Why we cannot overload subscript operator using friend function

CodePudding user response：

Not all operators can be overloaded. For those that can be overloaded there are rules whether they can be non-members or not. For details I refer you to https://en.cppreference.com/w/cpp/language/operators.

The operators that can be overloaded only as members are =,(),[] and ->.

CodePudding user response：

Let's add overloading to a language that is derived from C. Did you know that the built-in operator is symmetric? For raw arrays it's always true that a[i] == i[a]. It's an emergent property that may be cute, but hardly conducive for great code.

So, wouldn't it be a good idea to limit our overloading to only have our "container object" on the left? Many will say yes. And it is also true that the left-hand side of an expression is always taken as this for member lookup.

Now if we only want to allow the object on the lhs, the choice is easy: only allow defining the operator as a member. For a similar reason operator= is a member-only operator too, because the object being assigned to only makes sense (to the language designer) on the left.