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

void task(std::string const& name)
{
    std::cout << name << std::endl;
}

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

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

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