C++17 std::async non blocking execution - c++

Question

I have created this C++17 code that mimics something that I need.
std::cout << "start" << std::endl;
auto a = std::async([]() -> int {
std::this_thread::sleep_for(std::chrono::seconds{ 5 });
return 2;
});
std::cout << a.get() << std::endl;
std::cout << "stop" << std::endl;
The thread sleeps here but in real example I do heavy operations and I return an integer which can be 0, 1 or 7. This is my output:
start
2
stop
This is good! This code will not freeze my UI. I know that a.get() is a blocking operation but is there a way to be non blocking?
In other words: instead of
start
2
stop
can I get as output
start
stop
2
using async? I need async because I have found online that it is useful when return a value is needed. It is also easy to read! I do not want to use std::packaged_task, promises, futures etc because async is easy.
If it cannot be non blocking can I use something else?

Answer 1

This code will output what you need:
std::cout << "start" << std::endl;
std::thread{
[]() {
auto a = std::async([]() -> int {
std::this_thread::sleep_for(std::chrono::seconds{ 5 });
return 2;
});
std::cout << a.get() << std::endl;
}
}.detach();
std::cout << "stop" << std::endl;
If I were you I wouldn't use async at all. If you just need to display a value and you don't want to block, you should do something else. For example:
std::cout << "start" << std::endl;
std::thread{
[]() {
//do here what you need and print the result
}
}.detach();
/* ... or ...
auto t = std::thread{ ... };
t.detach();
*/
std::cout << "stop" << std::endl;
See that detach() makes the new thread "independent" and doesn't block while join() blocks. The point is that async must block because if it has to do an operation that takes a lot of time, it has to spend that time!