覆盖非虚拟方法 [英] Overriding non-virtual methods

问题描述

让我们在Visual C ++ 2010中假定这种情况:

Let's assume this scenario in Visual C++ 2010:

#include <iostream>
#include <conio.h>

using namespace std;

class Base
{
public:
    int b;
    void Display()
    {
        cout<<"Base: Non-virtual display."<<endl;
    };
    virtual void vDisplay()
    {
        cout<<"Base: Virtual display."<<endl;
    };
};

class Derived : public Base
{
public:
    int d;
    void Display()
    {
        cout<<"Derived: Non-virtual display."<<endl;
    };
    virtual void vDisplay()
    {
        cout<<"Derived: Virtual display."<<endl;
    };
};

int main()
{
    Base ba;
    Derived de;

    ba.Display();
    ba.vDisplay();
    de.Display();
    de.vDisplay();

    _getch();
    return 0;
};

从理论上讲，这个小应用程序的输出应为:

Theoretically, the output of this little application should be:

• 基本:非虚拟显示.
• 基本:虚拟显示.
• 基础:非虚拟展示.
• 派生:虚拟显示.

因为基类的Display方法不是虚拟方法，所以派生类不应覆盖它.对吧?

because the Display method of the Base class is not a virtual method so the Derived class should not be able to override it. Right?

问题在于，当我运行该应用程序时，它会打印以下内容:

The problem is that when I run the application, it prints this:

• 基本:非虚拟显示.
• 基本:虚拟显示.
• 派生:非虚拟显示.
• 派生:虚拟显示.

所以我不了解虚拟方法的概念，或者在Visual C ++中发生了奇怪的事情.

So either I didn't understand the concept of virtual methods or something strange happens in Visual C++.

有人可以帮我一个解释吗?

Could someone help me with an explanation?

推荐答案

是的，您有点误会了.

在这种情况下，派生类上同名的方法将隐藏父方法.您会想到，如果不是这种情况，尝试创建与基类非虚拟方法同名的方法会引发错误.这是允许的，这不是问题-如果您直接完成该方法就调用该方法，就可以了.

The method of the same name on the derived class will hide the parent method in this case. You would imagine that if this weren't the case, trying to create a method with the same name as a base class non-virtual method should throw an error. It is allowed and it's not a problem - and if you call the method directly as you have done it will be called fine.

但是，由于是非虚拟的，因此不会使用允许多态的C ++方法查找机制.因此，例如，如果您创建了派生类的实例，但通过指向基类的指针调用了"Display"方法，则将调用基类的方法，而对于"vDisplay"，则将调用派生方法.

But, being non-virtual, C++ method lookup mechanisms that allow for polymorphism won't be used. So for example if you created an instance of your derived class but called your 'Display' method via a pointer to the base class, the base's method will be called, whereas for 'vDisplay' the derived method would be called.

例如，尝试添加以下行:

For example, try adding these lines:

Base *b = &ba;
b->Display();
b->vDisplay();
b = &de;
b->Display();
b->vDisplay();

...并按预期观察输出:

...and observe the output as expected:

基本:非虚拟显示.
基础:虚拟显示.
基本:非虚拟展示.
派生:虚拟显示.

Base: Non-virtual display.
Base: Virtual display.
Base: Non-virtual display.
Derived: Virtual display.

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