Displaying results as soon as they are ready with std::async - c++

Question

I'm trying to discover asynchronous programming in C++. Here's a toy example I've been using:
#include <iostream>
#include <future>
#include <vector>
#include <chrono>
#include <thread>
#include <random>
// For simplicity
using namespace std;
int called_from_async(int m, int n)
{
this_thread::sleep_for(chrono::milliseconds(rand() % 1000));
return m * n;
}
void test()
{
int m = 12;
int n = 42;
vector<future<int>> results;
for(int i = 0; i < 10; i++)
{
for(int j = 0; j < 10; j++)
{
results.push_back(async(launch::async, called_from_async, i, j));
}
}
for(auto& f : results)
{
cout << f.get() << endl;
}
}
Now, the example is not really interesting, but it raises a question that is, to me, interesting. Let's say I want to display results as they "arrive" (I don't know what will be ready first, since the delay is random), how should I do it?
What I'm doing here is obviously wrong, since I wait for all the tasks in the order in which I created them - so I'll wait for the first to finish even if it's longer than the others.
I thought about the following idea: for each future, using wait_for on a small time and if it's ready, display the value. But I feel weird doing that:
while (any_of(results.begin(), results.end(), [](const future<int>& f){
return f.wait_for(chrono::seconds(0)) != future_status::ready;
}))
{
cout << "Loop" << endl;
for(auto& f : results)
{
auto result = f.wait_for(std::chrono::milliseconds(20));
if (result == future_status::ready)
cout << f.get() << endl;
}
}
This brings another issue: we'd call get several times on some futures, which is illegal:
terminate called after throwing an instance of 'std::future_error' what(): std::future_error: No associated state
So I don't really know what to do here, please suggest!

Answer 1

Use valid() to skip the futures for which you have already called get().
bool all_ready;
do {
all_ready = true;
for(auto& f : results) {
if (f.valid()) {
auto result = f.wait_for(std::chrono::milliseconds(20));
if (result == future_status::ready) {
cout << f.get() << endl;
}
else {
all_ready = false;
}
}
}
}
while (!all_ready);