This std::thread examples shows the code and screen output of two threads counting down. Below it, the code is discussed in more detail.

Download the Qt Creator project 'CppThreadExample1' (zip)


#include <iostream>

#include <thread>



void DoCountDown(const int id)

{

  for (int i=0; i!=10; ++i)

  {

    std::cout << "#" << id << ": " << (10-i) << '\n';

  }

}



int main()

{

  std::thread t1(DoCountDown,1);

  std::thread t2(DoCountDown,2);

  t1.join();

  t2.join();

}

Screen output:


##12: : 10

10#

#1: 29

: #9

#12: : 8

#8

2: #7

#12: : 7

#61

: #6

#21: : 5

5#1

: #4

#2: 14

: #3

#21: : 2

3#

1#: 2: 21



#2: 1

Note the following about the code:

Calling std::thread::join denotes 'wait for the std::thread to terminate' [1]
It is important that we wait for the std::thread to terminate, because 'when a std::thread goes out of scope the program is std::terminate()d unless its task has completed' [1]

Both std::threads do not wait for the other to finish writing to std::cout. Note that writing to std::clog makes the output less messier:

This example is checked for errors in helgrind example 1: two counting threads, where helgrind is shown to detect race conditions.

The next example, std::thread example 2: two counting threads using std::mutex demonstrates how to use std::mutexes to share std::cout nicely.

References

Bjarne Stroustrup's C++0x page

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) std::thread example 1: two counting threads

References