Question

What is the reason that the second class is not standard layout? (Visual Studio C++)

#include <iostream>
#include <type_traits>

struct A
{
    int i;
};

struct B : public A
{
};

std::cout << "is_standard_layout<B> == "
          << std::boolalpha
          << std::is_standard_layout<B>::value // gives false
          << std::endl;

Answer 1

Accorting to this MSVC supports built-in type traits since version 8 but this seems to say that you need version 11.

Section 9.7 defines a standard-layout class as a class that:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,
• has no virtual functions (10.3) and no virtual base classes (10.1),
• has the same access control (Clause 11) for all non-static data members,
• has no non-standard-layout base 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, and
• has no base classes of the same type as the first non-static data member.

There’s some explanation here.