How to none blocked join to std thread - c++

Question

I want to keep my code clean and do the things right, to any std::thread I need to do join or detach, but how can I wait (at the main thread) for another thread without blocking the execution of the main thread?
void do_computation()
{
// Calculate 1000 digits of Pi.
}
int main()
{
std::thread td1(&do_computation);
while (running)
{
// Check if thread td1 finish and if yes print a message
// Here are some stuff of the main to do...
// Print to UI, update timer etc..
}
// If the thread has not finished yet here, just kill it.
}

Answer 1

The answer is semaphores. You can use a binary semaphore to synchronize your threads.
You may use System V semaphores or pthread mutexes, but they are somehow legacy in C++. Using Tsuneo Yoshioka's answer, we could implement a C++ way of semaphore, though.
#include <mutex>
#include <condition_variable>
class Semaphore {
public:
Semaphore (int count_ = 0)
: count(count_) {}
inline void notify()
{
std::unique_lock<std::mutex> lock(mtx);
count++;
cv.notify_one();
}
inline void wait()
{
std::unique_lock<std::mutex> lock(mtx);
while(count == 0){
cv.wait(lock);
}
count--;
}
private:
std::mutex mtx;
std::condition_variable cv;
int count;
};
Your implementation may make use of the Semaphore class, like so.
void do_computation()
{
//calculate 1000 digits of Pi.
semaphore.notify();
}
int main()
{
Semaphore semaphore(0);
std::thread td1(&do_computation);
semaphore.wait();
}

Answer 2

You can use std::promise and std::future. More info here and here.
#include <vector>
#include <thread>
#include <future>
#include <numeric>
#include <iostream>
#include <chrono>
void accumulate(std::vector<int>::iterator first,
std::vector<int>::iterator last,
std::promise<int> accumulate_promise)
{
int sum = std::accumulate(first, last, 0);
accumulate_promise.set_value(sum); // Notify future
}
void do_work(std::promise<void> barrier)
{
std::this_thread::sleep_for(std::chrono::seconds(1));
barrier.set_value();
}
int main()
{
// Demonstrate using promise<int> to transmit a result between threads.
std::vector<int> numbers = { 1, 2, 3, 4, 5, 6 };
std::promise<int> accumulate_promise;
std::future<int> accumulate_future = accumulate_promise.get_future();
std::thread work_thread(accumulate, numbers.begin(), numbers.end(),
std::move(accumulate_promise));
accumulate_future.wait(); // wait for result
std::cout << "result=" << accumulate_future.get() << '\n';
work_thread.join(); // wait for thread completion
// Demonstrate using promise<void> to signal state between threads.
std::promise<void> barrier;
std::future<void> barrier_future = barrier.get_future();
std::thread new_work_thread(do_work, std::move(barrier));
barrier_future.wait();
new_work_thread.join();
}