555: Mainline

Here, by way of summary, is where you are:

• You know that you ordinarily use call-by-value parameters. C++ programs always allocate a chunk of memory for each call-by-value parameter and C++ programs always arrange for argument values to be copied into that chunk. The rationale is that you want to isolate each function's parameters from other parameters, local variables, and global variables that happen to have the same name.
• You know that you generally use call-by-reference parameters when a function's argument is to be an object that belongs to a subclass of the parameter's type. The rationale is that all of the object, rather than just the portion associated with the parameter's type, must be visible inside the function. Each call-by-reference parameter shares an existing memory chunk with a corresponding argument, rather than copying that argument into a newly allocated chunk; hence, all data in the existing memory chunk remains visible.