Thread Local Data

By using the keyword thread_local, you have thread local data also known as thread local storage. Each thread has its copy of the data. Thread-local data behaves like static variables. They are created at their first usage, and their lifetime is bound to the lifetime of the thread.

Thread local data

// threadLocal.cpp 
...
std::mutex coutMutex;
thread_local std::string s("hello from ");

void addThreadLocal(std::string const& s2){
  s+= s2;
  std::lock_guard<std::mutex> guard(coutMutex);
  std::cout << s << std::endl;
  std::cout << "&s: " << &s << std::endl;
  std::cout << std::endl;
}

std::thread t1(addThreadLocal, "t1");
std::thread t2(addThreadLocal, "t2");
std::thread t3(addThreadLocal, "t3");
std::thread t4(addThreadLocal, "t4");

Each thread has its copy of the thread_local string, therefore, each string s modifies its string independently...