// global flag, regular function void init(); std::once_flag flag; void f() { std::call_once(flag, init); } // function static flag, function object struct initializer { void operator()(); }; void g() { static std::once_flag flag2; std::call_once(flag2, initializer()); } // object flag, member function class information { std::once_flag verified; void verifier(); public: void verify() { std::call_once(verified, &information::verifier, *this); } };
— end example ]
Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached. Class condition_variable provides a condition variable that can only wait on an object of type unique_lock<mutex>, allowing maximum efficiency on some platforms. Class condition_variable_any provides a general condition variable that can wait on objects of user-supplied lock types.
Condition variables permit concurrent invocation of the wait, wait_for, wait_until, notify_one and notify_all member functions.
The execution of notify_one and notify_all shall be atomic. The execution of wait, wait_for, and wait_until shall be performed in three atomic parts:
the release of the mutex and entry into the waiting state;
the unblocking of the wait; and
the reacquisition of the lock.
The implementation shall behave as if notify_one, notify_all, and each part of the wait, wait_for, and wait_until executions are executed in some unspecified total order.
Condition variable construction and destruction need not be synchronized.
Header condition_variable synopsis
namespace std {
class condition_variable;
class condition_variable_any;
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);
enum class cv_status { no_timeout, timeout };
}
void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);
Effects: transfers ownership of the lock associated with lk into internal storage and schedules cond to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as if
lk.unlock(); cond.notify_all();
Synchronization: The call to notify_all_at_thread_exit and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with ([intro.multithread]) calls to functions waiting on cond.
Note: The supplied lock will be held until the thread exits, and care must be taken to ensure that this does not cause deadlock due to lock ordering issues. After calling notify_all_at_thread_exit it is recommended that the thread should be exited as soon as possible, and that no blocking or time-consuming tasks are run on that thread.
Note: It is the user's responsibility to ensure that waiting threads do not erroneously assume that the thread has finished if they experience spurious wakeups. This typically requires that the condition being waited for is satisfied while holding the lock on lk, and that this lock is not released and reacquired prior to calling notify_all_at_thread_exit.
namespace std {
class condition_variable {
public:
condition_variable();
~condition_variable();
condition_variable(const condition_variable&) = delete;
condition_variable& operator=(const condition_variable&) = delete;
void notify_one() noexcept;
void notify_all() noexcept;
void wait(unique_lock<mutex>& lock);
template <class Predicate>
void wait(unique_lock<mutex>& lock, Predicate pred);
template <class Clock, class Duration>
cv_status wait_until(unique_lock<mutex>& lock,
const chrono::time_point<Clock, Duration>& abs_time);
template <class Clock, class Duration, class Predicate>
bool wait_until(unique_lock<mutex>& lock,
const chrono::time_point<Clock, Duration>& abs_time,
Predicate pred);
template <class Rep, class Period>
cv_status wait_for(unique_lock<mutex>& lock,
const chrono::duration<Rep, Period>& rel_time);
template <class Rep, class Period, class Predicate>
bool wait_for(unique_lock<mutex>& lock,
const chrono::duration<Rep, Period>& rel_time,
Predicate pred);
typedef implementation-defined native_handle_type; // See [thread.req.native]
native_handle_type native_handle(); // See [thread.req.native]
};
}
The class condition_variable shall be a standard-layout class (Clause [class]).
Effects: Constructs an object of type condition_variable.
Throws: system_error when an exception is required ([thread.req.exception]).
Error conditions:
resource_unavailable_try_again — if some non-memory resource limitation prevents initialization.
Requires: There shall be no thread blocked on *this. [ Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. This relaxes the usual rules, which would have required all wait calls to happen before destruction. Only the notification to unblock the wait must happen before destruction. The user must take care to ensure that no threads wait on *this once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads of wait, wait_for, or wait_until that take a predicate. — end note ]
Effects: Destroys the object.
Effects: If any threads are blocked waiting for *this, unblocks one of those threads.
Effects: Unblocks all threads that are blocked waiting for *this.
void wait(unique_lock<mutex>& lock);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait or timed_wait) threads.
Effects:
Atomically calls lock.unlock() and blocks on *this.
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
The function will unblock when signaled by a call to notify_one() or a call to notify_all(), or spuriously.
If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Predicate>
void wait(unique_lock<mutex>& lock, Predicate pred);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait or timed_wait) threads.
Effects:
while (!pred()) wait(lock);
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
Throws: std::system_error when an exception is required ([thread.req.exception]).
template <class Clock, class Duration>
cv_status wait_until(unique_lock<mutex>& lock,
const chrono::time_point<Clock, Duration>& abs_time);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.
Effects:
Atomically calls lock.unlock() and blocks on *this.
When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), expiration of the absolute timeout ([thread.req.timing]) specified by abs_time, or spuriously.
If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
Returns: cv_status::timeout if the absolute timeout ([thread.req.timing]) specified by abs_time expired, otherwise cv_status::no_timeout.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Rep, class Period>
cv_status wait_for(unique_lock<mutex>& lock,
const chrono::duration<Rep, Period>& rel_time);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.
Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time);
Returns: cv_status::timeout if the relative timeout ([thread.req.timing]) specified by rel_time expired, otherwise cv_status::no_timeout.
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Clock, class Duration, class Predicate>
bool wait_until(unique_lock<mutex>& lock,
const chrono::time_point<Clock, Duration>& abs_time,
Predicate pred);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait or timed_wait) threads.
Effects:
while (!pred())
if (wait_until(lock, abs_time) == cv_status::timeout)
return pred();
return true;
Returns: pred()
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
[ Note: The returned value indicates whether the predicate evaluated to true regardless of whether the timeout was triggered. — end note ]
Throws: std::system_error when an exception is required ([thread.req.exception]).
template <class Rep, class Period, class Predicate>
bool wait_for(unique_lock<mutex>& lock,
const chrono::duration<Rep, Period>& rel_time,
Predicate pred);
Requires: lock.owns_lock() is true and lock.mutex() is locked by the calling thread, and either
no other thread is waiting on this condition_variable object or
lock.mutex() returns the same value for each of the lock arguments supplied by all concurrently waiting (via wait, wait_for, or wait_until) threads.
Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
[ Note: There is no blocking if pred() is initially true, even if the timeout has already expired. — end note ]
Postcondition: lock.owns_lock() is true and lock.mutex() is locked by the calling thread.
Returns: pred()
[ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]
Throws: system_error when an exception is required ([thread.req.exception]).
A Lock type shall meet the BasicLockable requirements ([thread.req.lockable.basic]). [ Note: All of the standard mutex types meet this requirement. If a Lock type other than one of the standard mutex types or a unique_lock wrapper for a standard mutex type is used with condition_variable_any, the user must ensure that any necessary synchronization is in place with respect to the predicate associated with the condition_variable_any instance. — end note ]
namespace std {
class condition_variable_any {
public:
condition_variable_any();
~condition_variable_any();
condition_variable_any(const condition_variable_any&) = delete;
condition_variable_any& operator=(const condition_variable_any&) = delete;
void notify_one() noexcept;
void notify_all() noexcept;
template <class Lock>
void wait(Lock& lock);
template <class Lock, class Predicate>
void wait(Lock& lock, Predicate pred);
template <class Lock, class Clock, class Duration>
cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);
template <class Lock, class Clock, class Duration, class Predicate>
bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time,
Predicate pred);
template <class Lock, class Rep, class Period>
cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
template <class Lock, class Rep, class Period, class Predicate>
bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time,
Predicate pred);
};
}
Effects: Constructs an object of type condition_variable_any.
Throws: bad_alloc or system_error when an exception is required ([thread.req.exception]).
Requires: There shall be no thread blocked on *this. [ Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. This relaxes the usual rules, which would have required all wait calls to happen before destruction. Only the notification to unblock the wait must happen before destruction. The user must take care to ensure that no threads wait on *this once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads of wait, wait_for, or wait_until that take a predicate. — end note ]
Effects: Destroys the object.
Effects: If any threads are blocked waiting for *this, unblocks one of those threads.
Effects: Unblocks all threads that are blocked waiting for *this.
template <class Lock>
void wait(Lock& lock);
Note: if any of the wait functions exits via an exception, it is unspecified whether the Lock is held. One can use a Lock type that allows to query that, such as the unique_lock wrapper.
Effects:
Atomically calls lock.unlock() and blocks on *this.
When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), or spuriously.
If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.
Postcondition: lock is locked by the calling thread.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Lock, class Predicate>
void wait(Lock& lock, Predicate pred);
Effects:
while (!pred()) wait(lock);
template <class Lock, class Clock, class Duration>
cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);
Effects:
Atomically calls lock.unlock() and blocks on *this.
When unblocked, calls lock.lock() (possibly blocking on the lock) and returns.
The function will unblock when signaled by a call to notify_one(), a call to notify_all(), expiration of the absolute timeout ([thread.req.timing]) specified by abs_time, or spuriously.
If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.
Postcondition: lock is locked by the calling thread.
Returns: cv_status::timeout if the absolute timeout ([thread.req.timing]) specified by abs_time expired, otherwise cv_status::no_timeout.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Lock, class Rep, class Period>
cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);
Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time);
Returns: cv_status::timeout if the relative timeout ([thread.req.timing]) specified by rel_time expired, otherwise cv_status::no_timeout.
Postcondition: lock is locked by the calling thread.
Throws: system_error when an exception is required ([thread.req.exception]).
template <class Lock, class Clock, class Duration, class Predicate>
bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);
Effects:
while (!pred())
if (wait_until(lock, abs_time) == cv_status::timeout)
return pred();
return true;
Returns: pred()
[ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]
template <class Lock, class Rep, class Period, class Predicate>
bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);
Effects: as if
return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));
[ Note: There is no blocking if pred() is initially true, even if the timeout has already expired. — end note ]
Postcondition: lock is locked by the calling thread.
Returns: pred()
[ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]
Throws: system_error when an exception is required ([thread.req.exception]).
[futures] describes components that a C++ program can use to retrieve in one thread the result (value or exception) from a function that has run in the same thread or another thread. [ Note: These components are not restricted to multi-threaded programs but can be useful in single-threaded programs as well. — end note ]
Header <future> synopsis
namespace std {
enum class future_errc {
broken_promise,
future_already_retrieved,
promise_already_satisfied,
no_state
};
enum class launch : unspecified {
async = unspecified,
deferred = unspecified,
implementation-defined
};
enum class future_status {
ready,
timeout,
deferred
};
template <> struct is_error_code_enum<future_errc> : public true_type { };
error_code make_error_code(future_errc e) noexcept;
error_condition make_error_condition(future_errc e) noexcept;
const error_category& future_category() noexcept;
class future_error;
template <class R> class promise;
template <class R> class promise<R&>;
template <> class promise<void>;
template <class R>
void swap(promise<R>& x, promise<R>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>;
template <class R> class future;
template <class R> class future<R&>;
template <> class future<void>;
template <class R> class shared_future;
template <class R> class shared_future<R&>;
template <> class shared_future<void>;
template <class> class packaged_task; // undefined
template <class R, class... ArgTypes>
class packaged_task<R(ArgTypes...)>;
template <class R>
void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&) noexcept;
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>;
template <class F, class... Args>
future<typename result_of<F(Args...)>::type>
async(F&& f, Args&&... args);
template <class F, class... Args>
future<typename result_of<F(Args...)>::type>
async(launch policy, F&& f, Args&&... args);
}
The enum type launch is an implementation-defined bitmask type ([bitmask.types]) with launch::async and launch::deferred denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by async() applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload of async() by adding their extensions to the launch policy under the “as if” rule. — end note ]
const error_category& future_category() noexcept;
Returns: A reference to an object of a type derived from class error_category.
The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "future".
error_code make_error_code(future_errc e) noexcept;
Returns: error_code(static_cast<int>(e), future_category()).
error_condition make_error_condition(future_errc e) noexcept;
Returns: error_condition(static_cast<int>(e), future_category()).
namespace std {
class future_error : public logic_error {
public:
future_error(error_code ec); // exposition only
const error_code& code() const noexcept;
const char* what() const noexcept;
};
}
const error_code& code() const noexcept;
Returns: The value of ec that was passed to the object's constructor.
const char *what() const noexcept;
Returns: An ntbs incorporating code().message().
Many of the classes introduced in this sub-clause use some state to communicate results. This shared state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly void) value or an exception. [ Note: Futures, promises, and tasks defined in this clause reference such shared state. — end note ]
[ Note: The result can be any kind of object including a function to compute that result, as used by async when policy is launch::deferred. — end note ]
An asynchronous return object is an object that reads results from an shared state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the shared state to be made ready. If a waiting function can return before the state is made ready because of a timeout ([thread.req.lockable]), then it is a timed waiting function, otherwise it is a non-timed waiting function.
An asynchronous provider is an object that provides a result to a shared state. The result of a shared state is set by respective functions on the asynchronous provider. [ Note: Such as promises or tasks. — end note ] The means of setting the result of a shared state is specified in the description of those classes and functions that create such a state object.
When an asynchronous return object or an asynchronous provider is said to release its shared state, it means:
if the return object or provider holds the last reference to its shared state, the shared state is destroyed; and
the return object or provider gives up its reference to its shared state.
When an asynchronous provider is said to make its shared state ready, it means:
first, the provider marks its shared state as ready; and
second, the provider unblocks any execution agents waiting for its shared state to become ready.
When an asynchronous provider is said to abandon its shared state, it means:
first, if that state is not ready, the provider
stores an exception object of type future_error with an error condition of broken_promise within its shared state; and then
makes its shared state ready;
second, the provider releases its shared state.
A shared state is ready only if it holds a value or an exception ready for retrieval. Waiting for a shared state to become ready may invoke code to compute the result on the waiting thread if so specified in the description of the class or function that creates the state object.
Calls to functions that successfully set the stored result of a shared state synchronize with ([intro.multithread]) calls to functions successfully detecting the ready state resulting from that setting. The storage of the result (whether normal or exceptional) into the shared state synchronizes with ([intro.multithread]) the successful return from a call to a waiting function on the shared state.
Some functions (e.g., promise::set_value_at_thread_exit) delay making the shared state ready until the calling thread exits. The destruction of each of that thread's objects with thread storage duration ([basic.stc.thread]) is sequenced before making that shared state ready.
Access to the result of the same shared state may conflict ([intro.multithread]). [ Note: this explicitly specifies that the result of the shared state is visible in the objects that reference this state in the sense of data race avoidance ([res.on.data.races]). For example, concurrent accesses through references returned by shared_future::get() ([futures.shared_future]) must either use read-only operations or provide additional synchronization. — end note ]
namespace std {
template <class R>
class promise {
public:
promise();
template <class Allocator>
promise(allocator_arg_t, const Allocator& a);
promise(promise&& rhs) noexcept;
promise(const promise& rhs) = delete;
~promise();
// assignment
promise& operator=(promise&& rhs) noexcept;
promise& operator=(const promise& rhs) = delete;
void swap(promise& other) noexcept;
// retrieving the result
future<R> get_future();
// setting the result
void set_value(see below);
void set_exception(exception_ptr p);
// setting the result with deferred notification
void set_value_at_thread_exit(const R& r);
void set_value_at_thread_exit(see below);
void set_exception_at_thread_exit(exception_ptr p);
};
template <class R>
void swap(promise<R>& x, promise<R>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>;
}
The implementation shall provide the template promise and two specializations, promise<R&> and promise<void>. These differ only in the argument type of the member function set_value, as set out in its description, below.
The set_value, set_exception, set_value_at_thread_exit, and set_exception_at_thread_exit member functions behave as though they acquire a single mutex associated with the promise object while updating the promise object.
template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>
: true_type { };
Requires: Alloc shall be an Allocator ([allocator.requirements]).
promise();
template <class Allocator>
promise(allocator_arg_t, const Allocator& a);
Effects: constructs a promise object and a shared state. The second constructor uses the allocator a to allocate memory for the shared state.
promise(promise&& rhs) noexcept;
Effects: constructs a new promise object and transfers ownership of the shared state of rhs (if any) to the newly-constructed object.
Postcondition: rhs has no shared state.
Effects: Abandons any shared state ([futures.state]).
promise& operator=(promise&& rhs) noexcept;
Effects: Abandons any shared state ([futures.state]) and then as if promise(std::move(rhs)).swap(*this).
Returns: *this.
void swap(promise& other) noexcept;
Effects: Exchanges the shared state of *this and other.
Postcondition: *this has the shared state (if any) that other had prior to the call to swap. other has the shared state (if any) that *this had prior to the call to swap.
Returns: A future<R> object with the same shared state as *this.
Throws: future_error if *this has no shared state or if get_future has already been called on a promise with the same shared state as *this.
void promise::set_value(const R& r);
void promise::set_value(R&& r);
void promise<R&>::set_value(R& r);
void promise<void>::set_value();
Effects: atomically stores the value r in the shared state and makes that state ready ([futures.state]).
void set_exception(exception_ptr p);
Effects: atomically stores the exception pointer p in the shared state and makes that state ready ([futures.state]).
Throws: future_error if its shared state already has a stored value or exception.
void promise::set_value_at_thread_exit(const R& r);
void promise::set_value_at_thread_exit(R&& r);
void promise<R&>::set_value_at_thread_exit(R& r);
void promise<void>::set_value_at_thread_exit();
Effects: Stores the value r in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.
Throws: future_error if an error condition occurs.
void promise::set_exception_at_thread_exit(exception_ptr p);
Effects: Stores the exception pointer p in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.
Throws: future_error if an error condition occurs.
template <class R>
void swap(promise<R>& x, promise<R>& y);
Effects: x.swap(y).
The class template future defines a type for asynchronous return objects which do not share their shared state with other asynchronous return objects. A default-constructed future object has no shared state. A future object with shared state can be created by functions on asynchronous providers ([futures.state]) or by the move constructor and shares its shared state with the original asynchronous provider. The result (value or exception) of a future object can be set by calling a respective function on an object that shares the same shared state.
[ Note: Member functions of future do not synchronize with themselves or with member functions of shared_future. — end note ]
The effect of calling any member function other than the destructor, the move-assignment operator, or valid on a future object for which valid() == false is undefined. [ Note: Implementations are encouraged to detect this case and throw an object of type future_error with an error condition of future_errc::no_state. — end note ]
namespace std {
template <class R>
class future {
public:
future() noexcept;
future(future &&) noexcept;
future(const future& rhs) = delete;
~future();
future& operator=(const future& rhs) = delete;
future& operator=(future&&) noexcept;
shared_future<R> share();
// retrieving the value
see below get();
// functions to check state
bool valid() const noexcept;
void wait() const;
template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
};
}
The implementation shall provide the template future and two specializations, future<R&> and future<void>. These differ only in the return type and return value of the member function get, as set out in its description, below.
Postcondition: valid() == false.
future(future&& rhs) noexcept;
Effects: move constructs a future object that refers to the shared state that was originally referred to by rhs (if any).
Effects:
releases any shared state ([futures.state]);
destroys *this.
future& operator=(future&& rhs) noexcept;
Effects:
releases any shared state ([futures.state]).
move assigns the contents of rhs to *this.
Returns: shared_future<R>(std::move(*this)).
Postcondition: valid() == false.
R future::get();
R& future<R&>::get();
void future<void>::get();
Note: as described above, the template and its two required specializations differ only in the return type and return value of the member function get.
Effects: wait()s until the shared state is ready, then retrieves the value stored in the shared state.
Returns:
future::get() returns the value stored in the object's shared state. If the type of the value is MoveAssignable the returned value is moved, otherwise it is copied.
future<R&>::get() returns the reference stored as value in the object's shared state.
future<void>::get() returns nothing.
Throws: the stored exception, if an exception was stored in the shared state.
Postcondition: valid() == false.
Returns: true only if *this refers to a shared state.
Effects: blocks until the shared state is ready.
template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
Effects: none if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the relative timeout ([thread.req.timing]) specified by rel_time has expired.
Returns:
future_status::deferred if the shared state contains a deferred function.
future_status::ready if the shared state is ready.
future_status::timeout if the function is returning because the relative timeout ([thread.req.timing]) specified by rel_time has expired.
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
Effects: none if the shared state contains a deferred function ([futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout ([thread.req.timing]) specified by abs_time has expired.
Returns:
future_status::deferred if the shared state contains a deferred function.
future_status::ready if the shared state is ready.
future_status::timeout if the function is returning because the absolute timeout ([thread.req.timing]) specified by abs_time has expired.
The function template async provides a mechanism to launch a function potentially in a new thread and provides the result of the function in a future object with which it shares a shared state.
template <class F, class... Args>
future<typename result_of<F(Args...)>::type>
async(F&& f, Args&&... args);
template <class F, class... Args>
future<typename result_of<F(Args...)>::type>
async(launch policy, F&& f, Args&&... args);
Requires: F and each Ti in Args shall satisfy the MoveConstructible requirements. INVOKE(DECAY_COPY(std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...) ([func.require], [thread.thread.constr]) shall be a valid expression.
Effects: The first function behaves the same as a call to the second function with a policy argument of launch::async | launch::deferred and the same arguments for F and Args. The second function creates a shared state that is associated with the returned future object. The further behavior of the second function depends on the policy argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):
if policy & launch::async is non-zero — calls
INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
([func.require], [thread.thread.constr])
as if in a new thread of execution represented by a thread object
with the calls to DECAY_COPY() being evaluated in the thread that called async.
Any return value
is stored as the result in the
shared state. Any exception propagated from
the execution of
INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
is stored as the exceptional result in the shared state.
The thread object is
stored in the shared state
and affects the behavior of any asynchronous return objects that
reference that state.
if policy & launch::deferred is non-zero —
Stores DECAY_COPY(std::forward<F>(f)) and
DECAY_COPY(std::forward<Args>(args))...
in the shared state. These copies of f and args constitute
a deferred function. Invocation of the deferred function evaluates
INVOKE(g, xyz) where g is the stored value of
DECAY_COPY(std::forward<F>(f)) and xyz is the stored copy of
DECAY_COPY(std::forward<Args>(args)).... The shared state is not
made ready until the function has completed. The first call to a
non-timed waiting function ([futures.state])
on an asynchronous return object referring to
this shared state shall invoke the
deferred function in the thread that called the waiting function.
Once evaluation of INVOKE(g, xyz) begins, the function is no longer
considered deferred.
[ Note: If this policy is specified together with other policies, such as when using a
policy value of launch::async | launch::deferred, implementations should defer
invocation or the selection of the policy when no more concurrency can be effectively
exploited. — end note ]
Returns: An object of type future<typename result_of<F(Args...)>::type> that refers to the shared state created by this call to async.
Synchronization: Regardless of the provided policy argument,
the invocation of async synchronizes with ([intro.multithread]) the invocation of f. [ Note: This statement applies even when the corresponding future object is moved to another thread. — end note ]; and
the completion of the function f is sequenced before ([intro.multithread]) the shared state is made ready. [ Note: f might not be called at all, so its completion might never happen. — end note ]
If the implementation chooses the launch::async policy,
a call to a waiting function on an asynchronous return object that shares the shared state created by this async call shall block until the associated thread has completed, as if joined ([thread.thread.member]);
the associated thread completion synchronizes with ([intro.multithread]) the return from the first function that successfully detects the ready status of the shared state or with the return from the last function that releases the shared state, whichever happens first.
Throws: system_error if policy is launch::async and the implementation is unable to start a new thread.
Error conditions:
resource_unavailable_try_again — if policy is launch::async and the system is unable to start a new thread.
Remarks: The first signature shall not participate in overload resolution if decay<F>::type is std::launch.
[ Example:
int work1(int value);
int work2(int value);
int work(int value) {
auto handle = std::async([=]{ return work2(value); });
int tmp = work1(value);
return tmp + handle.get(); // #1
}
[ Note: Line #1 might not result in concurrency because the async call uses the default policy, which may use launch::deferred, in which case the lambda might not be invoked until the get() call; in that case, work1 and work2 are called on the same thread and there is no concurrency. — end note ] — end example ]
The class template packaged_task defines a type for wrapping a function or callable object so that the return value of the function or callable object is stored in a future when it is invoked.
When the packaged_task object is invoked, its stored task is invoked and the result (whether normal or exceptional) stored in the shared state. Any futures that share the shared state will then be able to access the stored result.
namespace std {
template<class> class packaged_task; // undefined
template<class R, class... ArgTypes>
class packaged_task<R(ArgTypes...)> {
public:
// construction and destruction
packaged_task() noexcept;
template <class F>
explicit packaged_task(F&& f);
template <class F, class Allocator>
explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
~packaged_task();
// no copy
packaged_task(packaged_task&) = delete;
packaged_task& operator=(packaged_task&) = delete;
// move support
packaged_task(packaged_task&& rhs) noexcept;
packaged_task& operator=(packaged_task&& rhs) noexcept;
void swap(packaged_task& other) noexcept;
bool valid() const noexcept;
// result retrieval
future<R> get_future();
// execution
void operator()(ArgTypes... );
void make_ready_at_thread_exit(ArgTypes...);
void reset();
};
template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>;
}
Effects: constructs a packaged_task object with no shared state and no stored task.
template <class F>
packaged_task(F&& f);
template <class F, class Allocator>
explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);
Requires: INVOKE(f, t1, t2, ..., tN, R), where t1, t2, ..., tN are values of the corresponding types in ArgTypes..., shall be a valid expression. Invoking a copy of f shall behave the same as invoking f.
Effects: constructs a new packaged_task object with a shared state and initializes the object's stored task with std::forward<F>(f). The constructors that take an Allocator argument use it to allocate memory needed to store the internal data structures.
Throws: any exceptions thrown by the copy or move constructor of f, or std::bad_alloc if memory for the internal data structures could not be allocated.
packaged_task(packaged_task&& rhs) noexcept;
Effects: constructs a new packaged_task object and transfers ownership of rhs's shared state to *this, leaving rhs with no shared state. Moves the stored task from rhs to *this.
Postcondition: rhs has no shared state.
packaged_task& operator=(packaged_task&& rhs) noexcept;
Effects:
releases any shared state ([futures.state]).
packaged_task(std::move(rhs)).swap(*this).
Effects: Abandons any shared state. ([futures.state]).
void swap(packaged_task& other) noexcept;
Effects: exchanges the shared states and stored tasks of *this and other.
Postcondition: *this has the same shared state and stored task (if any) as other prior to the call to swap. other has the same shared state and stored task (if any) as *this prior to the call to swap.
Returns: true only if *this has a shared state.
Returns: A future object that shares the same shared state as *this.
Throws: a future_error object if an error occurs.
void operator()(ArgTypes... args);
Effects: INVOKE(f, t1, t2, ..., tN, R), where f is the stored task of *this and t1, t2, ..., tN are the values in args.... If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored. The shared state of *this is made ready, and any threads blocked in a function waiting for the shared state of *this to become ready are unblocked.
Throws: a future_error exception object if there is no shared state or the stored task has already been invoked.
Synchronization: a successful call to operator() synchronizes with ([intro.multithread]) a call to any member function of a future or shared_future object that shares the shared state of *this. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with ([intro.multithread]) the successful return from any member function that detects that the state is set to ready. [ Note: operator() synchronizes and serializes with other functions through the shared state. — end note ]
void make_ready_at_thread_exit(ArgTypes... args);
Effects: INVOKE(f, t1, t2, ..., tN, R), where f is the stored task and t1, t2, ..., tN are the values in args.... If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored. In either case, this shall be done without making that state ready ([futures.state]) immediately. Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.
Throws: future_error if an error condition occurs.
Effects: as if *this = packaged_task(std::move(f)), where f is the task stored in *this. [ Note: This constructs a new shared state for *this. The old state is abandoned ([futures.state]). — end note ]
template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
Effects: x.swap(y)
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>
: true_type { };
Requires: Alloc shall be an Allocator ([allocator.requirements]).