C++: new base class,but it can access the derived class's property

Question

#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;
}

Answer 1

正如各种评论者所指出的那样，你可以欺骗编译器让你这样做。

关键是

fish *p_fish = (fish *)p_animal;

这一行基本上迫使编译器接受 p_animal指向的，它现在指向一条鱼。

如果您能够访问那里的属性，那么它基本上是偶然的，并且不同的编译器可能会给您不同的结果。

如果你写过

fish *p_fish = p_animal;

然后编译器会抱怨。

Answer 2

对您的代码提出一些意见。

在此示例中，您可能需要虚拟功能。请参阅以下修改代码：

#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;
}

我希望这可以帮到你。