c++类的继承问题

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