title: c++ stl library what are these methods with underscores?
body: <p>I have some methods which are not referenced in any of the cpp references over the internet</p>
  
  <p>For example in "memory"
  Shared_ptr's have a method called "_Expired"</p>
  
  <p>It returns a boolean if the ptr is expired or not</p>
  
  <p>I thought that only weakptr's have this...</p>