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C - C++ Threads - Transfer ownership of threads at runtime

std::move can be called to move the ownership of the underlying thread resource from one std::thread object to another.

Problem

#include <iostream>
#include <string>
#include <thread>
#include <functional>
 
void Func1()
{
  std::this_thread::sleep_for(std::chrono::milliseconds(5000));
  std::cout << "Finished Func1"<< std::endl;
}
 
int main()
{
  std::thread t1(Func1);
  t1.join();
 
  std::cout << "Continue Main" << std::endl;
 
  return 0;
}

NOTE: The Continue Main message is only displayed once the function Func1 finishes.

This shows that the main() thread is blocked until the t1 thread completes.

Solution

To not have to wait for a thread to finish, pass the thread to another function which will wait for the thread to finish and execute some action once the execution is done.

#include <iostream>
#include <string>
#include <thread>
#include <functional>
 
void Func1()
{
  std::this_thread::sleep_for(std::chrono::milliseconds(5000));
  std::cout << "Finished Func1"<< std::endl;
}
 
void Func2(std::thread t1)
{
  t1.join();
  std::cout << "Finished Func2" << std::endl;
}
 
int main()
{
  std::thread t1(Func1);
  std::thread t2(Func2, std::move(t1));
 
  // Do a bunch of other processing without waiting for t1 to finish.
  // Instead now the responsibility of monitoring the t1 thread is passed to t2.
 
  std::cout << "Continue Main" << std::endl;
 
  // Finally wait for t2 to finish.
  t2.join();
 
  return 0;
}

NOTE: Notice that now the Continue Main message is displayed even before the thread function finishes.

A join() blocks the thread that called it, in this example the main() thread.

This may result in the application freezing.

To overcome this issue, ownership of the thread is passed to a different thread, in this example, t2, allowing the main() thread to not be blocked.

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C - C++ Threads - Transfer ownership of threads at runtime

Problem

Solution