Is this allowed?

#include <iostream>

class Foo {
public:
    int a;
    int b;
};

class Bar : public Foo
{
   // NO data members added
   public:
   inline int sum() const { return a+b;};
};

int main(){
    Foo * foo = new Foo{10,20};
    Bar * bar = reinterpret_cast<Bar *>(foo);
    std::cout << bar->sum() << std::endl;
}

Note that the above code works in GCC, CLANG and MSVC. The question is, is this undefined behaviour or is this allowed? My actual use case is a subclass of boost::shared_ptr which only adds some helper methods but the above code is a simple demonstration of concept.

See https://godbolt.org/z/zzoxj7Maz for a live version