C++中Override的兩個例子

/*

 * 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;

}

結果：

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

可以看到，雖然傳回型別不完全相同，但是父類中傳回值是一個指標或引用，子類override時可以返回這個指標（或引用）的派生,可見這種情況下，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;

}

結果：

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

可見這種情況下，override並沒有發生，hide發生了

將

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

改為：

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

則可以override成功！