Enhance the definition of the class StringVar given in Displays 11.11 and

11.12 by adding all of the following:

■ Member function copy_piece, which returns a specified substring;

member function one_char, which returns a specified single character;

and member function set_char, which changes a specified character.

■ An overloaded version of the == operator (note that only the string values

have to be equal; the values of max_length need not be the same).

■ An overloaded version of + that performs concatenation of strings of

type StringVar.

■ An overloaded version of the extraction operator >> that reads one

word (as opposed to input_line, which reads a whole line). (continued)

If you did the section on overloading the assignment operator, then add it

as well. Also write a suitable test program and thoroughly test your class

definition.