为什么我们需要C ++中的虚拟函数？ [英] Why do we need Virtual Functions in C++?

问题描述

我正在学习C ++，我正在学习虚拟函数。

I'm learning C++ and I'm just getting into Virtual Functions.

根据我在书中和在线阅读，虚函数

From what I've read (in the book and online), virtual functions are functions in the a base class that you can override in derived classes.

但在本书的前面，当学习基本继承时，我能够重写派生类中的基函数没有使用 virtual 的类。

But earlier in the book, when learning about basic inheritance, I was able to override base functions in derived classes without using virtual.

我知道有更多的虚拟函数，它似乎是重要的，所以我想清楚它是什么。我只是在网上找不到一个直接的答案。

So what am I missing here? I know there is more to virtual functions and it seems to be important so I want to be clear on what it is exactly. I just can't find a straight answer online.

推荐答案

我是一个C ++新手自己，但这里是我的理解不仅仅是什么虚拟函数，而是为什么它们是必需的：

I'm a C++ newbie myself, but here is how I understood not just what virtual functions are, but why they're required:

让我们假设你有这两个类：

Let's say you have these two classes:

class Animal
{
public:
void eat() { std::cout << "I'm eating generic food."; }
}

class Cat : public Animal
{
public:
void eat() { std::cout << "I'm eating a rat."; }
}

在您的主要功能中：

Animal *animal = new Animal;
Cat *cat = new Cat;

animal->eat(); // outputs: "I'm eating generic food."
cat->eat();    // outputs: "I'm eating a rat."

到目前为止好不好？动物吃普通食物，猫吃大鼠，所有没有虚拟。

So far so good right? Animals eat generic food, cats eat rats, all without virtual.

现在让我们改变一点， code> eat（）通过中间函数调用（这个例子中的一个简单函数）：

Let's change it a little now so that eat() is called via an intermediate function (a trivial function just for this example):

//this can go at the top of the main.cpp file
void func(Animal *xyz) { xyz->eat(); }

现在我们的主要功能是：

Now our main function is:

Animal *animal = new Animal;
Cat *cat = new Cat;

func(animal); // outputs: "I'm eating generic food."
func(cat);    // outputs: "I'm eating generic food."

噢噢...我们将一个Cat传递给了 func code>，但它不会吃老鼠。如果你重载 func（）所以它需要一个猫*？如果你必须从动物得到更多的动物，他们都需要自己的 func（）。

Uh oh... we passed a Cat into func(), but it won't eat rats. Should you overload func() so it takes a Cat* ? If you have to derive more animals from Animal they would all need their own func().

让 eat（）一个虚拟函数：

class Animal
{
public:
virtual void eat() { std::cout << "I'm eating generic food."; }
}
class Cat : public Animal
{
public:
void eat() { std::cout << "I'm eating a rat."; }
}

主要：

func(animal); // outputs: "I'm eating generic food."
func(cat);    // outputs: "I'm eating a rat."

完成。

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