Constructor initialization order and reference passing

Question

Hi I have a question about the constructor initialization order. Given below

struct B {}
struct A
{
    B& b;
    A(B& b) : b(b) {}
}
struct C
{
    B b;
    A a;
    C() : b(),a(b) {}
}
struct D
{
    A a;
    B b;
    D() : a(b),b() {}
}

I know that C is valid as b gets initialized before a. But what about D? b wouldn't have been constructed yet, but the address should already be known, so it should be safe?

Thanks

Answer 1

They're both valid because A doesn't call into B at all. If A accessed a data member or member function of B, then that would be invalid. In the existing case of A, it's impossible to produce an invalid example.