Why does this const member function allow a member variable to be modified?

Question

class String
{

    private:
        char* rep;

    public:
        String (const char*);
        void toUpper() const;
};


String :: String (const char* s)
{
             


        

Answer 1

The reason is that you don't change rep. If you would, you would find rep = ...; somewhere in your code. This is the difference between

char*const rep;

and

const char* rep;

In your case, the first one is done if you execute a const member-function: The pointer is const. So, you won't be able to reset the pointer. But you will very well be able to change what the pointer points to.

Now, remember rep[i] = ...; is the same as *(rep + i) = ...;. Thus, what you change is not the pointer, but what the pointer points to. You are allowed, since the pointer is not of the second case type.

Solution

1. The const meaning you are looking at is physical constness. However, a const member-function means your object is logical const. If a change to some content will change the logical constness of your object, for example if it changes some static variable that your object depends upon, your compiler cannot know that your class now has another logical value. And neither it can know that the logical value changes dependent on what a pointer points to: The compiler doesn't try to check logical constness in a const member function, since it cannot know what those member variables mean. This stuff is termed const-correctness.
2. Use an object that is not just a reference or a pointer: A const member function will make that object const, and will disallow you to change its content. std::string, as proposed by some, or an array of characters (note that an array will disallow you changing its content, as opposed to just a pointer), would be an appropriate choice.
2.