How do I make a function asynchronous in C++?

This can be done portably with modern C++ or even with old C++ and some boost. Both boost and C++11 include sophisticated facilities to obtain asynchronous values from threads, but if all you want is a callback, just launch a thread and call it.

1998 C++/boost approach:

#include <iostream>
#include <string>
#include <boost/thread.hpp>
void callback(const std::string& data)
{
    std::cout << "Callback called because: " << data << '\n';
}
void task(int time)
{
    boost::this_thread::sleep(boost::posix_time::seconds(time));
    callback("async task done");
}
int main()
{
    boost::thread bt(task, 1);
    std::cout << "async task launched\n";
    boost::this_thread::sleep(boost::posix_time::seconds(5));
    std::cout << "main done\n";
    bt.join();
}

2011 C++ approach (using gcc 4.5.2, which needs this #define)

#define _GLIBCXX_USE_NANOSLEEP
#include <iostream>
#include <string>
#include <thread>
void callback(const std::string& data)
{
    std::cout << "Callback called because: " << data << '\n';
}
void task(int time)
{
    std::this_thread::sleep_for(std::chrono::seconds(time));
    callback("async task done");
}
int main()
{
    std::thread bt(task, 1);
    std::cout << "async task launched\n";
    std::this_thread::sleep_for(std::chrono::seconds(5));
    std::cout << "main done\n";
    bt.join();
}