#if defined(_MSC_VER) && !defined(__clang__)
// ms-specific keywords
#define _ABSTRACT abstract
#else
#define _ABSTRACT
#endif

class BaseClass _ABSTRACT
{
public:
    virtual void Method() = 0;
};

class ExtClass : public BaseClass
{
public:
    virtual void Method() {}
};

int main()
{
    ExtClass* a = new ExtClass();
    BaseClass* b = a;
    b->Method();
    return 0;
}