CODEWITHSUNDEEP
c++multithreadingasynchronousstdasync
Alston
Alston

Reputation: 1226

C++ std::async does not spawn a new thread

C++11

int main(int argc, char** argv) {
    std::async(std::launch::async, [](){ 
        while(true) cout << "async thread" <<endl; 
    });
    while(true) cout << "main thread" << endl;
    return 0;
}

I expected the output should be something interleaved with async thread and main thread since there should be 2 different threads.

But it's not.

It outputs:

async thread
async thread
async thread
async thread
...

I guess there's only one thread. Can someone tell me why it isn't spawn a new thread for std::async? Thanks.

Upvotes: 2

Views: 2554

Answers (1)

Alston
Alston

Reputation: 1226

Change to this:

auto _ = std::async(std::launch::async, [](){ 
    while(true) cout << "async thread" <<endl; 
});

Document:

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

Upvotes: 6

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