Shared Data

To make the point clear, you only need to think about synchronisation if you have shared, mutable data because shared, mutable data is prone to data races. If you have concurrent non-synchronised read and write access to data, your program has undefined behaviour.

The easiest way to visualise concurrent, unsynchronised read and write operations is to write something to std::cout.

Let’s have a look.

Writing unsynchronised to std::cout

 1 // coutUnsynchronised.cpp
 2 
 3 #include <chrono>
 4 #include <iostream>
 5 #include <thread>
 6 
 7 class Worker{
 8 public:
 9   Worker(std::string n):name(n){};
10     void operator() (){
11       for (int i = 1; i <= 3; ++i){
12         // begin work
13         std::this_thread::sleep_for(std::chrono::milliseconds(200));
14         // end work
15         std::cout << name << ": " << "Work " << i << " done !!!" << std::endl;
16       }...