覆盖方法时 virtual 关键字是什么意思? [英] What does the virtual keyword mean when overriding a method?

问题描述

关键字 virtual 在覆盖方法时有什么作用?我没有使用它，一切正常.

What does the keyword virtual do when overriding a method? I'm not using it and everything works fine.

每个编译器在这方面的行为都一样吗?

Does every compiler behave the same in this regard?

我应该使用它还是不使用它?

Should I use it or not?

推荐答案

没有它你不能重写成员函数.

You cannot override a member function without it.

您只能隐藏一个.

struct Base {
   void foo() {}
};

struct Derived : Base {
   void foo() {}
};

Derived::foo不会覆盖Base::foo；它只是隐藏它，因为它具有相同的名称，例如:

Derived::foo does not override Base::foo; it simply hides it because it has the same name, such that the following:

Derived d;
d.foo();

调用Derived::foo.

virtual 启用多态性，以便您实际上覆盖函数:

struct Base {
   virtual void foo() {}
};

struct Derived : Base {
   virtual void foo() {} // * second `virtual` is optional, but clearest
};

Derived d;
Base& b = d;
b.foo();

这会调用Derived::foo，因为它现在覆盖 Base::foo —你的对象是多态的.

This invokes Derived::foo, because this now overrides Base::foo — your object is polymorphic.

(您还必须为此使用引用或指针，因为切片问题.)

(You also have to use references or pointers for this, due to the slicing problem.)

• Derived::foo 不需要重复 virtual 关键字，因为 Base::foo 已经使用了它.这是由标准保证的，您可以信赖它.但是，有些人认为为了清楚起见最好将其保留.

• Derived::foo doesn't need to repeat the virtual keyword because Base::foo has already used it. This is guaranteed by the standard, and you can rely on it. However, some think it best to keep that in for clarity.

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