condition_variable Class

Use the condition_variable class to wait for an event when you have a mutex of type unique_lock<mutex>. Objects of this type may have better performance than objects of type condition_variable_any<unique_lock<mutex>>.

Syntax

class condition_variable;

Members

Constructors

Name	Description
condition_variable	Constructs a condition_variable object.

Functions

Name	Description
native_handle	Returns the implementation-specific type representing the condition_variable handle.
notify_all	Unblocks all threads that are waiting for the condition_variable object.
notify_one	Unblocks one of the threads that are waiting for the condition_variable object.
wait	Blocks a thread.
wait_for	Blocks a thread, and sets a time interval after which the thread unblocks.
wait_until	Blocks a thread, and sets a maximum point in time at which the thread unblocks.

condition_variable

Constructs a condition_variable object.

condition_variable();

Remarks

If not enough memory is available, the constructor throws a system_error object that has a not_enough_memory error code. If the object cannot be constructed because some other resource is not available, the constructor throws a system_error object that has a resource_unavailable_try_again error code.

native_handle

Returns the implementation-specific type that represents the condition_variable handle.

native_handle_type native_handle();

Return Value

native_handle_type is defined as a pointer to Concurrency Runtime internal data structures.

notify_all

Unblocks all threads that are waiting for the condition_variable object.

void notify_all() noexcept;

notify_one

Unblocks one of the threads that are waiting on the condition_variable object.

void notify_one() noexcept;

wait

Blocks a thread.

void wait(unique_lock<mutex>& Lck);

template <class Predicate>
void wait(unique_lock<mutex>& Lck, Predicate Pred);

Parameters

Lck
A unique_lock<mutex> object.

Pred
Any expression that returns true or false.

Remarks

The first method blocks until the condition_variable object is signaled by a call to notify_one or notify_all. It can also wake up spuriously.

In effect, the second method executes the following code.

while(!Pred())
    wait(Lck);

wait_for

Blocks a thread, and sets a time interval after which the thread unblocks.

template <class Rep, class Period>
cv_status wait_for(
    unique_lock<mutex>& Lck,
    const chrono::duration<Rep, Period>& Rel_time);

template <class Rep, class Period, class Predicate>
bool wait_for(
    unique_lock<mutex>& Lck,
    const chrono::duration<Rep, Period>& Rel_time,
    Predicate Pred);

Parameters

Lck
A unique_lock<mutex> object.

Rel_time
A chrono::duration object that specifies the amount of time before the thread wakes up.

Pred
Any expression that returns true or false.

Return Value

The first method returns cv_status::timeout if the wait terminates when Rel_time has elapsed. Otherwise, the method returns cv_status::no_timeout.

The second method returns the value of Pred.

Remarks

The first method blocks until the condition_variable object is signaled by a call to notify_one or notify_all or until the time interval Rel_time has elapsed. It can also wake up spuriously.

In effect, the second method executes the following code.

while(!Pred())
    if(wait_for(Lck, Rel_time) == cv_status::timeout)
    return Pred();

return true;

wait_until

Blocks a thread, and sets a maximum point in time at which the thread unblocks.

template <class Clock, class Duration>
cv_status wait_until(
    unique_lock<mutex>& Lck,
    const chrono::time_point<Clock, Duration>& Abs_time);

template <class Clock, class Duration, class Predicate>
bool wait_until(
    unique_lock<mutex>& Lck,
    const chrono::time_point<Clock, Duration>& Abs_time,
    Predicate Pred);

cv_status wait_until(
    unique_lock<mutex>& Lck,
    const xtime* Abs_time);

template <class Predicate>
bool wait_until(
    unique_lock<mutex>& Lck,
    const xtime* Abs_time,
    Predicate Pred);

Parameters

Lck
A unique_lock<mutex> object.

Abs_time
A chrono::time_point object.

Pred
Any expression that returns true or false.

Return Value

Methods that return a cv_status type return cv_status::timeout if the wait terminates when Abs_time elapses. Otherwise, the methods return cv_status::no_timeout.

Methods that return a bool return the value of Pred.

Remarks

The first method blocks until the condition_variable object is signaled by a call to notify_one or notify_all or until Abs_time. It can also wake up spuriously.

In effect, the second method executes the following code

while(!Pred())
    if(wait_until(Lck, Abs_time) == cv_status::timeout)
    return Pred();

return true;

The third and fourth methods use a pointer to an object of type xtime to replace the chrono::time_point object. The xtime object specifies the maximum amount of time to wait for a signal.

See also

Header Files Reference
<condition_variable>