覆盖vs虚拟 [英] Overriding vs Virtual

问题描述

在函数前使用保留字virtual的目的是什么？如果我想要一个子类来覆盖父函数，我只是声明相同的函数，如 void draw（）{} 。

  class Parent {
 public：
 void say（）{
 std :: cout << 1; 
} 
}; 
 
 class Child：public Parent {
 public：
 void say（）
 {
 std :: cout< 2; 
} 
}; 
 
 int main（）
 {
 child * a = new Child（）; 
 a-> say（）; 
 return 0; 
} 
  

输出为2.

$ b b

所以再次，为什么在 say（）？的标题中需要保留字 virtual / p>

感谢一堆。

解决方案

多态性工作我认为。主要的想法是，你想要抽象每个对象的具体类型。换句话说：你想要在不知道它是一个孩子的情况下调用Child实例！

下面是一个例子：
假设你有Child 和类Child2和Child3你想通过他们的基类（Parent）引用它们。

 父母[3] 
 parents [0] = new Child（）; 
 parents [1] = new Child2（）; 
 parents [2] = new Child3（）; 
 
 for（int i = 0; i <3; ++ i）
 parents [i]  - > say（）; 
  

你可以想象，这是非常强大的。它允许你根据需要扩展Parent多次，并且使用Parent指针的函数仍然可以工作。为了这个工作像其他人提到你需要声明方法为虚拟。

What is the purpose of using the reserved word virtual in front of functions? If I want a child class to override a parent function, I just declare the same function such as void draw(){}.

class Parent { 
public:
    void say() {
        std::cout << "1";
    }
};

class Child : public Parent {
public:
    void say()
    {
        std::cout << "2";
    }
};

int main()
{
    Child* a = new Child();
    a->say();
    return 0;
}

The output is 2.

So again, why would the reserved word virtual be necessary in the header of say() ?

Thanks a bunch.

解决方案

This is the classic question of how polymorphism works I think. The main idea is that you want to abstract the specific type for each object. In other words: You want to be able to call the Child instances without knowing it's a child!

Here is an example: Assuming you have class "Child" and class "Child2" and "Child3" you want to be able to refer to them through their base class (Parent).

Parent[3] parents;
parents[0] = new Child();
parents[1] = new Child2();
parents[2] = new Child3();

for (int i=0;i<3;++i)
 parents[i]->say();

As you can imagine, this is very powerful. It let's you extend the Parent as many times as you want and functions that take a Parent pointer will still work. For this to work as others mention you need to declare the method as virtual.

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