C++ 中的函数覆盖在没有“虚拟"的情况下工作 [英] Function overriding in C++ works without 'virtual'

问题描述

我有一个包含一些函数的类(没有一个是虚拟的)，另外还有 2 个类公开继承了该类.在两个子类中，我都覆盖了基类的相同功能.

I have a class that contains some functions (none are virtual) and 2 more classes publicly inherit that class. In both the sub classes I override the same function of the base class.

在 main(位于同一文件)中创建所有三个类的对象后，我使用基类对象调用原始函数，并使用派生类对象调用覆盖函数.

After creating objects of all three classes in main (located at the same file), I call the original function with the baseclass object and the overridden functions with the derivedclass objects.

我期望所有 3 个函数调用都运行来自基类的原始函数(因为我没有在代码中的任何地方使用虚拟")，但我实际上让该函数的每个版本都根据类中的类工作它被定义(3个不同的版本).

I was expecting all 3 function calls to run the original function from the base class (since I didn't use 'virtual' anywhere in the code), but I actually get each version of that function working according to the class in which it was defined (3 different versions).

我有 Base & 类推导如下:

I have the classes Base & Derived as follows:

struct Base
{
   void foo();
};

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

主要内容:

int main()
{
   Derived d;
   d.foo();
}

如果不使用虚拟"，我认为 d.foo() 应该运行 Base::foo().

I thought d.foo() should run Base::foo() if not using 'virtual'.

推荐答案

这不是压倒一切"……而且也没有必要.

This is not "overriding"... and it doesn't need to be.

struct Base
{
   void foo();
};

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

int main()
{
   Derived d;
   d.foo();
}

如果我理解正确，那么您期望它执行 Base::foo()，因为函数不是虚拟的，因此一个不会覆盖另一个.

If I understand you correctly, then you were expecting this to execute Base::foo(), because the functions are not virtual and therefore one does not override the other.

但是，在这里，您不需要虚拟分派:继承规则只是声明您将获得运行它的对象类型的正确函数.

But, here, you do not need virtual dispatch: the rules of inheritance simply state that you'll get the right function for the type of the object you run it on.

当您需要虚拟分派/覆盖时，情况略有不同:当您使用间接时:

When you need virtual dispatch/overriding is a slightly different case: it's when you use indirection:

int main()
{
   Base* ptr = new Derived();
   ptr->foo();
   delete ptr;
}

在上面的代码片段中，结果将是 Base::foo() 被调用，因为表达式 ptr->foo() 不知道*ptr 是真的一个Derived.它只知道 ptr 是一个 Base*.

In the above snippet, the result will be that Base::foo() is called, because the expression ptr->foo() doesn't know that *ptr is really a Derived. All it knows is that ptr is a Base*.

这就是添加virtual(并在这样做时使一个函数覆盖另一个函数)产生奇迹的地方.

This is where adding virtual (and, in doing so, making the one function override the other) makes magic happen.

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