c++ - Why should I use std::async?

Question

I'm trying to explore all the options of the new C++11 standard in depth, while using std::async and reading its definition, I noticed 2 things, at least under linux with gcc 4.8.1 :

• it's called async, but it got a really "sequential behaviour", basically in the row where you call the future associated with your async function foo, the program blocks until the execution of foo it's completed.
• it depends on the exact same external library as others, and better, non-blocking solutions, which means pthread, if you want to use std::async you need pthread.

at this point it's natural for me asking why choosing std::async over even a simple set of functors ? It's a solution that doesn't even scale at all, the more future you call, the less responsive your program will be.

Am I missing something ? Can you show an example that is granted to be executed in an async, non blocking, way ?

Answer 1

• it's called async, but it got a really "sequential behaviour",

No, if you use the std::launch::async policy then it runs asynchronously in a new thread. If you don't specify a policy it might run in a new thread.

basically in the row where you call the future associated with your async function foo, the program blocks until the execution of foo it's completed.

It only blocks if foo hasn't completed, but if it was run asynchronously (e.g. because you use the std::launch::async policy) it might have completed before you need it.

• it depends on the exact same external library as others, and better, non-blocking solutions, which means pthread, if you want to use std::async you need pthread.

Wrong, it doesn't have to be implemented using Pthreads (and on Windows it isn't, it uses the ConcRT features.)

at this point it's natural for me asking why choosing std::async over even a simple set of functors ?

Because it guarantees thread-safety and propagates exceptions across threads. Can you do that with a simple set of functors?

It's a solution that doesn't even scale at all, the more future you call, the less responsive your program will be.

Not necessarily. If you don't specify the launch policy then a smart implementation can decide whether to start a new thread, or return a deferred function, or return something that decides later, when more resources may be available.

Now, it's true that with GCC's implementation, if you don't provide a launch policy then with current releases it will never run in a new thread (there's a bugzilla report for that) but that's a property of that implementation, not of std::async in general. You should not confuse the specification in the standard with a particular implementation. Reading the implementation of one standard library is a poor way to learn about C++11.

Can you show an example that is granted to be executed in an async, non blocking, way ?

This shouldn't block:

auto fut = std::async(std::launch::async, doSomethingThatTakesTenSeconds);
auto result1 = doSomethingThatTakesTwentySeconds();
auto result2 = fut.get();

By specifying the launch policy you force asynchronous execution, and if you do other work while it's executing then the result will be ready when you need it.