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<int> p;
std::future<int> f = p.get_future();
std::future<void> 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();
}

Answer 2

You don't choose to use a promise instead of the others, you use a promise to fulfill a future in conjunction with the others. The code sample at cppreference.com gives an example of using all four:

#include <iostream>
#include <future>
#include <thread>
 
int main()
{
    // future from a packaged_task
    std::packaged_task<int()> task([](){ return 7; }); // wrap the function
    std::future<int> f1 = task.get_future();  // get a future
    std::thread(std::move(task)).detach(); // launch on a thread
 
    // future from an async()
    std::future<int> f2 = std::async(std::launch::async, [](){ return 8; });
 
    // future from a promise
    std::promise<int> p;
    std::future<int> f3 = p.get_future();
    std::thread( [](std::promise<int>& p){ p.set_value(9); }, 
                 std::ref(p) ).detach();
 
    std::cout << "Waiting...";
    f1.wait();
    f2.wait();
    f3.wait();
    std::cout << "Done!\nResults are: "
              << f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n';
}

prints

Waiting...Done!

Results are: 7 8 9

Futures are used with all three threads to get their results, and a promise is used with the third one to fulfill a future by means other than a return value. Also, a single thread can fulfill multiple futures with different values via promises, which it can't do otherwise.

An easy way to think of it is that you can either set a future by returning a value or by using a promise. future has no set method; that functionality is provided by promise. You choose what you need based on what the situation allows.