The meaning of the VC++ compiler warning C4250 'class1' : inherits 'class2::member' via dominance is clear to me. (But see here for an explanation.)
I have currently the problem that I get this warning when serializing a class hierarchy that has an abstract base class with boost::serialization (1.44.0).
Please note that my classes do not form any kind of diamond-like inheritance hierarchy that could cause this warning, but the warning is caused by the instantiation of boost::detail::is_virtual_base_of_impl<...> when serializing instances of derived classes. (Which seems to be using is_virtual_base_of from Boost.TypeTraits.)
Here is a minimal code sample to reproduce the warning on Visual Studio 2005. Note that the code should be dropped as-is into one cpp-file and it should compile.
Note also the two points in the code that I have marked by comments that trigger the warning. If BOOST_CLASS_EXPORTis not used then the warning is not triggerd, but more interestingly the warning is also not triggered, if the derived class does not use virtual inheritance! (So maybe I do not understand C4250 after all.)
// -- std includes --
#include <iostream>
#include <sstream>
#include <string>
// -- boost serialization --
#define BOOST_SERIALIZATION_DYN_LINK
#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/export.hpp>
// Base with serialization support
struct Base
{
virtual ~Base() {};
virtual void DoStuff() const {
std::cout << "Base@[" << static_cast<const void*>(this) << "]::DoStuff() called\n";
}
template<class Archive> // serialization support!
void serialize(Archive & ar, const unsigned int file_version) { /*empty*/ }
};
// (The only) Specific class with ser. support
struct Concrete2 : virtual/*!C4250!*/ public Base
{
virtual void DoStuff() const {
std::cout << "Concrete2@[" << static_cast<const void*>(this) << "]::DoStuff() called\n";
}
template<class Archive> // serialization support!
void serialize(Archive & ar, const unsigned int ver) {
ar & boost::serialization::base_object<Base>(*this);
// This is just a test - no members neccessary
std::cout << "Concrete2::serialize!" << typeid(ar).name() << "\n";
}
};
// Without guid export -> *no* C4250, even *with* virtual inheritance
// (however, can't be serialized via base class pointer anymore)
BOOST_CLASS_EXPORT(Concrete2);
BOOST_CLASS_TRACKING(Concrete2, boost::serialization::track_never);
int main() {
using namespace std;
Concrete2 obj1;
obj1.DoStuff();
// The following test code is not neccessary to generate the warning ...
// (but is neccessary to show if base-pointer serialization works at runtime)
Base* ref1 = &obj1;
ostringstream out_buf;
boost::archive::text_oarchive out_archive(out_buf);
out_archive << ref1;
const string buf = out_buf.str();
cout << "Serialized obj:\n~~~~\n";
cout << buf;
cout << "\n~~~~~\n";
std::istringstream in_buf(buf);
boost::archive::text_iarchive in_archive(in_buf);
// Concrete2 obj2;
Base* ref2;
in_archive >> ref2;
if(ref2)
ref2->DoStuff();
delete ref2;
}
And here is the compiler warning (ugh!):
1>...\boost_library-1_44_0\boost\type_traits\is_virtual_base_of.hpp(61) : warning C4250: 'boost::detail::is_virtual_base_of_impl<Base,Derived,tag>::X' : inherits 'Concrete2::Concrete2::DoStuff' via dominance
1> with
1> [
1> Base=type,
1> Derived=Concrete2,
1> tag=boost::mpl::bool_<true>
1> ]
1> ...\boostserializewarningtest\vbc.cpp(27) : see declaration of 'Concrete2::DoStuff'
...
1> ...\boost_library-1_44_0\boost\mpl\eval_if.hpp(40) : see reference to class template instantiation 'boost::mpl::if_<T1,T2,T3>' being compiled
1> with
1> [
1> T1=boost::is_virtual_base_of<type,Concrete2>,
1> T2=boost::mpl::identity<boost::serialization::void_cast_detail::void_caster_virtual_base<Concrete2,type>>,
1> T3=boost::mpl::identity<boost::serialization::void_cast_detail::void_caster_primitive<Concrete2,type>>
1> ]
1> ...\boost_library-1_44_0\boost\serialization\void_cast.hpp(279) : see reference to class template instantiation 'boost::mpl::eval_if<C,F1,F2>' being compiled
1> with
1> [
1> C=boost::is_virtual_base_of<type,Concrete2>,
1> F1=boost::mpl::identity<boost::serialization::void_cast_detail::void_caster_virtual_base<Concrete2,type>>,
1> F2=boost::mpl::identity<boost::serialization::void_cast_detail::void_caster_primitive<Concrete2,type>>
1> ]
1> ...\boost_library-1_44_0\boost\serialization\base_object.hpp(68) : see reference to function template instantiation 'const boost::serialization::void_cast_detail::void_caster &boost::serialization::void_cast_register<Derived,Base>(const Derived *,const Base *)' being compiled
1> with
1> [
1> Derived=Concrete2,
1> Base=type
1> ]
...
1> ...\boost_library-1_44_0\boost\serialization\export.hpp(128) : while compiling class template member function 'void boost::archive::detail::`anonymous-namespace'::guid_initializer<T>::export_guid(boost::mpl::false_) const'
1> with
1> [
1> T=Concrete2
1> ]
1> ...\boostserializewarningtest\vbc.cpp(40) : see reference to class template instantiation 'boost::archive::detail::`anonymous-namespace'::guid_initializer<T>' being compiled
1> with
1> [
1> T=Concrete2
1> ]
The reason is in fact the is_virtual_base_of check from boost type traits. This check-construct will generate warning C4250 if the check is successful, as can be seen by this example:
...
struct base {
virtual void mf() { };
};
struct derived_normal : public base {
virtual void mf() { };
};
struct derived_virt : virtual public base {
virtual void mf() { };
};
int main() {
using namespace std;
cout << "boost::is_virtual_base_of<base, derived_normal>::value reports: ";
// The following line DOES NOT cause C4250
cout << boost::is_virtual_base_of<base, derived_normal>::value << endl;
cout << "boost::is_virtual_base_of<base, derived_virt> reports: ";
// The following line causes C4250:
cout << boost::is_virtual_base_of<base, derived_virt>::value << endl;
...
FWIW, the usage of this type-traits tool in boost serialization goes like this:
BOOST_EXPORT_CLASS ->
BOOST_CLASS_EXPORT_IMPLEMENT ->
struct guid_initializer (in export.hpp) ->void_cast_register -> is_virtual_base_of is used hereAs far as I can tell the warning is completely harmless in this case and can be prevented by wrapping the header in:
#pragma warning( push )
#pragma warning( disable : 4250 ) // C4250 - 'class1' : inherits 'class2::member' via dominance
#include ...
#pragma warning( pop ) // C4250