问题
In C++, must operator []() always be a member function? If yes, why?
I read "An operator must be a member function" in book
"The C++ Programming Language Special Edition" page 287.
回答1:
From the C++ draft:
13.5.5 Subscripting [over.sub]
operator[] shall be a non-static member function with exactly one parameter. It implements the subscripting syntax
postfix-expression [ expression ]
Thus, a subscripting expression x[y] is interpreted as x.operator for a class object x of type T if T::operator exists and if the operator is selected as the best match function by the overload resolution mechanism (13.3.3).
I can't find it explicit in the spec, but I'm assuming the reason is because it's expected to return an lvalue.
Scratch that: it didn't make sense. It returns an lvalue of the subscripted type not the object type.
来源：https://stackoverflow.com/questions/17210718/in-c-must-operator-be-a-member-function