FAQ 23.05: Why do subscript operators usually come in pairs?

They usually occur in pairs so that users can access elements of a const object.

Classes that have a subscript operator often have a pair of subscript operators: a const version and a non-const version. The const version normally returns the element by value or by const reference, and the non-const version normally returns the element by non-const reference. The code for the two versions is normally quite similar if not identical.