Is it safe to return from function before all std::futures are finished?

Question

I have code like that:

int function()
{
    std::vector<std::future<int>> futures;
    for (const auto& elem : elements)
    {
        futures.push_back(std::async(&MyClass::foo, myClass, elem);
    }
    for (auto& f : futures)
    {
        const int x = f.get();
        if (x != 0)
            return x;
    }
}

Can I return from function when there are unfinished async calls? I'm interested only in one non zero value. Should I wait until all async calls are finished? Is this code safe?

Answer 1

The destructor of std::future (when initialized from a call to std::async) blocks until the async task is complete. (See here)

So your function return statement will not complete until all of the tasks have finished.