What I am trying to do is each thread gets to print out its 5 characters before potentially yielding to another thread. It works but it does not do what I want. THanks for help.

for (unsigned i = 0; i < _repCount; ++i) {  
    unique_lock<mutex> lck(mtx);
     cv.wait(lck, []{return !inUse; });
        inUse = true;
    for (auto c : _printMe) {
      cout << c;
      cout.flush();  
    }
    inUse = false ;  
  }

You seem to have made this more complicated than it should be. In the following code, so long as the mutex named mtx is shared by all threads, each thread will wait to take the mutex, output _printMe, release the mutex. Each thread will output _printMe repeatedly; repcout times. There is no guarantee that the threads will take turns :)

for (unsigned i = 0; i < _repCount; ++i)
{  
    lock_guard<mutex> lck(mtx);
    for (auto c : _printMe) 
    {
       cout << c;
       cout.flush();  
    }
}