Determining if a struct is empty (= has no members) [duplicate]

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• Is there an easy way to tell if a class/struct has no data members? 4 answers

Consider plain-old-data structs - with no methods explicitly defined, no final, no inheritance etc. They either have some data members, or are empty - and I want to tell the two cases apart, generically.

That is, I want a constexpr function template <typename T> magic() for which magic<T>() is true for empty structs, and false for non-empty ones (and I don't care about other types). Alternatively, magic_alt<T>::value is true for empty structs and magic_alt<T>::value is false otherwise.

Can this be done: (1) in C++11? (2) In C++17 or C++20?