Question

In general, how do you declare the index [ ] operator of a class for both read and write accesss?

I tried something like

/**
 * Read index operator.
 */
T& operator[](T u);

/**
 * Write index operator
 */
const T& operator[](T u);

which gives me the error

../src/Class.h:44:14: error: 'const T& Class::operator[](T)' cannot be overloaded
../src/Class.h:39:8: error: with 'T& Class::operator[](T)'

Answer 1

Your mutable version is fine:

T& operator[](T u);

but the const version should be a const member function as well as returning a const reference:

const T& operator[](T u) const;
                         ^^^^^

This not only distinguishes it from the other overload, but also allows (read-only) access to const instances of your class. In general, overloaded member functions can be distinguished by their parameter types and const/volatile qualifications, but not by their return types.