But do we need the lifetime of b to be started? Isn't it enough that a1's lifetime is started? Taking of address of a1 happens after that. [0] Upd: There is a

But do we need the lifetime of b to be started? Isn't it enough that a1's lifetime is started? Taking of address of a1 happens after that. [0] Upd: There is an interesting sentence in [class.cdtor] but I don't think it applies here because B has no constructors: "For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior."[1] [0] https://eel.is/c++draft/dcl.init.aggr#7 https://eel.is/c++draft/dcl.init.aggr#7 [1] https://eel.is/c++draft/class.cdtor#1 https://eel.is/c++draft/class.cdtor#1