为什么C ++允许将指向基础对象的指针转换为派生类的指针 [英] Why C++ allow converting pointer to base object into pointer of derived class

问题描述

可能重复：

使用C ++中的Static_cast进行下转

我很困惑，我们什么时候需要将指针转换为基础对象转换为派生类的指针？
任何人都可以帮我给我一些例子。

I'm very confused by when do we need to convert pointer to base object into pointer of derived class? Can anyone do me a favor to give me some example?

推荐答案

想象你有一个基类 Animal 和两个派生类 Cat 和 Dog 。

Imagine you have a base class Animal and two derived classes, Cat and Dog.

// Assume methods are public.
class Animal { virtual void Attack(); }
class Dog : public Animal { void Meow(); }
class Cat : public Animal { void Bark(); }

我们可以使用 base class 指针引用我们的派生对象。这有助于我们现在可以将它们包含为相同类型。

We can use base class pointers to reference our derived objects. This helps as we can now contain them as the same type.

Animal* myCat = new Cat;
Animal* myDog = newDog;
std::vector<Animal*> myAnimals;
myAnimals.push_back(myCat);
myAnimals.push_back(myDog);

这很有用，因为我们可以调用所有各种动物的基础成员函数，类类型。

This is useful, as we can call base member functions all all kinds of animals, regardless of their derived class types.

// Call Attack() on every cat and dog.
for_each(myAnimals.begin(), myAnimals.end(), [](auto& e) { e->Attack(); });

您可以使用动态投射测试其中一个基本指针被转换为派生类指针。

You can use dynamic casting to test if one of the base pointers can be converted into a derived class pointer.

Cat* realCat = dynamic_cast<Cat*> myAnimals[0]; // Success. Pointer is valid.
Cat* fakeCat = dynamic_cast<Cat*> myAnimals[1]; // Failure, it's not a cat. NULL.

现在可以调用您的成员方法，例如 Meow code>从派生类指针。这是不可能的，因为 Animal 没有这些方法。

You can now call your member methods, such as Meow() from the derived class pointers. This was not possible before, as Animal does not have these methods.

realCat->Meow(); // Valid.
myCat->Meow(); // Animal*, there is not Meow() method.

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