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A simple approach to generate Fibonacci series via multi-threading

This is a very simple approach taken to generate the Fibonacci series through multithreading. Here instead of a function, used a function object. The code is very simple and self-explanatory. 

#include <iostream>
#include <mutex>
#include <thread>

class Fib {
public:
    Fib() : _num0(1), _num1(1) {}
    unsigned long operator()();

private:
    unsigned long _num0, _num1;
    std::mutex mu;
};

unsigned long Fib::operator()() {
    mu.lock(); // critical section, exclusive access to the below code by locking the mutex
    unsigned long  temp = _num0;
    _num0 = _num1;
    _num1 = temp + _num0;
    mu.unlock();
    return temp;
}

int main()
{
    Fib f;
    
    int i = 0;
    unsigned long res = 0, res2= 0, res3 = 0;
    std::cout << "Fibonacci series: ";

    while (i <= 15) {
        std::thread t1([&] { res = f(); }); // Capturing result to respective variable via lambda
        std::thread t2([&] { res2 = f(); });
        std::thread t3([&] { res3 = f(); });

        t1.join();
        t2.join();
        t3.join();
        
        // Arranging in order, since threads execution can be random, 
        // So the result may not be in order to display

        if (res > res3) std::swap(res, res3);
        if (res > res2) std::swap(res, res2);
        if (res2 > res3) std::swap(res2, res3);

        std::cout << res << " " << res2 << " " << res3 << " ";
        i += 3;
    }
}

We can use std::lock_guard like below to give RAII style mechanism for owning the mutex during the scoped block.

unsigned long Fib::operator()() 
{
     // critical section, exclusive access to the below code by locking a mutex by lock_guard
    std::lock_guard<std::mutex> lock(mu); 
    unsigned long  temp = _num0;
    _num0 = _num1;
    _num1 = temp + _num0;
    return temp;
}
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