13 Overloading [over]

operator[] shall be a non-static member function with exactly one parameter. It implements the subscripting syntax

postfix-expression [ expression ]

or

postfix-expression [ 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 ]