C++ unique_lock 01

Table of contents

struct Box {
  explicit Box( int num ) : num_things{ num }
  {
  }

  int        num_things;
  std::mutex m;
};

void transfer( Box &from, Box &to, int num )
{
  // don't actually take the locks yet
  std::unique_lock lock1{ from.m, std::defer_lock };
  std::unique_lock lock2{ to.m, std::defer_lock };

  // lock both unique_locks without deadlock
  std::lock( lock1, lock2 );

  from.num_things -= num;
  to.num_things += num;

  // 'from.m' and 'to.m' mutexes unlocked in 'unique_lock' dtors
}

int main()
{
  Box box1{ 100 };
  Box box2{ 50 };

  std::thread t1{ transfer, std::ref( box1 ), std::ref( box2 ), 10 };
  std::thread t2{ transfer, std::ref( box2 ), std::ref( box1 ), 5 };

  t1.join();
  t2.join();

  fmt::print( "box1: {}     box2: {}\n", box1.num_things, box2.num_things );

  return 0;
}

Possible output

box1: 95     box2: 55

References

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