Search code examples
c++boostboost-thread

return value in boost thread without using boost::promise

int xxx()
{
    return 5;
}

int main()
{
    boost::thread th;
    th = boost::thread(xxx);
    th.join();
    return 0;
}

How to catch the value returned by the xxx() method without the use of boost::promise?

Solution

  • Since you say you can't change xxx, call another function which puts the result somewhere accessible. A promise is probably still the best option, but you could write it directly to a local variable if you're careful with synchronisation. For example

    int result;
th = boost::thread([&]{result = xxx();});

// Careful! result will be modified by the thread.
// Don't access it without synchronisation.

th.join();

// result is now safe to access.

    As mentioned in the comments, there's a handy async function which gives you a future from which to retrieve an asynchronously-called function's return value:

    auto future = boost::async(boost::launch::async, xxx);
int result = future.get();