16 Overloading [over]

16.5 Overloaded operators [over.oper]

16.5.5 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]). [Example:

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]