POD-ness with nested structs/classes

Question

I have a question concerning POD-ness. I expected that if B is non-POD and B is a member in A, so would A be non-POD. However the following code example outputs "10", hence B is correctly considered non-POD but A is.

struct A
{
    int i;
    struct B
    {
        std::string s;
    };
};
std::cout << std::is_pod<A>::value;
std::cout << std::is_pod<A::B>::value;

Is this a bug in GCC? I'm using "c++ (GCC) 7.3.1 20180312". I don't see the sense in this behaviour. Lets say I wanted to optimize buffer allocations and use the POD-check in order to determine whether I would have to use new or can use malloc/realloc for a specific type. I would be totally wrong to use malloc to allocate storage for A. Best regards

Answer 1

A has a type A::B in it.

Instances of A have no instance of A::B in it. There is only a definition of the type, but no instantiation of it.

Add B b; to A and your anomaly goes away.