The operator Keyword

How do we teach a normal C++ operator to act on a user-defined operand? The keyword
operator is used to overload the ++ operator in this declarator:
void operator ++ ()
The return type (void in this case) comes first, followed by the keyword operator, followed
by the operator itself (++), and finally the argument list enclosed in parentheses (which are
empty here). This declarator syntax tells the compiler to call this member function whenever
the ++ operator is encountered, provided the operand (the variable operated on by the ++) is of
type Counter.
We saw in Chapter 5, “Functions,” that the only way the compiler can distinguish between
overloaded functions is by looking at the data types and the number of their arguments. In the
same way, the only way it can distinguish between overloaded operators is by looking at the
data type of their operands. If the operand is a basic type such as an int, as in
++intvar;
then the compiler will use its built-in routine to increment an int. But if the operand is a
Counter variable, the compiler will know to use our user-written operator++() instead.