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Embedded Programming with Modern C++

- Examples

Understand how to implement thread-safe singletons in C++ using std::call_once, std::once_flag, and the Meyers Singleton pattern. Learn to manage multithreading challenges effectively and ensure safe initialization in concurrent environments.

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Example 1

The short example demonstrates the application of std::call_once and the std::once_flag. Both are declared in the header <mutex>.

C++
// safeInitializationCallOnce.cpp
#include <iostream>
#include <thread>
#include <mutex>
 
std::once_flag onceFlag;
 
void do_once(){
  std::call_once(onceFlag, [](){ std::cout << "Only once." << std::endl; });
}
 
int main(){
  
  std::cout << std::endl;
  
  std::thread t1(do_once);
  std::thread t2(do_once);
  std::thread t3(do_once);
  std::thread t4(do_once);
 
  t1.join();
  t2.join();
  t3.join();
  t4.join();
  
  std::cout << std::endl;
}

Explanation

  • The program starts four threads (lines 17 - 20). Each of them invokes do_once. The string “only once” is, as a result, displayed only once. ...