#include <iostream>
#include <future>
#include <thread>
#include <vector>
#include <chrono>
#include <stdexcept>
 
void thread_func() {
    std::cout << "new thread \n";  
    
    if (rand()%2 == 0) throw std::runtime_error("my exception");
}
 
int main()
{
    std::vector<std::future<void>> thread_pool(4);
    
    // occupy all cores
    for(auto &i : thread_pool) i = std::async(std::launch::async, thread_func);
    
    // start a new task in freed thread (for example 20 times)
    for(int k = 0; k < 20;) {
        for(auto &i : thread_pool)
            if(i.wait_for(std::chrono::seconds(0)) == std::future_status::ready) 
            {
                try { i.get(); } catch(...) { std::cout << "exception was in thread \n"; }
                i = std::async(std::launch::async, thread_func);
                ++k;
            }
        std::this_thread::yield();
    }
    
    return 0;
}