How to detect if a class is final in C++11?

Question

Code first.

#include <iostream>  using namespace std;  struct A final {}; struct B {};  int main() {      cout << is_final<A>::value << endl; // Output true     cout << is_final<B>::value << endl; // Output false      return 0;  } 

How to implement the class is_final?

Answer 1

As the implementer of GCC's __is_final intrinisic (for PR 51365) I'm pretty sure it can't be done in a library, it needs compiler support.

You can do some very clever things with C++11's SFINAE for expressions feature but to detect whether a class is final you'd need to derive from it, and instantiate the derived type, in a template argument deduction context, but deriving from a class is done in a declaration not an expression.

Also, you should think about whether you only want to know if the final pseudo-keyword was used, or if a class is un-derivable for other reasons, such as having only private constructors.