id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
11310	is_virtual_base_of has false positives caused by sizeof alignment	leandro.gracia.gil@…	John Maddock	"The is_virtual_base_of<Base, Derived> type trait can report virtual bases in situations when there isn't any. Here's an example code snippet where it fails:

{{{
#include <iomanip>
#include <iostream>
#include <boost/type_traits.hpp>

struct A { int a; };
struct B : public virtual A {};
struct C : public A { virtual ~C() {} };

int main() {
  std::cout << std::boolalpha;
  std::cout << ""Is A a virtual base of B? "" << boost::is_virtual_base_of<A, B>::value << std::endl;
  std::cout << ""Is A a virtual base of C? "" << boost::is_virtual_base_of<A, C>::value << std::endl;
  return 0;
}
}}}

In the above code both checks are true despite A being a non-virtual base of C. The reason for this is that alignment is affecting the results from sizeof and it’s making two different sizes seem equal.

In particular:
{{{
sizeof(is_virtual_base_of_impl<A, C, mpl::true_>::boost_type_traits_internal_struct_X) == 16
sizeof(is_virtual_base_of_impl<A, C, mpl::true_>::boost_type_traits_internal_struct_Y) == 16
}}}

However, if we redefine C as packed (example for gcc / clang):
{{{
struct C : public A { virtual ~C() {} } __attribute__((packed);
}}}

Then the sizes do mismatch as expected and the result is correct:
{{{
sizeof(is_virtual_base_of_impl<A, C, mpl::true_>::boost_type_traits_internal_struct_X) == 16
sizeof(is_virtual_base_of_impl<A, C, mpl::true_>::boost_type_traits_internal_struct_Y) == 12
}}}

The solution for this problem would be to define the auxiliary types ''boost_type_traits_internal_struct_X'' and ''boost_type_traits_internal_struct_Y'' as packed in such a way that works across all supported compilers. This might be some combination of ''_ _attribute_ _((packed))'' for gcc/clang, ''#pragma pack'' for the Visual C++ compiler and others."	Bugs	closed	To Be Determined	type_traits	Boost 1.57.0	Problem	duplicate