title: Difference between shared mutex and mutex (why do both exist in C++ 11)?
body: <p>Haven't got an example online to demonstrate this vividly. Saw an example at <a href="http://en.cppreference.com/w/cpp/header/shared_mutex" rel="noreferrer">http://en.cppreference.com/w/cpp/header/shared_mutex</a> but 
  it is still unclear. Can somebody help?</p>