dynamic_cast <new_type> (expression)
dynamic_cast运算符，应该算是四个里面最特殊的一个，因为它涉及到编译器的属性设置，而且牵扯到的面向对象的多态性跟程序运行时的状态也有关系，所以不能完全的使用传统的转换方式来替代。但是也因此它是最常用，最不可缺少的一个运算符。
与static_cast一样，dynamic_cast的转换也需要目标类型和源对象有一定的关系：继承关系。 更准确的说，dynamic_cast是用来检查两者是否有继承关系。因此该运算符实际上只接受基于类对象的指针和引用的类转换。从这个方面来看，似乎dynamic_cast又和reinterpret_cast是一致的，但实际上，它们还是存在着很大的差别。
还是用代码来解释，让编译器来说明吧。
/////////////////////////////////////////////////////////////////////////////
// cast_operator_comparison.cpp
// Language: C++
// Complier: Visual Studio 2010, Xcode3.2.6
// Platform: MacBook Pro 2010 // Application: none // Author: Ider, Syracuse University ider.cs@gmail.com /////////////////////////////////////////////////////////////////////////// #include <string> #include <iostream> using namespace std; class Parents { public: Parents(string n="Parent"){ name = n;} virtual ~Parents(){} virtual void Speak() { cout << "\tI am " << name << ", I love my children." << endl; } void Work() { cout << "\tI am " << name <<", I need to work for my family." << endl;; } protected: string name; }; class Children : public Parents { public: Children(string n="Child"):Parents(n){ } virtual ~Children(){} virtual void Speak() { cout << "\tI am " << name << ", I love my parents." << endl; } /* **Children inherit Work() method from parents, **it could be treated like part-time job. */ void Study() { cout << "\tI am " << name << ", I need to study for future." << endl;; } private: //string name; //Inherit "name" member from Parents }; class Stranger { public: Stranger(string n="stranger"){name = n;} virtual ~Stranger(){} void Self_Introduce() { cout << "\tI am a stranger" << endl; } void Speak() { //cout << "I am a stranger" << endl; cout << "\tDo not talk to "<< name << ", who is a stranger." << endl; } private: string name; }; int main() { /******* cast from child class to base class *******/ cout << "dynamic_cast from child class to base class:" << endl; Children * daughter_d = new Children("Daughter who pretend to be my mother"); Parents * mother_d = dynamic_cast<Parents*> (daughter_d); //right, cast with polymorphism mother_d->Speak(); mother_d->Work(); //mother_d->Study(); //Error, no such method cout << "static_cast from child class to base class:" << endl; Children * son_s = new Children("Son who pretend to be my father"); Parents * father_s = static_cast<Parents*> (son_s); //right, cast with polymorphism father_s->Speak(); father_s->Work(); //father_s->Study(); //Error, no such method cout << endl; /******* cast from base class to child class *******/ cout << "dynamic_cast from base class to child class:" << endl; Parents * father_d = new Parents("Father who pretend to be a my son"); Children * son_d = dynamic_cast<Children*> (father_d); //no error, but not safe if (son_d) { son_d->Speak(); son_d->Study(); } else cout << "\t[null]" << endl; cout << "static_cast from base class to child class:" << endl; Parents * mother_s = new Parents("Mother who pretend to be a my daugher"); Children * daughter_s = static_cast<Children*> (mother_s); //no error, but not safe if (daughter_s) { daughter_s->Speak(); daughter_s->Study(); } else cout << "\t[null]" << endl; cout << endl; /******* cast between non-related class *******/ cout << "dynamic_cast to non-related class:" << endl; Stranger* stranger_d = dynamic_cast<Stranger*> (daughter_d); if (stranger_d) { stranger_d->Self_Introduce(); stranger_d->Speak(); } else cout <<"\t[null]"<<endl; //Stranger* stranger_s = static_cast<Stranger*> (son_s); //Error, invalid cast cout << "reinterpret_cast to non-related class:" << endl; Stranger* stranger_r = reinterpret_cast<Stranger*> (son_s); if (stranger_r) { stranger_d->Self_Introduce(); //stranger_d->Speak(); //This line would cause program crush, //as "name" could not be found corretly. } else cout << "\t[null]" << endl; cout << endl; /******* cast back*******/ cout << "use dynamic_cast to cast back from static_cast:" << endl; Children* child_s = dynamic_cast<Children*> (father_s); if (child_s) { child_s->Speak(); child_s->Work(); } else cout << "\t[null]" << endl; //cout<<typeid(stranger_r).name()<<endl; cout << "use dynamic_cast to cast back from reinterpret_cast:" << endl; Children* child_r = dynamic_cast<Children*> (stranger_r); if (child_r) { child_r->Speak(); child_r->Work(); } else cout << "\t[null]" << endl; delete daughter_d; delete son_s; delete father_d; delete mother_s; return 0; } /********************* Result *********************/ //dynamic_cast from child class to base class: // I am Daughter who pretend to be my mother, I love my parents. // I am Daughter who pretend to be my mother, I need to work for my family. //static_cast from child class to base class: // I am Son who pretend to be my father, I love my parents. // I am Son who pretend to be my father, I need to work for my family. // //dynamic_cast from base class to child class: // [null] //static_cast from base class to child class: // I am Mother who pretend to be a my daugher, I love my children. // I am Mother who pretend to be a my daugher, I need to study for future. // //dynamic_cast to non-related class: // [null] //reinterpret_cast to non-related class: // I am a stranger // //use dynamic_cast to cast back from static_cast: // I am Son who pretend to be my father, I love my parents.