Quick A: It only need to take the address.

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Why does unary operator & not require a complete type?

What if stru has overloaded operator&()?

Then it is unspecified whether the overload will be called (See Oliv's comment for standard quote).

How could unary operator & does not require a complete type?

That's how the standard has defined the language. The built-in address-of operator doesn't need to know the definition of the type, since that has no effect on where to get the address of the object.

One consideration for why it is a good thing: Compatibility with C.