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I have some question about behavior of std::async function with std::launch::async policy & std::future object returned from async. In following code, main thread waits for the completion of foo() on the thread created by async call. #include <thread> #include <future> #include <iostream> void foo() { std::cout << "foo:begin" << std::endl; std::this_thread::sleep_for(std::chrono::seconds(10)); std::cout << "foo:done" << std::endl; } int main() { std::cout << "main:begin" << std::endl; { auto f = std::async(std::launch::async, foo); // dtor f::~f blocks until completion of foo()... why?? } std::this_thread::sleep_for(std::chrono::seconds(2)); std::cout << "main:done" << std::endl; } And I know http://www.stdthread.co.uk/doc/headers/future/async.html says The destructor of the last future object associated with the asynchronous state of the returned std::future shall block until the future is ready. My question is: Q1. Does this behavior conform to the current C++ standard? Q2. If Q1's answer is yes, which statements say that?

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