C++：新的基類，但它可以訪問派生類的屬性

-7

#include <iostream> 
using namespace std; 
class animal 
{ 

    public: 
    void breathe() 
    { 
    cout << "breathe!" << endl; 
    } 

    int height; 
}; 
class fish : public animal 
{ 
    public: 
    void breathe() 
    { 
    cout << "fish breathe!" << endl; 
    } 
    int weight; 
}; 
int main() 
{ 
    animal *p_animal = new animal(); 
    fish *p_fish = (fish *)p_animal; 

    p_fish->breathe(); 
    p_fish->weight = 2; 
    cout << p_fish->weight; //I new a animal instance,but why does it has weight property? 

    int temp; 
    cin >> temp; 
}

來源

2015-04-23 King

編譯器不會阻止你自己拍攝。你走出去的方式來做到這一點，代碼現在是越野車。 – chris

你正在使用你不應該的內存。您的代碼受未定義的行爲影響。 –

它沒有「重量」屬性，你只是假裝它。假裝可以讓你快樂一段時間，直到現實提醒你，世界不是那樣的。然後你學會不要對編譯器說謊。 – molbdnilo

對你的代碼的一些意見。

在這個例子中，你可能需要虛擬功能。看到這個修改你的代碼有一些意見：

#include <iostream> 

class Animal 
{ 
    public: 
    virtual ~Animal() {} // Always define virtual destructor in this case 
    virtual void breathe() const // This fonction doesn't modify the object 
    {        // then, add 'const' at the end 
     std::cout << "breathe!" << std::endl; 
    } 
    unsigned int height; // An animal can't have a negative height ;-) 
         // then, use an unsigned type. 
}; 

class Fish : public Animal 
{ 
    public: 
    virtual ~Fish() {} 
    virtual void breathe() const 
    { 
     std::cout << "fish breathe!" << std::endl; 
    } 
    unsigned int weight; 
}; 

int main() 
{ 
    Animal* p_animal = new Animal; // Don't put() for empty constructor 
    Animal* p_fish = new Fish; 

    p_animal->breathe(); // this prints "breathe!" 
    p_fish->breathe(); // this prints "fish breathe!" even if the pointer is 
         // an pointer to Animal. It's the behaviour of virtual 
         // functions : when the code is executed, the real type 
         // is checked and the corresponding function is called. 

    p_fish->height = 10; // This works 
    // p_fish->weight = 5; // This doesn't compile, it's an pointer to Animal 

    Fish* p_real_fish = dynamic_cast<Fish*>(p_fish); 
    p_real_fish->height = 10; // This works 
    p_real_fish->weight = 5; // This works too 

    delete p_animal; // Don't forget to release the memory ! 
    delete p_fish; 
}

我希望這可以幫助你。

來源

2015-04-23 07:15:06 Caduchon

正如各種評論者指出的那樣，你在欺騙編譯器來讓你這樣做。

重點線是

fish *p_fish = (fish *)p_animal;

這條線基本上強制編譯器接受無論 p_animal指着，它現在指向一條魚。

如果您可以訪問那裏的屬性，那麼它基本上是機會的，而不同的編譯器可能會給你不同的結果。

如果你寫了

fish *p_fish = p_animal;

然後編譯器會抱怨。

來源

2015-04-23 05:56:31 Arunas

C++：新的基類，但它可以訪問派生類的屬性

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