具有多个使用者的线程队列

Question

I'm doing threaded queue with consumer getting items from queue and producer put items into queue. The number of consumer created is from user input. I tried following two ways. First version works well but the second is not. I cannot use first version because of user input constraint. Actually I didn't get the difference between the two. Is the first one creating three threads at the same time while second one create one by one? Any clue on this?

auto c1 = async(launch::async, consumer);
auto c2 = async(launch::async, consumer);
auto c3 = async(launch::async, consumer);

for(int i = 1; i <= 3; i++)
    auto c1 = async(launch::async, consumer);

Answer 1

Both are, in a way, equal, as both starts three asynchronous tasks one after another.

Both are, in a way, different, as for the loop the variable c1 is confined to a lifetime inside the loop and will go out of scope (and be destructed) every time the loop iterates. And it's probably this that makes all the difference: The object c1 is destructed and have to wait for the consumer function to finish before it can be destructed.