title: What do we need unary_function and binary_function for?
body: <p>I read the tutorials about the binary and unary functions. I understood the structure of them, but I couldn't imagine in which case I need these functions. Can you give an example for usage of them. </p>
  
  <p><a href="http://www.cplusplus.com/reference/std/functional/unary_function/" rel="nofollow">http://www.cplusplus.com/reference/std/functional/unary_function/</a></p>
  
  <p><a href="http://www.cplusplus.com/reference/std/functional/binary_function/" rel="nofollow">http://www.cplusplus.com/reference/std/functional/binary_function/</a></p>