12 Overloading [over]

12.6 Overloaded operators [over.oper]

12.6.5 Subscripting [over.sub]

operator[] shall be a non-static member function with exactly one parameter.
It implements the subscripting syntax
postfix-expression [ expr-or-braced-init-list ]
Thus, a subscripting expression x[y] is interpreted as x.operator[](y) for a class object x of type T if T​::​operator[](T1) exists and if the operator is selected as the best match function by the overload resolution mechanism ([over.match.best]).
struct X {
  Z operator[](std::initializer_list<int>);
X x;
x[{1,2,3}] = 7;                 // OK: meaning x.operator[]({1,2,3})
int a[10];
a[{1,2,3}] = 7;                 // error: built-in subscript operator
— end example