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最新评论
Let's see what if we want to clone some thing when virtual and class hierarchy is considered.
// it is ok if we know in advance what is the type of object we are cloning. NotQuery *pnq; NotQuery *pnq2 = new NotQuery(*pnq); // however what if the object type we don't know const Query* pq = pnq->op() ; // suppose that pnq-> returns some Query instance. const Query * pq3 = new ???(*pq);
So, why not create anthor function which is called clone, which is virtual and each derived class know how to create it self.
so we have the following code.
class Query { public: virtual Query* clone() = 0; }; class NameQuery : public Query { public: NameQuery(string name) : _name(name) { } NameQuery(const NameQuery &rhs) : _name(rhs._name) {} virtual Query* clone() // invokes the NameQuery constructor { return new NameQuery(*this); } private: string _name; };
You see in this impl, we have a clone method which return Query * pointer. and you will probably use the code as below.
void test_virtually_virtual_new() { Query *pq = new NameQuery("Rike"); // so this is not working as the // Query::clone() returns Qeury //NameQuery * pq2 = pq->clone(); NameQuery *pnq2 = static_cast<NameQuery *> (pq->clone()); // but why this is needed of the static cast? NameQuery *pnq3 = static_cast<NameQuery *>(pnq2->clone()); }
this is not neat, expecially when we already have a NameQuery poiter, we have to do the downcast...
But remember we have discussed there is an exception to the function signature mismatch, where virtual function can have different return types in the base and in the derived class.
Here is how we reimplemented it.
class NameQuery : public Query { public: NameQuery(string name) : _name(name) { } NameQuery(const NameQuery &rhs) : _name(rhs._name) {} //virtual Query* clone() // // invokes the NameQuery constructor //{ return new NameQuery(*this); } // while the following is what you should do virtual NameQuery *clone() { return new NameQuery(*this); } private: string _name; };
Now, we can use the virtual new as follow.
void test_virtually_virtual_new_return_overloaded() { Query *pq = new NameQuery("Rike"); NameQuery* pnq2 = static_cast<NameQuery *> (pq->clone()); // there is no need to do the static_cast as you saw before. // NameQuery* pnq3 = pnq2->clone(); }
发表评论
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不安装Visual Studio,只用Windows SDK搭建VC环境
2013-12-31 21:52 15288首先你需要下载的是 Microsoft Windows S ... -
rpath - runtime search path
2013-04-03 11:36 965RPath is a very interesting to ... -
C++ - autogenerated copy constructor and assignment operator gotchas
2013-01-24 13:32 744It has been changed that the s ... -
c++ - rethrow a exception gotchas
2012-12-23 10:57 924As in my prevoius example in j ... -
c++ -typeid operator
2012-10-15 22:30 1033typeid is the one of the meager ... -
c++ - dynamic_cast revisit
2012-10-14 21:21 738There are several built-in type ... -
c++ - virtual inheritance example 1
2012-10-14 15:25 784we have discussed the virtual i ... -
c++ - virtual inheritance
2012-10-12 08:58 917As we have discussed in the pos ... -
c++ type of inheritance
2012-09-28 08:58 724There are 3 types of inheritanc ... -
c++ - virtual destructor
2012-09-27 22:01 928As we all know that virtual des ... -
c++ - vritual function and default arguments
2012-09-27 08:56 951As we all know that we virtual ... -
c++ - template specialization and partial specialization
2012-09-26 22:38 1289in this post, we are going to e ... -
c++ - member template in class template
2012-09-26 08:19 907class member template can be us ... -
c++ template class and the pattern to use its friends
2012-09-25 23:47 960template class may defined thei ... -
c++ - Friend declaration in class Template
2012-09-25 08:47 1183There are three kinds of friend ... -
c++ - class template default parameters
2012-09-25 08:18 807the template has parameter, it ... -
c++ - operator new and delete and an example of linked list stores by new/delete
2012-09-24 07:53 555The operator new and delete ope ... -
c++ - delete(void *, size_t) or delete(void *)
2012-09-24 07:18 1136In my previous dicuss, we have ... -
c++ - placement operator new() and the operator delete()
2012-09-23 15:22 839A class member operator new() c ... -
c++ - overloaded subscript operator - []
2012-09-23 08:50 1132You can overload the subscript ...
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