std::async

 

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Defined in header <future>
template< class Function, class... Args> std::future<typename std::result_of<Function(Args...)>::type>     async( Function&& f, Args&&... args );	(1)	(since C++11)
template< class Function, class... Args > std::future<typename std::result_of<Function(Args...)>::type>     async( std::launch policy, Function&& f, Args&&... args );	(2)	(since C++11)

The template function async provides the mechanisms for asynchronous function execution and for lazy evaluation of functions.

1) behaves the same as async(std::launch::async | std::launch::deferred, f, args...)

2) If policy & std::launch::async != 0 (the async bit is set), spawns a new thread of execution as if by std::thread(f, 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 policy & std::launch::deferred != 0 (the deferred bit is set), converts 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 f(args...) to be executed in the current thread. All further accesses to the same std::future will return the result immediately.

If both std::launch::async and std::launch::deferred bits are set in policy, it is up to the implementation whether to perform asynchronous execution or lazy evaluation.

Parameters

f	-	function or function 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

Return value

std::future referring to the return value of the function.

Exceptions

Throws std::system_error with error condition std::errc::resource_unavailable_try_again if the launch policy is std::launch::async and the implementation is unable to start a new thread.

Example

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <future>
 
template<typename RAIter>
int parallel_sum(RAIter beg, RAIter end)
{
    typename RAIter::difference_type 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