学步园

#include<iostream><br /> #include<vector><br /> #include<algorithm><br /> using namespace std;<br /> class Base {<br /> virtual int fun(){}<br /> };<br /> class Derived: public Base {<br /> int fun(){}<br /> };</p> <p>int main()<br /> {<br /> Base* bp1 = new Base();<br /> Base* bp2 = new Derived();<br /> Derived* dp = new Derived();</p> <p> cout << (typeid(*bp1) == typeid(Base)) << endl;//1<br /> cout << (typeid(*bp1) == typeid(Derived)) << endl;//0<br /> cout << (typeid(*bp2) == typeid(Base)) << endl;//0<br /> cout << (typeid(*bp2) == typeid(Derived)) << endl;//1<br /> cout << (typeid(*dp) == typeid(Base)) << endl;//0<br /> cout << (typeid(*dp) == typeid(Derived)) << endl;//1</p> <p> return 0;<br /> } 

如果不定义虚函数，影响的是指向子类型的父类型指针

#include<iostream><br /> #include<vector><br /> #include<algorithm><br /> using namespace std;<br /> class Base {<br /> };<br /> class Derived: public Base {<br /> };</p> <p>int main()<br /> {<br /> Base* bp1 = new Base();<br /> Base* bp2 = new Derived();<br /> Derived* dp = new Derived();</p> <p> cout << (typeid(*bp1) == typeid(Base)) << endl;//1<br /> cout << (typeid(*bp1) == typeid(Derived)) << endl;//0<br /> cout << (typeid(*bp2) == typeid(Base)) << endl;//1<br /> cout << (typeid(*bp2) == typeid(Derived)) << endl;//0<br /> cout << (typeid(*dp) == typeid(Base)) << endl;//0<br /> cout << (typeid(*dp) == typeid(Derived)) << endl;//1</p> <p> return 0;<br /> } 

原因很简单，只有父类中定义了虚函数，才能实现运行时绑定。此时使用

Base* bp2 = new Derived();

typeid(*dp)得到的是Derived而不是Base