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

void task(std::string const& name, std::mutex& mutex)
{
    mutex.lock();
    std::cout << name << std::endl;
    mutex.unlock();
}

int main()
{
    std::string a { "A" };
    std::string b { "B" };

    std::mutex mutex { };
    std::thread t1 { task, std::cref(a), std::ref(mutex) };
    std::thread t2 { task, std::cref(b), std::ref(mutex) };

    t1.join();
    t2.join();
}