Lazy Initialization

• Common pattern in functional programming
• A variable is only initialized when it is first used
• This is useful when the variable is expensive to construct
  • e.g. it sets up a network connection
• Can be used in multi-threaded code
  • But we need to avoid data races

#include <mutex>

class Test {
public:
  void func() { /*...*/ }
};

Test *ptest = nullptr; // Variable to be lazily initialized
std::mutex mut;

void process() {
  std::unique_lock<std::mutex> uniq_lck(mut);

  if (!ptest)
    ptest = new Test;
  uniq_lck.unlock();
  ptest->func();
}

• Every thread that calls process() locks the mutex
  • Locking the mutex blocks every other thread that calls process()
• The lock is only needed while ptest is being initialized
  • Once ptest has been initialized, locking the mutex is unnecessary
  • Causes a loss of performance