Question

I would like to determine at compile time if a pointer to Derived can be cast from a pointer to Base without dynamic_cast<>. Is this possible using templates and metaprogramming? This isn’t exactly the same problem as determining if Base is a virtual base class of Derived, because Base could be the super class of a virtual base class of Derived.

Thanks,
Tim
Update:
I felt good about this method:

#include <iostream>

using namespace std;

class Foo
{
};

class Bar : public Foo
{
};

class Baz : public virtual Foo
{
};

class Autre : public virtual Bar
{
};

typedef char Small;
class Big { char dummy[2]; };

template<typename B, typename D>
struct is_static_castable
{
    const B* foo;
    char bar[1];
    static Small test(char(*)[sizeof(static_cast<const D*>(foo)) == sizeof(const D*)]);
    static Big test(...);
    enum { value = (sizeof(test(&bar)) == sizeof(Small)) };
};

int main()
{

    cout << "Foo -> Bar: " << is_static_castable<Foo, Bar>::value << "\n";
    cout << "Foo -> Baz: " << is_static_castable<Foo, Baz>::value << "\n";
    cout << "Foo -> Autre: " << is_static_castable<Foo, Autre>::value << "\n";
}

But it doesn’t work with gcc:

multi-fun.cpp: In instantiation of ‘is_static_castable<Foo, Baz>’:
multi-fun.cpp:38:   instantiated from here
multi-fun.cpp:29: error: cannot convert from base ‘Foo’ to derived type ‘Baz’ via virtual base ‘Foo’
multi-fun.cpp:29: error: array bound is not an integer constant
multi-fun.cpp: In instantiation of ‘is_static_castable<Foo, Autre>’:
multi-fun.cpp:39:   instantiated from here
multi-fun.cpp:29: error: cannot convert from base ‘Foo’ to derived type ‘Autre’ via virtual base ‘Bar’
multi-fun.cpp:29: error: array bound is not an integer constant

Am I confused about what can be done with the sizeof() trick?

Answer 1

I had the same problem, once. Unfortunately, I’m not quite sure about the virtual-problem. But: Boost has a class named is_base_of (see here) which would enable you to do smth. like the following

BOOST_STATIC_ASSERT((boost::is_base_of<Foo, Bar>::value));

Furthermore, there’s a class is_virtual_base_of in Boost’s type_traits, maybe that’s what you’re looking for.