c++: const functions

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.
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.

// constant_member_function.cpp
class Date
{
 public:
   Date( int mn, int dy, int yr );
   int getMonth() const;     // A read-only function
   void setMonth( int mn );  // A write function can't be const

 private:
   int month;
};

int Date::getMonth() const
{
   return month;        // Doesn't modify anything
}