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

Question

I want to call a function which will be asynchronous (I will give a callback when this task is done).

I want to do this in single thread.

Answer 1

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(); } 

Answer 2

You can't in plain C++. You'll need to use an OS-specific mechanism, and you need a point where execution is suspended in a way that allows the OS to execute the callback. E.g. for Windows, QueueUserAPC - the callback will be executed when you e.g. SleepEx or WaitForSingleObjectEx