boost/type_traits/is_virtual_base_of.hpp
// (C) Copyright Daniel Frey and Robert Ramey 2009. // (C) Copyright Balint Cserni 2017 // Use, modification and distribution are subject to the Boost Software License, // Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at // http://www.boost.org/LICENSE_1_0.txt). // // See http://www.boost.org/libs/type_traits for most recent version including documentation. #ifndef BOOST_TT_IS_VIRTUAL_BASE_OF_HPP_INCLUDED #define BOOST_TT_IS_VIRTUAL_BASE_OF_HPP_INCLUDED #include <boost/type_traits/is_base_of.hpp> #include <boost/type_traits/is_same.hpp> #include <boost/type_traits/make_void.hpp> #include <utility> namespace boost { namespace detail { #ifdef BOOST_MSVC #pragma warning( push ) #pragma warning( disable : 4584 4250 4594) #elif defined(__GNUC__) && (__GNUC__ >= 4) #pragma GCC system_header #endif #if !defined(BOOST_NO_SFINAE_EXPR) && !defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) && !defined(BOOST_NO_CXX11_NULLPTR) && !BOOST_WORKAROUND(BOOST_GCC, < 40800) // Implementation based on the standard's rules of explicit type conversions. // A pointer to an object of *derived* class type may be explicitly converted to a pointer to an *unambiguous* *base* class type. // A pointer to an object of an *unambiguous* *non-virtual* *base* class type may be explicitly converted to a pointer of a *derived* class type. // Therefore Derived has a virtual base Base if and only if // (1) a Derived* can be converted to Base* (so the base class is unambiguous, which comes necessarily from virtual inheritance) // (2) a Base* cannot be converted to Derived* (so the base class is either ambiguous or virtual) // With both conditions true, Base must be a virtual base of Derived. // The "is_base_of" is only needed so the compiler can (but is not required to) error out if the types are incomplete. // This is in league with the the expected behaviour. template<class T, class U> constexpr bool is_virtual_base_impl(...) { return true; } // C-style casts have the power to ignore inheritance visibility while still act as a static_cast. // They can also fall back to the behaviour of reinterpret_cast, which allows is_virtual_base_of to work on non-class types too. // Note that because we are casting pointers there can be no user-defined operators to interfere. template<class T, class U, typename boost::make_void<decltype((U*)(std::declval<T*>()))>::type* = nullptr> constexpr bool is_virtual_base_impl(int) { return false; } } // namespace detail template<class T, class U> struct is_virtual_base_of : public boost::integral_constant< bool, boost::is_base_of<T, U>::value && detail::is_virtual_base_impl<T, U>(0) && !detail::is_virtual_base_impl<U, T>(0) > {}; #else template<typename Base, typename Derived, typename tag> struct is_virtual_base_of_impl { BOOST_STATIC_CONSTANT(bool, value = false); }; template<typename Base, typename Derived> struct is_virtual_base_of_impl<Base, Derived, true_type> { union max_align { unsigned u; unsigned long ul; void* v; double d; long double ld; #ifndef BOOST_NO_LONG_LONG long long ll; #endif }; #ifdef BOOST_BORLANDC struct boost_type_traits_internal_struct_X : public virtual Derived, public virtual Base { boost_type_traits_internal_struct_X(); boost_type_traits_internal_struct_X(const boost_type_traits_internal_struct_X&); boost_type_traits_internal_struct_X& operator=(const boost_type_traits_internal_struct_X&); ~boost_type_traits_internal_struct_X()throw(); max_align data[4]; }; struct boost_type_traits_internal_struct_Y : public virtual Derived { boost_type_traits_internal_struct_Y(); boost_type_traits_internal_struct_Y(const boost_type_traits_internal_struct_Y&); boost_type_traits_internal_struct_Y& operator=(const boost_type_traits_internal_struct_Y&); ~boost_type_traits_internal_struct_Y()throw(); max_align data[4]; }; #else struct boost_type_traits_internal_struct_X : public Derived, virtual Base { boost_type_traits_internal_struct_X(); boost_type_traits_internal_struct_X(const boost_type_traits_internal_struct_X&); boost_type_traits_internal_struct_X& operator=(const boost_type_traits_internal_struct_X&); ~boost_type_traits_internal_struct_X()throw(); max_align data[16]; }; struct boost_type_traits_internal_struct_Y : public Derived { boost_type_traits_internal_struct_Y(); boost_type_traits_internal_struct_Y(const boost_type_traits_internal_struct_Y&); boost_type_traits_internal_struct_Y& operator=(const boost_type_traits_internal_struct_Y&); ~boost_type_traits_internal_struct_Y()throw(); max_align data[16]; }; #endif BOOST_STATIC_CONSTANT(bool, value = (sizeof(boost_type_traits_internal_struct_X) == sizeof(boost_type_traits_internal_struct_Y))); }; template<typename Base, typename Derived> struct is_virtual_base_of_impl2 { typedef boost::integral_constant<bool, (boost::is_base_of<Base, Derived>::value && !boost::is_same<Base, Derived>::value)> tag_type; typedef is_virtual_base_of_impl<Base, Derived, tag_type> imp; BOOST_STATIC_CONSTANT(bool, value = imp::value); }; } // namespace detail template <class Base, class Derived> struct is_virtual_base_of : public integral_constant<bool, (::boost::detail::is_virtual_base_of_impl2<Base, Derived>::value)> {}; #endif template <class Base, class Derived> struct is_virtual_base_of<Base&, Derived> : public false_type{}; template <class Base, class Derived> struct is_virtual_base_of<Base, Derived&> : public false_type{}; template <class Base, class Derived> struct is_virtual_base_of<Base&, Derived&> : public false_type{}; #ifdef BOOST_MSVC #pragma warning( pop ) #endif } // namespace boost #endif