C++ Virtual关键字的具体使用

#include<iostream>

class Parent {
public:
  void print() {
    std::cout << "Parent" << std::endl;
  }
};

class Child : Parent {
public:
  void print() {
    std::cout << "Child" << std::endl;
  }
};

int main() {
  Child c;
  c.print();
  return 0;
}

#include<iostream>

class Parent {
public:
  void print() {
    std::cout << "Parent" << std::endl;
  }
};

// 注意这里必须是 public Parent
// 不然会报错 cannot cast 'Child' to its private base class 'Parent'
class Child : public Parent {
public:
  void print() {
    std::cout << "Child" << std::endl;
  }
};

int main() {
  Parent* p = new Child();
  p->print();
  return 0;
}

#include<iostream>

class Parent {
public:
  virtual void print() {
    std::cout << "Parent" << std::endl;
  }
};

class Child : public Parent {
public:
  virtual void print() {
    std::cout << "Child" << std::endl;
  }
};

int main() {
  Parent* p = new Child();
  p->print();
  return 0;
}

#include <iostream.h>
class Base
{
public:
virtual void f(float x){ cout << "Base::f(float) " << x << endl; }
void g(float x){ cout << "Base::g(float) " << x << endl; }
void h(float x){ cout << "Base::h(float) " << x << endl; }
};
 
class Derived : public Base
{
public:
virtual void f(float x){ cout << "Derived::f(float) " << x << endl; }
void g(int x){ cout << "Derived::g(int) " << x << endl; }
void h(float x){ cout << "Derived::h(float) " << x << endl; }
};
 
void main(void)
{
Derived d;
Base *pb = &d;
Derived *pd = &d;
// Good : behavior depends solely on type of the object
pb->f(3.14f); // Derived::f(float) 3.14
pd->f(3.14f); // Derived::f(float) 3.14
// Bad : behavior depends on type of the pointer
pb->g(3.14f); // Base::g(float) 3.14
pd->g(3.14f); // Derived::g(int) 3 (surprise!)
// Bad : behavior depends on type of the pointer
pb->h(3.14f); // Base::h(float) 3.14 (surprise!)
pd->h(3.14f); // Derived::h(float) 3.14
}