async Function
Represents an asynchronous provider.
template<class Fn, class... ArgTypes>
future<typename result_of<Fn(ArgTypes...)>::type>;
async(Fn&& fn, ArgTypes&&... args);
template<class Fn, class... ArgTypes>
future<typename result_of<Fn(ArgTypes...)>::type>;
async(launch policy, Fn&& fn, ArgTypes&&... args);
Parameters
- policy
A launch value.
Remarks
Definitions of abbreviations:
dfn |
The result of calling decay_copy(forward<Fn>(fn)). |
dargs |
The results of the calls decay_copy(forward<ArgsTypes>(args…)). |
Ty |
The type result_of<Fn(ArgTypes…)>::type. |
The first template function returns async(launch::any, fn, args…).
The second function returns a future<Ty> object whose associated asynchronous state holds a result together with the values of dfn and dargs and a thread object to manage a separate thread of execution.
Unless decay<Fn>::type is a type other than launch, the second function does not participate in overload resolution.
If policy is launch::any, the function might choose launch::async or launch::deferred. In this implementation, the function uses launch::async.
If policy is launch::async, the function creates a thread that evaluates INVOKE(dfn, dargs..., Ty). The function returns after it creates the thread without waiting for results. If the system can't start a new thread, the function throws a system_error that has an error code of resource_unavailable_try_again.
If policy is launch::deferred, the function marks its associated asynchronous state as holding a deferred function and returns. The first call to any non-timed function that waits for the associated asynchronous state to be ready in effect calls the deferred function by evaluating INVOKE(dfn, dargs..., Ty).
In all cases, the associated asynchronous state of the future object is not set to ready until the evaluation of INVOKE(dfn, dargs…, Ty) completes, either by throwing an exception or by returning normally. The result of the associated asynchronous state is an exception if one was thrown, or any value that's returned by the evaluation.
Note
For a future—or the last shared_future—that's attached to a task started with std::async, the destructor blocks if the task has not completed; that is, it blocks if this thread did not yet call .get() or .wait() and the task is still running. If a future obtained from std::async is moved outside the local scope, other code that uses it must be aware that its destructor may block for the shared state to become ready.
The pseudo-function INVOKE is defined in <functional>.
Requirements
Header: future
Namespace: std