C++中Override的两个例子

/*
 * main.cc
 *
 *  Created on: 2008-8-5
 *      Author: Administrator
 */
#include <iostream>

class Base
{
public:
    virtual Base *clone(){ std::cout << "Base::clone() \n"; return new Base;}
};

class Derived : public Base
{
public:
    Derived *clone(){ std::cout << "Derived::clone() \n"; return new Derived;}
};

int main(int arg, char **args)
{
    Derived *pd = new Derived;
    pd->clone();

    Base *pb = pd;
    pb->clone();

    return 0;
}

结果：

Derived::clone()
Derived::clone()

可以看到，虽然返回类型不完全相同，但是父类中返回值是一个指针或引用，子类override时可以返回这个指针（或引用）的派生,可见这种情况下，override成功了

再看下例：

#include <iostream>

class Base
{
public:
    virtual Base *clone(){ std::cout << "Base::clone() \n"; return new Base;}
    virtual void copy(Base&){ std::cout << "Base::copy() \n"; }
};

class Derived : public Base
{
public:
    Derived *clone(){ std::cout << "Derived::clone() \n"; return new Derived;}
    void copy(Derived&) { std::cout << "Derived::copy() \n"; }
};

int main(int arg, char **args)
{
    Derived d;
    Base b;

    Derived *pd = new Derived;
    pd->copy(d);

    Base *pb = pd;
    pb->copy(b);

    return 0;
}

结果：

Derived::copy()
Base::copy()

可见这种情况下，override并没有发生，hide发生了

将

void copy(Derived&) { std::cout << "Derived::copy() \n"; }

改为：

void copy(Base&) { std::cout << "Derived::copy() \n"; }

则可以override成功！