When is it a good idea to use std::promise over the other std::thread mechanisms?

Question

I am trying to establish some heuristics to help me decide the appropriate std::thread class to use.

As I understand it, from highest level (simplest to

Answer 1

When you have two levels of async, you need to use a promise. Eg:

void fun()
{
std::promise p;
std::future f = p.get_future();
std::future f2;
auto f3 = std::async([&]{
   // Do some other computation
   f2 = std::async([&]{ p.set_value(42);});
   // Do some other work
});
   // Do some other work
   // Now wait for the result of async work;
std::cout << f.get();
   // Do some other work again 
// Wait for async threads to complete
f3.wait();
f2.wait();
}