How do you overload a member function on const?

You can provide both const and non-const overloads of a member function; the compiler picks the const version for const objects and the non-const version for non-const objects. This is common for accessors that return a const reference for const objects and a mutable reference otherwise, avoiding code duplication (or using a const_cast helper).

struct V {
    int& at(int i)             { return d[i]; }
    const int& at(int i) const { return d[i]; }
    int d[10];
};