#include <cstdio>
#include <memory>
 
struct Animal { virtual void sayHello() = 0; }; 
 
template<typename...> struct Dog; //forward decl.
 
//template to defer compilation of func body
template<typename...> struct Cat : Animal { 
    void sayHello() { printf("I am a Cat!\n"); new(this) Dog<>; }
};
 
template<typename...> struct Dog : Animal { 
    void sayHello() { printf("I am a Dog!\n"); new(this) Cat<>; }
};
 
int main() {
    Cat<> cat;
    Animal& animal = cat;
 
    animal.sayHello(); //I am a Cat
    animal.sayHello(); //I am a Dog
 
    return 0;
}
 

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