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─────C/C++ Language─────────────────────────────────────────────────────────
 
     Declaring a member function with the const keyword specifies that
     the function is a "read-only" function that does not modify the
     object for which it is called.
 
     Declaring Constant Member Functions
 
     To declare a constant member function, place the const keyword
     after the closing parenthesis of the argument list. The const
     keyword is required in both the declaration and the definition.
     A constant member function cannot modify any data members or
     call any member functions that aren't constant.
 
        class Date
        {
        public:
            Date( int mn, int dy, int yr );
            int getMonth() const;       // A read-only function
            void setMonth( int mn );    // A write function;
                                        //    cannot be const
        private:
            int month;
        };
 
        int Date::getMonth() const
        {
            return month;        // Doesn't modify anything
        }
 
        void Date::setMonth( int mn )
        {
            month = mn;          // Modifies data member
        }
 
     Declaring Constant Objects
 
     To declare a constant object, place the const keyword at the start
     of an object declaration:
 
        const Date birthday( 3, 4, 1985 );
 
     You can only call constant member functions for a constant
     object. This ensures that the object is never modified.
 
        birthday.getMonth();    // Okay
        birthday.setMonth( 4 ); // Error
 
     You can call either constant or nonconstant member functions for
     a nonconstant object. You can also overload a member function
     using the const keyword; this allows a different version of the
     function to be called for constant and nonconstant objects.
 
     You cannot declare constructors or destructors with the const
     fkeyword.
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