12 Overloading [over]

12.6 Overloaded operators [over.oper]

12.6.4 Subscripting [over.sub]

1
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A subscripting operator function is a function named operator[] that is a non-static member function with exactly one parameter.
For an expression of the form 
postfix-expression [ expr-or-braced-init-list ]
the operator function is selected by overload resolution ([over.match.oper]).
If a member function is selected, the expression is interpreted as 
postfix-expression . operator [] ( expr-or-braced-init-list )
2
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[ 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
 ]