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 << " I am " << name << ", I love my children." << endl;
}
void Work()
{
cout << " I 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 << " I am " << name << ", I love my parents." << endl;
}
/*
**Children inherit Work() method from parents,
**it could be treated like part-time job.
*/
void Study()
{
cout << " I 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 << " I am a stranger" << endl;
}
void Speak()
{
//cout << "I am a stranger" << endl;
cout << " Do 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 << " [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 << " [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 <<" [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 << " [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 << " [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 << " [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