Why isn't this a POD type?

Question

I ran the below with g++ -std=c++0x pod_test.cpp on g++ 4.6.2 (mingw). I get an error on A4. Why isn\'t A4 POD?

#include 
#include          


        

Answer 1

It's not POD because it breaks this rule for standard layout classes:

— either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members

Only one class in the inheritance lattice can have non-static data members. In this case, both A and A4 have.