基类的虚函数没有实现而引起的错误

      先看一个错误的程序：

#include <iostream>using namespace std;class Basic{public:virtual void fun(); };class A : public Basic{public:void fun(){cout << "hi" << endl;}};int main(){A a;return 0;}

        可以修改为：

#include <iostream>using namespace std;class Basic{public:virtual void fun() = 0;  // 纯虚函数};class A : public Basic{public:void fun(){cout << "hi" << endl;}};int main(){A a;return 0;}

      或者：

#include <iostream>using namespace std;class Basic{public:virtual void fun(){}};class A : public Basic{public:void fun(){cout << "hi" << endl;}};int main(){A a;return 0;}