What can a 'const' method change?

Question

C++ methods allow a const qualifier to indicate that the object is not changed by the method. But what does that mean? Eg. if the instance variables are pointer

Answer 1

const basically prevents changing the class instance members' values inside the function. This is useful for more clearer interface, but pose restrictions when using inheritance for example. It's sometimes a little bit deceiving (or a lot actually), as in the example you posted.

const would be most appropriate for Get functions, where it is obvious that the caller is reading a value and has no intentions of changing the object state. In this case you would want to limit the inherited implementations as well to adhere to constness, to avoid confusion and hidden bugs when using polymorphism.

For example

class A{
     int i;
   public:
     virtual int GetI() {return i;};
}

class B : public A{
   public:
     int GetI() { i = i*2; return i;}; // undesirable
}

changing A to:

virtual int GetI() const {return i;};

solves the problem.