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# std::async

From cppreference.com
< cpp‎ | thread

Defined in header `<future>` |  |
| (1) |
template< class Function, class... Args> std::future<std::result_of_t<std::decay_t<Function>(std::decay_t<Args>...)>> async( Function&& f, Args&&... args ); |  | (since C++11) (until C++17)
template< class Function, class... Args> std::future<std::invoke_result_t<std::decay_t<Function>, std::decay_t<Args>...>> async( Function&& f, Args&&... args ); |  | (since C++17)
| (2) |
template< class Function, class... Args > std::future<std::result_of_t<std::decay_t<Function>(std::decay_t<Args>...)>> async( std::launch policy, Function&& f, Args&&... args ); |  | (since C++11) (until C++17)
template< class Function, class... Args > std::future<std::invoke_result_t<std::decay_t<Function>, std::decay_t<Args>...>> async( std::launch policy, Function&& f, Args&&... args ); |  | (since C++17)

The template function `async` runs the function `f` asynchronously (potentially in a separate thread which may be part of a thread pool) and returns a std::future that will eventually hold the result of that function call.

1) Behaves the same as async(std::launch::async | std::launch::deferred, f, args...). In other words, `f` may be executed in another thread or it may be run synchronously when the resulting std::future is queried for a value.
2) Calls a function `f` with arguments `args` according to a specific launch policy `policy`:

  - If the async flag is set (i.e. policy & std::launch::async != 0), then `async` executes the callable object `f` on a new thread of execution (with all thread-locals initialized) as if spawned by std::thread(std::forward<F>(f), std::forward<Args>(args)...), except that if the function `f` returns a value or throws an exception, it is stored in the shared state accessible through the std::future that `async` returns to the caller.
  - If the deferred flag is set (i.e. policy & std::launch::deferred != 0), then `async` converts `f` and `args...` the same way as by std::thread constructor, but does not spawn a new thread of execution. Instead, lazy evaluation is performed: the first call to a non-timed wait function on the std::future that `async` returned to the caller will cause the copy of `f` to be invoked (as an rvalue) with the copies of `args...` (also passed as rvalues) in the current thread (which does not have to be the thread that originally called `std::async`). The result or exception is placed in the shared state associated with the future and only then it is made ready. All further accesses to the same std::future will return the result immediately.
  - If both the std::launch::async and std::launch::deferred flags are set in `policy`, it is up to the implementation whether to perform asynchronous execution or lazy evaluation.

If neither std::launch::async nor std::launch::deferred, nor any implementation-defined policy flag is set in `policy`, the behavior is undefined. | (since C++14)

In any case, the call to `std::async` synchronizes-with (as defined in std::memory_order) the call to `f`, and the completion of `f` is sequenced-before making the shared state ready. If the `async` policy is chosen, the associated thread completion synchronizes-with the successful return from the first function that is waiting on the shared state, or with the return of the last function that releases the shared state, whichever comes first.

### [edit] Parameters

f | - | `Callable` object to call
args... | - | parameters to pass to `f`
policy | - | bitmask value, where individual bits control the allowed methods of execution
Bit | Explanation
std::launch::async | enable asynchronous evaluation
std::launch::deferred | enable lazy evaluation
Type requirements
- `Function, Args` must meet the requirements of `MoveConstructible`.

### [edit] Return value

std::future referring to the shared state created by this call to `std::async`.

### [edit] Exceptions

Throws std::system_error with error condition std::errc::resource_unavailable_try_again if the launch policy equals std::launch::async and the implementation is unable to start a new thread (if the policy is `async|deferred` or has additional bits set, it will fall back to deferred or the implementation-defined policies in this case), or std::bad_alloc if memory for the internal data structures could not be allocated.

### [edit] Notes

The implementation may extend the behavior of the first overload of std::async by enabling additional (implementation-defined) bits in the default launch policy.

Examples of implementation-defined launch policies are the sync policy (execute immediately, within the async call) and the task policy (similar to async, but thread-locals are not cleared)

If the `std::future` obtained from `std::async` is not moved from or bound to a reference, the destructor of the std::future will block at the end of the full expression until the asynchronous operation completes, essentially making code such as the following synchronous:

    std::async(std::launch::async, []{ f(); }); // temporary's dtor waits for f()
    std::async(std::launch::async, []{ g(); }); // does not start until f() completes

(note that the destructors of std::futures obtained by means other than a call to std::async never block)

### [edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2021 | C++11 | return type incorrect and value category of arguments unclear in the deferred case | corrected return type and clarified that rvalues are used

### [edit] Example

Run this code

    #include <iostream>
    #include <vector>
    #include <algorithm>
    #include <numeric>
    #include <future>

    template <typename RAIter>
    int parallel_sum(RAIter beg, RAIter end)
    {
        auto len = end - beg;
        if(len < 1000)
            return std::accumulate(beg, end, 0);

        RAIter mid = beg + len/2;
        auto handle = std::async(std::launch::async,
                                 parallel_sum<RAIter>, mid, end);
        int sum = parallel_sum(beg, mid);
        return sum + handle.get();
    }

    int main()
    {
        std::vector<int> v(10000, 1);
        std::cout << "The sum is " << parallel_sum(v.begin(), v.end()) << '\n';
    }

Output:

    The sum is 10000

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