675: Mainline

Because your class-object parameters should be call-by-reference parameters, rather than call-by-value parameters, you may want to use a trick to prevent yourself from inadvertently introducing a call-by-value class-object parameter.

• Define your own copy constructor, thus displacing the default copy constructor that the C++ compiler would otherwise create.
• Ensure that your copy constructor cannot be called by placing it in the private part of its class definition.

Because your copy constructors are in the private parts of the class definitions in which they appear, any attempt at copying—introduced inadvertently by a call-by-value parameter—leads to a compiler error.