C++渣渣，遇到这个题目，请问 Dan(std::function<Niao* ()> createNiao) 是什么用法？

还有应该怎么标志，才能知道产生 Egg 的是哪个类（ Niao 还是 Yazi ）?

#include <stdexcept> #include <functional>  class Dan;  class Niao { public:     virtual ~Niao() {};     virtual Dan* lay() = 0; };  class Dan { public:     Dan(std::function<Niao* ()> createNiao)     {         throw std::logic_error("Waiting to be implemented");     }      Niao* hatch()     {         throw std::logic_error("Waiting to be implemented");     } };  class Yazi : public Niao { public:     Yazi()     {     }      Dan* lay()     {         throw std::logic_error("Waiting to be implemented");     } };  #ifndef RunTests int main() {     Niao* yazi = new Yazi();      Dan* dan1 = yazi->lay();     Niao* childYazi1 = dan1->hatch(); } #endif