c++ - c++ exception to return type match and virtually virtual new operator

最新推荐文章于 2013-09-05 14:03:04 发布

iteye_13882

最新推荐文章于 2013-09-05 14:03:04 发布

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分类专栏： c++ 文章标签： c++

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本文探讨了通过指针或引用复制对象的方法，并提出了一种使用克隆接口来实现对象复制的设计模式。文中还详细解释了如何在派生类中正确实现此模式，确保能够创建与原始对象类型相匹配的新实例。

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How do we duplicate a object through its pointer or reference?　suppose

NotQuery *pnq;

NotQuery *pnq2 = new NotQuery(*pnq);

however, what if we have this ?

const Query *pq = pnq->op();

// ???

we cannot dclare a virutal new instance of operator new, the new operator is a static member, and it applies before the constructor.

THINK：??? you can overload new operator inside the class, , however, we cannot make it virtual still it does not answer the time/order problem.

the solution is a surrogate, somehting like this:

public Query {
public:
   virtual Query * clone() = 0;
};

however, if we cannot change the return type in the derived class, then we might see this

NameQuery *png = new NameQuery("Rike");
NameQuery *pnq2 = static_cast<NameQuery*> (pnq->clone());

BUT!!!,there is an exception to that "requirement that return type of the derived class must match exactly that of the base class".

so, in the derived class, you can write

public NameQuery : public Query 
{
public:
  virtual NameQuery* clone() { return new NameQuery(*this); }
};

Below shows the whole code ::

class Base 
{
public :
	Base() { }
	Base(Base &base) {}
	virtual Base *clone();
protected:
	Base *print();
private:

};

class Derived : public Base {
public:
	Derived() {}
	Derived (Derived &derived) : Base(derived) {} 
	virtual Derived *clone();
protected:
	Base *print();

};

Base* Base::clone(){ 
	return new Base(*this);
}

Derived* Derived::clone() {
	return new Derived(*this);
}