33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

33.6.1 General [thread.mutex.general]

Subclause [thread.mutex] provides mechanisms for mutual exclusion: mutexes, locks, and call once.

These mechanisms ease the production of race-free programs ([intro.multithread]).

33.6.2 Header <mutex> synopsis [mutex.syn]

namespace std { // [thread.mutex.class], class mutex class mutex; // [thread.mutex.recursive], class recursive_mutex class recursive_mutex; // [thread.timedmutex.class] class timed_mutex class timed_mutex; // [thread.timedmutex.recursive], class recursive_timed_mutex class recursive_timed_mutex; struct defer_lock_t { explicit defer_lock_t() = default; }; struct try_to_lock_t { explicit try_to_lock_t() = default; }; struct adopt_lock_t { explicit adopt_lock_t() = default; }; inline constexpr defer_lock_t defer_lock { }; inline constexpr try_to_lock_t try_to_lock { }; inline constexpr adopt_lock_t adopt_lock { }; // [thread.lock], locks template<class Mutex> class lock_guard; template<class... MutexTypes> class scoped_lock; template<class Mutex> class unique_lock; template<class Mutex> void swap(unique_lock<Mutex>& x, unique_lock<Mutex>& y) noexcept; // [thread.lock.algorithm], generic locking algorithms 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; template<class Callable, class... Args> void call_once(once_flag& flag, Callable&& func, Args&&... args); }

33.6.3 Header <shared_mutex> synopsis [shared.mutex.syn]

33.6.4 Mutex requirements [thread.mutex.requirements]

33.6.4.1 In general [thread.mutex.requirements.general]

A mutex object facilitates protection against data races and allows safe synchronization of data between execution agents .

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.

Both recursive and non-recursive mutexes are supplied.

33.6.4.2 Mutex types [thread.mutex.requirements.mutex]

33.6.4.2.1 General [thread.mutex.requirements.mutex.general]

The mutex types are the standard library types mutex, recursive_mutex, timed_mutex, recursive_timed_mutex, shared_mutex, and shared_timed_mutex.

They meet the requirements set out in [thread.mutex.requirements.mutex].

In this description, m denotes an object of a mutex type.

[Note 1:

The mutex types meet the Cpp17Lockable requirements ([thread.req.lockable.req]).

— end note]

The mutex types meet Cpp17DefaultConstructible and Cpp17Destructible.

If initialization of an object of a mutex type fails, an exception of type system_error is thrown.

The mutex types are neither copyable nor movable.

The error conditions for error codes, if any, reported by member functions of the mutex types are as follows:

(3.1)
resource_unavailable_try_again — if any native handle type manipulated is not available.
(3.2)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(3.3)
invalid_argument — if any native handle type manipulated as part of mutex construction is incorrect.

The implementation provides 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 appears to occur in a single total order.

[Note 2:

This can be viewed as the modification order of the mutex.

— end note]

[Note 3:

Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization can be used to ensure that mutex objects are initialized and visible to other threads.

— end note]

The expression m.lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_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.

Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Postconditions: The calling thread owns the mutex.

Return type: void.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(12.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(12.2)
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.

The expression m.try_lock() is well-formed and has the following semantics:

Preconditions: If m is of type mutex, timed_mutex, shared_mutex, or shared_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 4:

This spurious failure is normally uncommon, but allows interesting implementations based on a simple compare and exchange ([atomics]).

— end note]

An implementation should ensure that try_lock() does not consistently return false in the absence of contending mutex acquisitions.

Synchronization: If try_lock() returns true, prior unlock() operations on the same object synchronize with this operation.

[Note 5:

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]

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Nothing.

The expression m.unlock() is well-formed and has the following semantics:

Preconditions: The calling thread owns the mutex.

Effects: Releases the calling thread's ownership of the mutex.

Return type: void.

Synchronization: This operation synchronizes with subsequent lock operations that obtain ownership on the same object.

Throws: Nothing.

33.6.4.2.2 Class mutex [thread.mutex.class]

🔗

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(); using native_handle_type = implementation-defined; // 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 1:

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 meets all of the mutex requirements ([thread.mutex.requirements]).

It is a standard-layout class ([class.prop]).

[Note 2:

A program can 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 might 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.

33.6.4.2.3 Class recursive_mutex [thread.mutex.recursive]

🔗

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(); using native_handle_type = implementation-defined; // 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 meets all of the mutex requirements ([thread.mutex.requirements]).

It is a standard-layout class ([class.prop]).

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() fail, and additional calls to lock() 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 behavior of a program is undefined if:

(4.1)
it destroys a recursive_mutex object owned by any thread or
(4.2)
a thread terminates while owning a recursive_mutex object.

33.6.4.3 Timed mutex types [thread.timedmutex.requirements]

33.6.4.3.1 General [thread.timedmutex.requirements.general]

The timed mutex types are the standard library types timed_mutex, recursive_timed_mutex, and shared_timed_mutex.

They 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, and abs_time denotes an object of an instantiation of time_point .

[Note 1:

The timed mutex types meet the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

— end note]

The expression m.try_lock_for(rel_time) is well-formed and has the following semantics:

Preconditions: If m is of type timed_mutex or shared_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 returns within the timeout specified by rel_time only if it has obtained ownership of the mutex object.

[Note 2:

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]

Synchronization: If try_lock_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

The expression m.try_lock_until(abs_time) is well-formed and has the following semantics:

Preconditions: If m is of type timed_mutex or shared_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 returns before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained ownership of the mutex object.

[Note 3:

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]

Synchronization: If try_lock_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if ownership was obtained, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

33.6.4.3.2 Class timed_mutex [thread.timedmutex.class]

🔗

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(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); using native_handle_type = implementation-defined; // 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 meets all of the timed mutex requirements ([thread.timedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if:

(3.1)
it destroys a timed_mutex object owned by any thread,
(3.2)
a thread that owns a timed_mutex object calls lock(), try_lock(), try_lock_for(), or try_lock_until() on that object, or
(3.3)
a thread terminates while owning a timed_mutex object.

33.6.4.3.3 Class recursive_timed_mutex [thread.timedmutex.recursive]

🔗

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(); // blocking 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(); using native_handle_type = implementation-defined; // 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 meets all of the timed mutex requirements ([thread.timedmutex.requirements]).

It is a standard-layout class ([class.prop]).

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() fail, and additional calls to lock() 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.

The behavior of a program is undefined if:

(4.1)
it destroys a recursive_timed_mutex object owned by any thread, or
(4.2)
a thread terminates while owning a recursive_timed_mutex object.

33.6.4.4 Shared mutex types [thread.sharedmutex.requirements]

33.6.4.4.1 General [thread.sharedmutex.requirements.general]

The standard library types shared_mutex and shared_timed_mutex are shared mutex types.

Shared mutex types meet the requirements of mutex types ([thread.mutex.requirements.mutex]) and additionally meet the requirements set out below.

In this description, m denotes an object of a shared mutex type.

[Note 1:

The shared mutex types meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

— end note]

In addition to the exclusive lock ownership mode specified in [thread.mutex.requirements.mutex], shared mutex types provide a shared lock ownership mode.

Multiple execution agents can simultaneously hold a shared lock ownership of a shared mutex type.

But no execution agent holds a shared lock while another execution agent holds an exclusive lock on the same shared mutex type, and vice-versa.

The maximum number of execution agents which can share a shared lock on a single shared mutex type is unspecified, but is at least 10000.

If more than the maximum number of execution agents attempt to obtain a shared lock, the excess execution agents block until the number of shared locks are reduced below the maximum amount by other execution agents releasing their shared lock.

The expression m.lock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread.

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: Prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Postconditions: The calling thread has a shared lock on the mutex.

Return type: void.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(10.1)
operation_not_permitted — if the thread does not have the privilege to perform the operation.
(10.2)
resource_deadlock_would_occur — if the implementation detects that a deadlock would occur.

The expression m.unlock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread holds a shared lock on the mutex.

Effects: Releases a shared lock on the mutex held by the calling thread.

Return type: void.

Synchronization: This operation synchronizes with subsequent lock() operations that obtain ownership on the same object.

Throws: Nothing.

The expression m.try_lock_shared() is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Attempts to obtain shared ownership of the mutex for the calling thread without blocking.

If shared ownership is not obtained, there is no effect and try_lock_shared() immediately returns.

An implementation may fail to obtain the lock even if it is not held by any other thread.

Synchronization: If try_lock_shared() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Nothing.

33.6.4.4.2 Class shared_mutex [thread.sharedmutex.class]

🔗

namespace std { class shared_mutex { public: shared_mutex(); ~shared_mutex(); shared_mutex(const shared_mutex&) = delete; shared_mutex& operator=(const shared_mutex&) = delete; // exclusive ownership void lock(); // blocking bool try_lock(); void unlock(); // shared ownership void lock_shared(); // blocking bool try_lock_shared(); void unlock_shared(); using native_handle_type = implementation-defined; // see [thread.req.native] native_handle_type native_handle(); // see [thread.req.native] }; }

The class shared_mutex provides a non-recursive mutex with shared ownership semantics.

The class shared_mutex meets all of the shared mutex requirements ([thread.sharedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if:

(3.1)
it destroys a shared_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_mutex.

shared_mutex may be a synonym for shared_timed_mutex.

33.6.4.5 Shared timed mutex types [thread.sharedtimedmutex.requirements]

33.6.4.5.1 General [thread.sharedtimedmutex.requirements.general]

The standard library type shared_timed_mutex is a shared timed mutex type.

Shared timed mutex types meet the requirements of timed mutex types ([thread.timedmutex.requirements]), shared mutex types ([thread.sharedmutex.requirements]), and additionally meet the requirements set out below.

In this description, m denotes an object of a shared timed 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 .

[Note 1:

The shared timed mutex types meet the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

— end note]

The expression m.try_lock_shared_for(rel_time) is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: Attempts to obtain shared lock ownership for the calling thread 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_shared()).

The function returns within the timeout specified by rel_time only if it has obtained shared ownership of the mutex object.

[Note 2:

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]

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: If try_lock_shared_for() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

The expression m.try_lock_shared_until(abs_time) is well-formed and has the following semantics:

Preconditions: The calling thread has no ownership of the mutex.

Effects: The function attempts to obtain shared ownership of the mutex.

If abs_time has already passed, the function attempts to obtain shared ownership without blocking (as if by calling try_lock_shared()).

The function returns before the absolute timeout ([thread.req.timing]) specified by abs_time only if it has obtained shared ownership of the mutex object.

[Note 3:

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]

If an exception is thrown then a shared lock has not been acquired for the current thread.

Synchronization: If try_lock_shared_until() returns true, prior unlock() operations on the same object synchronize with ([intro.multithread]) this operation.

Return type: bool.

Returns: true if the shared lock was acquired, otherwise false.

Throws: Timeout-related exceptions ([thread.req.timing]).

33.6.4.5.2 Class shared_timed_mutex [thread.sharedtimedmutex.class]

🔗

namespace std { class shared_timed_mutex { public: shared_timed_mutex(); ~shared_timed_mutex(); shared_timed_mutex(const shared_timed_mutex&) = delete; shared_timed_mutex& operator=(const shared_timed_mutex&) = delete; // exclusive ownership void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); // shared ownership void lock_shared(); // blocking bool try_lock_shared(); template<class Rep, class Period> bool try_lock_shared_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_shared_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock_shared(); }; }

The class shared_timed_mutex provides a non-recursive mutex with shared ownership semantics.

The class shared_timed_mutex meets all of the shared timed mutex requirements ([thread.sharedtimedmutex.requirements]).

It is a standard-layout class ([class.prop]).

The behavior of a program is undefined if:

(3.1)
it destroys a shared_timed_mutex object owned by any thread,
(3.2)
a thread attempts to recursively gain any ownership of a shared_timed_mutex, or
(3.3)
a thread terminates while possessing any ownership of a shared_timed_mutex.

33.6.5 Locks [thread.lock]

33.6.5.1 General [thread.lock.general]

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 1:

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 inline constexpr defer_lock_t defer_lock { }; inline constexpr try_to_lock_t try_to_lock { }; inline constexpr adopt_lock_t adopt_lock { }; }

33.6.5.2 Class template lock_guard [thread.lock.guard]

🔗

namespace std { template<class Mutex> class lock_guard { public: using mutex_type = Mutex; explicit lock_guard(mutex_type& m); lock_guard(mutex_type& m, adopt_lock_t); ~lock_guard(); lock_guard(const lock_guard&) = delete; lock_guard& operator=(const lock_guard&) = delete; private: mutex_type& pm; // exposition only }; }

An object of type lock_guard controls the ownership of a lockable object within a scope.

A lock_guard object maintains ownership of a lockable object throughout the lock_guard object's lifetime .

The behavior of a program is undefined if the lockable object referenced by pm does not exist for the entire lifetime of the lock_guard object.

The supplied Mutex type shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

🔗

explicit lock_guard(mutex_type& m);

Effects: Initializes pm with m.

Calls m.lock().

🔗

lock_guard(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a non-shared lock on m.

Effects: Initializes pm with m.

Throws: Nothing.

🔗

~lock_guard();

Effects: Equivalent to: pm.unlock()

33.6.5.3 Class template scoped_lock [thread.lock.scoped]

🔗

namespace std { template<class... MutexTypes> class scoped_lock { public: using mutex_type = see below; // Only if sizeof...(MutexTypes) == 1 is true explicit scoped_lock(MutexTypes&... m); explicit scoped_lock(adopt_lock_t, MutexTypes&... m); ~scoped_lock(); scoped_lock(const scoped_lock&) = delete; scoped_lock& operator=(const scoped_lock&) = delete; private: tuple<MutexTypes&...> pm; // exposition only }; }

An object of type scoped_lock controls the ownership of lockable objects within a scope.

A scoped_lock object maintains ownership of lockable objects throughout the scoped_lock object's lifetime .

The behavior of a program is undefined if the lockable objects referenced by pm do not exist for the entire lifetime of the scoped_lock object.

(1.1)
If sizeof...(MutexTypes) is one, let Mutex denote the sole type constituting the pack MutexTypes.

Mutex shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

The member typedef-name mutex_type denotes the same type as Mutex.
(1.2)
Otherwise, all types in the template parameter pack MutexTypes shall meet the Cpp17Lockable requirements ([thread.req.lockable.req]) and there is no member mutex_type.

🔗

explicit scoped_lock(MutexTypes&... 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, lock(m...).

🔗

explicit scoped_lock(adopt_lock_t, MutexTypes&... m);

Preconditions: The calling thread holds a non-shared lock on each element of m.

Effects: Initializes pm with tie(m...).

Throws: Nothing.

🔗

~scoped_lock();

Effects: For all i in [0, sizeof...(MutexTypes)), get<i>(pm).unlock().

33.6.5.4 Class template unique_lock [thread.lock.unique]

33.6.5.4.1 General [thread.lock.unique.general]

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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 }; }

An object of type unique_lock controls the ownership of a lockable object within a scope.

Ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another unique_lock object.

Objects of type unique_lock are not copyable but are movable.

The behavior of a program is undefined if the contained pointer pm is not null and the lockable object pointed to by pm does not exist for the entire remaining lifetime ([basic.life]) of the unique_lock object.

The supplied Mutex type shall meet the Cpp17BasicLockable requirements ([thread.req.lockable.basic]).

[Note 1:

unique_lock<Mutex> meets the Cpp17BasicLockable requirements.

If Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]), unique_lock<Mutex> also meets the Cpp17Lockable requirements; if Mutex meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]), unique_lock<Mutex> also meets the Cpp17TimedLockable requirements.

— end note]

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

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unique_lock() noexcept;

Postconditions: pm == nullptr and owns == false.

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explicit unique_lock(mutex_type& m);

Effects: Calls m.lock().

Postconditions: pm == addressof(m) and owns == true.

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unique_lock(mutex_type& m, defer_lock_t) noexcept;

Postconditions: pm == addressof(m) and owns == false.

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unique_lock(mutex_type& m, try_to_lock_t);

Preconditions: The supplied Mutex type meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

Effects: Calls m.try_lock().

Postconditions: pm == addressof(m) and owns == res, where res is the value returned by the call to m.try_lock().

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unique_lock(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a non-shared lock on m.

Postconditions: pm == addressof(m) and owns == true.

Throws: Nothing.

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: 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).

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: 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).

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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.

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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 1:

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.

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~unique_lock();

Effects: If owns calls pm->unlock().

33.6.5.4.3 Locking [thread.lock.unique.locking]

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void lock();

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]).

Error conditions:

(4.1)
operation_not_permitted — if pm is nullptr.
(4.2)
resource_deadlock_would_occur — if on entry owns is true.

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bool try_lock();

Preconditions: The supplied Mutex meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

Effects: As if by pm->try_lock().

Postconditions: owns == res, where res is the value returned by pm->try_lock().

Returns: The value returned by pm->try_lock().

Throws: Any exception thrown by pm->try_lock().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(10.1)
operation_not_permitted — if pm is nullptr.
(10.2)
resource_deadlock_would_occur — if on entry owns is true.

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: As if by pm->try_lock_until(abs_time).

Postconditions: owns == res, where res is the value returned by pm->try_lock_until(abs_time).

Returns: The value returned by pm->try_lock_until(abs_time).

Throws: Any exception thrown by pm->try_lock_until(abstime).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(16.1)
operation_not_permitted — if pm is nullptr.
(16.2)
resource_deadlock_would_occur — if on entry owns is true.

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

Preconditions: The supplied Mutex type meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

Effects: As if by pm->try_lock_for(rel_time).

Postconditions: owns == res, where res is the value returned by pm->try_lock_for(rel_time).

Returns: The value returned by pm->try_lock_for(rel_time).

Throws: Any exception thrown by pm->try_lock_for(rel_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(22.1)
operation_not_permitted — if pm is nullptr.
(22.2)
resource_deadlock_would_occur — if on entry owns is true.

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void unlock();

Effects: As if by pm->unlock().

Postconditions: owns == false.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(26.1)
operation_not_permitted — if on entry owns is false.

33.6.5.4.4 Modifiers [thread.lock.unique.mod]

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void swap(unique_lock& u) noexcept;

Effects: Swaps the data members of *this and u.

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mutex_type* release() noexcept;

Postconditions: pm == 0 and owns == false.

Returns: The previous value of pm.

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

Effects: As if by x.swap(y).

33.6.5.4.5 Observers [thread.lock.unique.obs]

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bool owns_lock() const noexcept;

Returns: owns.

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explicit operator bool() const noexcept;

Returns: owns.

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mutex_type *mutex() const noexcept;

Returns: pm.

33.6.5.5 Class template shared_lock [thread.lock.shared]

33.6.5.5.1 General [thread.lock.shared.general]

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namespace std { template<class Mutex> class shared_lock { public: using mutex_type = Mutex; // [thread.lock.shared.cons], construct/copy/destroy shared_lock() noexcept; explicit shared_lock(mutex_type& m); // blocking shared_lock(mutex_type& m, defer_lock_t) noexcept; shared_lock(mutex_type& m, try_to_lock_t); shared_lock(mutex_type& m, adopt_lock_t); template<class Clock, class Duration> shared_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time); template<class Rep, class Period> shared_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time); ~shared_lock(); shared_lock(const shared_lock&) = delete; shared_lock& operator=(const shared_lock&) = delete; shared_lock(shared_lock&& u) noexcept; shared_lock& operator=(shared_lock&& u) noexcept; // [thread.lock.shared.locking], locking void lock(); // blocking bool try_lock(); template<class Rep, class Period> bool try_lock_for(const chrono::duration<Rep, Period>& rel_time); template<class Clock, class Duration> bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time); void unlock(); // [thread.lock.shared.mod], modifiers void swap(shared_lock& u) noexcept; mutex_type* release() noexcept; // [thread.lock.shared.obs], observers bool owns_lock() const noexcept; explicit operator bool () const noexcept; mutex_type* mutex() const noexcept; private: mutex_type* pm; // exposition only bool owns; // exposition only }; }

An object of type shared_lock controls the shared ownership of a lockable object within a scope.

Shared ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another shared_lock object.

Objects of type shared_lock are not copyable but are movable.

The supplied Mutex type shall meet the Cpp17SharedLockable requirements ([thread.req.lockable.shared]).

[Note 1:

shared_lock<Mutex> meets the Cpp17Lockable requirements ([thread.req.lockable.req]).

If Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]), shared_lock<Mutex> also meets the Cpp17TimedLockable requirements ([thread.req.lockable.timed]).

— end note]

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

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shared_lock() noexcept;

Postconditions: pm == nullptr and owns == false.

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explicit shared_lock(mutex_type& m);

Effects: Calls m.lock_shared().

Postconditions: pm == addressof(m) and owns == true.

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shared_lock(mutex_type& m, defer_lock_t) noexcept;

Postconditions: pm == addressof(m) and owns == false.

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shared_lock(mutex_type& m, try_to_lock_t);

Effects: Calls m.try_lock_shared().

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared().

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shared_lock(mutex_type& m, adopt_lock_t);

Preconditions: The calling thread holds a shared lock on m.

Postconditions: pm == addressof(m) and owns == true.

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: Calls m.try_lock_shared_until(abs_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_until(abs_time).

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: Calls m.try_lock_shared_for(rel_time).

Postconditions: pm == addressof(m) and owns == res where res is the value returned by the call to m.try_lock_shared_for(rel_time).

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~shared_lock();

Effects: If owns calls pm->unlock_shared().

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shared_lock(shared_lock&& sl) noexcept;

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this construction), sl.pm == nullptr and sl.owns == false.

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

Effects: If owns calls pm->unlock_shared().

Postconditions: pm == sl_p.pm and owns == sl_p.owns (where sl_p is the state of sl just prior to this assignment), sl.pm == nullptr and sl.owns == false.

33.6.5.5.3 Locking [thread.lock.shared.locking]

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void lock();

Effects: As if by pm->lock_shared().

Postconditions: owns == true.

Throws: Any exception thrown by pm->lock_shared().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(4.1)
operation_not_permitted — if pm is nullptr.
(4.2)
resource_deadlock_would_occur — if on entry owns is true.

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bool try_lock();

Effects: As if by pm->try_lock_shared().

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared().

Returns: The value returned by the call to pm->try_lock_shared().

Throws: Any exception thrown by pm->try_lock_shared().

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(9.1)
operation_not_permitted — if pm is nullptr.
(9.2)
resource_deadlock_would_occur — if on entry owns is true.

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by pm->try_lock_shared_until(abs_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_until(abs_time).

Returns: The value returned by the call to pm->try_lock_shared_until(abs_time).

Throws: Any exception thrown by pm->try_lock_shared_until(abs_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(15.1)
operation_not_permitted — if pm is nullptr.
(15.2)
resource_deadlock_would_occur — if on entry owns is true.

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

Preconditions: Mutex meets the Cpp17SharedTimedLockable requirements ([thread.req.lockable.shared.timed]).

Effects: As if by pm->try_lock_shared_for(rel_time).

Postconditions: owns == res, where res is the value returned by the call to pm->try_lock_shared_for(rel_time).

Returns: The value returned by the call to pm->try_lock_shared_for(rel_time).

Throws: Any exception thrown by pm->try_lock_shared_for(rel_time).

system_error when an exception is required ([thread.req.exception]).

Error conditions:

(21.1)
operation_not_permitted — if pm is nullptr.
(21.2)
resource_deadlock_would_occur — if on entry owns is true.

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void unlock();

Effects: As if by pm->unlock_shared().

Postconditions: owns == false.

Throws: system_error when an exception is required ([thread.req.exception]).

Error conditions:

(25.1)
operation_not_permitted — if on entry owns is false.

33.6.5.5.4 Modifiers [thread.lock.shared.mod]

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void swap(shared_lock& sl) noexcept;

Effects: Swaps the data members of *this and sl.

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mutex_type* release() noexcept;

Postconditions: pm == nullptr and owns == false.

Returns: The previous value of pm.

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

Effects: As if by x.swap(y).

33.6.5.5.5 Observers [thread.lock.shared.obs]

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bool owns_lock() const noexcept;

Returns: owns.

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explicit operator bool() const noexcept;

Returns: owns.

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mutex_type* mutex() const noexcept;

Returns: pm.

33.6.6 Generic locking algorithms [thread.lock.algorithm]

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template<class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

Preconditions: Each template parameter type meets the Cpp17Lockable requirements.

[Note 1:

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() is called for all prior arguments with no further calls to try_lock().

Returns: -1 if all calls to try_lock() returned true, otherwise a zero-based index value that indicates the argument for which try_lock() returned false.

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template<class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

Preconditions: Each template parameter type meets the Cpp17Lockable requirements.

[Note 2:

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 does not result in deadlock, but is otherwise unspecified.

[Note 3:

A deadlock avoidance algorithm such as try-and-back-off can 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() is called for any argument that had been locked by a call to lock() or try_lock().

33.6.7 Call once [thread.once]

33.6.7.1 Struct once_flag [thread.once.onceflag]

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namespace std { struct once_flag { constexpr once_flag() noexcept; once_flag(const once_flag&) = delete; once_flag& operator=(const once_flag&) = delete; }; }

The class once_flag is an opaque data structure that call_once uses to initialize data without causing a data race or deadlock.

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constexpr once_flag() noexcept;

Synchronization: The construction of a once_flag object is not synchronized.

Postconditions: 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.

33.6.7.2 Function call_once [thread.once.callonce]

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template<class Callable, class... Args>
  void call_once(once_flag& flag, Callable&& func, Args&&... args);

Mandates: is_invocable_v<Callable, Args...> is true.

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 calls INVOKE(std::forward<Callable>(func), std::forward<Args>(args)...) ([func.require]).

If such a call to func throws an exception the execution is exceptional, otherwise it is returning.

An exceptional execution propagates the exception to the caller of call_once.

Among all executions of call_once for any given once_flag: at most one is a returning execution; if there is a returning execution, it is the last active execution; and there are passive executions only if there is a returning execution.

[Note 1:

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 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 1: // 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]