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4 A structure is a class defined with the class-key struct; its members and base classes (clause 10) are public by default
(clause 11). A union is a class defined with the class-key union; its members are public by default and it holds only one
data member at a time (9.5). [Note: aggregates of class type are described in 8.5.1. —end note ] A POD-struct is an
aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or
reference, and has no user-declared copy assignment operator and no user-declared destructor. Similarly, a POD-union
is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such
types) or reference, and has no user-declared copy assignment operator and no user-declared destructor. A POD class is
a class that is either a POD-struct or a POD-union.
