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 const qualifier means it will not change any members of the class.

In this case rep is the only member of the class and I see no attempt to modify this member. Anything pointed at or referenced outside the class is not considered as part of the class.

A solution to this problem would be to replace the char* with a std::string.
Then you would only be able to call const members of std::string from within toUpper()

For example (use std::string)

class String
{
    std::string rep;

    void toUpper() const
    { 
        for (int i = 0; rep [i]; i++)
            rep[i] = toupper(rep[i]);

        // Can only use const member functions on rep.
        // So here we use 'char const& std::string::operator[](size_t) const'
        // There is a non const version but we are not allowed to use it
        // because this method is const.

        // So the return type is 'char const&'
        // This can be used in the call to toupper()
        // But not on the lhs of the assignemnt statement

    }
}