需求：希望某些类提供拷贝自身对象的功能，实现如下

template <typename T>
class A
{
public:
	T *clone() {
		return new T(static_cast<T &>(*this));
	}

private:
	friend T;
	A() = default;
};

class B : public A<B>
{
public:
	B(int value) { m_value = value; }
	B(const B &other) {
		m_value = other.m_value;
		std::cout << "B(const B &other)" << std::endl;
	}
	void setValue(int value) { m_value = value; }
	int getValue() { return m_value; }

private:
	int m_value;
};

int main()
{
	B *b1 = new B(1);
	B *b2 = b1->clone();
	std::cout << b2->getValue() << std::endl;
	system("pause");
	return 0;
}