and copy assignment operator for classes with a copy constructor
This is often described as the rule of 3 (or rule of 5 in C++11 if move
constructors and assignment operators are also defined) and makes good sense in
ensuring consistency. For classes in which the default bitwise copy constructor
or assignment operator are appropriate these are now specified explicitly using
the "= default" keyword if the other is explicitly defined fulfilling the rule
of 3 without the need to define the body of the function.
