虚函数动态绑定问题 [英] Virtual function dynamic binding issue

问题描述

我陷入了继承和多态的实践项目，我们被要求通过编码派生的struct xhost来扩展基础结构主机。



我们不允许修改host.h：

I'm stuck in a practice project on inheritance and polymorphism, where we were asked to expand the base struct host by coding its derived struct xhost.

We were not allowed to modify host.h:

#ifndef HOST_H
#define HOST_H

#include <string>
#include <iostream>

struct host {
/////////snap//////////
private:
    virtual void handle_recv(const host&, const std::string&) const {};
};

#endif

和我的xhost看起来像这样（部分）1.cpp：

And my xhost looks like this in (part of) 1.cpp:

#include "host.h"
using namespace std;

struct xhost : public host
{
    virtual void send(const host &other, const string &s) const
    {
        string message = s;

        for (int i = 0; i != splugList.size(); i++)
        {
            message = (*(splugList[i]))(message);
        }

        cout << "[" << name << " -> " << other.name << "] " << message << std::endl;

        other.handle_recv(*this, s);//////////////////////ERROR HERE
    }

/////////snap//////////

private:
    virtual void handle_recv(const host &other, const string &s) const
    {
        string message = s;

        for (int i = 0; i != rplugList.size(); i++)
        {
            (*(rplugList[i]))(other, *this, message);
        }
    }
};

编译器说我试图访问私有成员函数基类主机中的handle_recv，这当然是一个错误。但是，我不明白为什么要访问基类函数;我认为，因为handle_recv是一个虚函数而另一个是引用，它会动态绑定到xhost中定义的handle_recv而不是主机中的那个？

The compiler said I was trying to access private member function handle_recv in base class host, which is of course an error. However, I don't understand why it was base class function being accessed; I thought that since handle_recv is a virtual function and other is a reference, it would dynamically bind to handle_recv defined in xhost but not the one in host?

推荐答案

好的我我想我现在明白了...不完全理解实践项目的规范（并没有注意到它）是我的坏事。规格很长，所以我没有将所有这些都包含在我的问题中，这也导致了进一步的混淆。



无论它值多少我都会尝试记录这个混乱的原因：



xhost 中定义的发送功能应该是：

1.在/// ERROR行上方执行操作后，检查参数引用其他是否绑定到主机或 xhost 。

2.如果其他绑定到 xhost ：调用其他（即xhost）的成员函数handle_recv。

如果其他绑定到主机： DO NOTHING 。 （f％^& *％

Okay I think I figure it out now... It was my bad for not fully understanding the specifications of the practice project (and not noticing it). The specs were very long, so I didn't include all of them in my question, which also led to further confusions.

For whatever it's worth I'll try to document the cause of this mess:

What send function defined in xhost was expected to do was:
1. After doing stuff above the ///ERROR line, check whether the argument reference other is bound to a host or an xhost.
2. If other is bound to an xhost : call other(i.e. an xhost)'s member function handle_recv.
And if other is bound to a host : DO NOTHING. (f%^&*%

## @ ##

me ....）



所以Philippe Mori所说的是正确的，但与这里的混乱无关（这是100％我的坏。抱歉......）。 解决方案可以简单地用以下内容替换/// ERROR行：

me....)

So what Philippe Mori said was correct, but not really related to the mess here (which is 100% my bad. Sorry...). The "solution" could simply be replacing the ///ERROR line by something like:

if (typeid(other) == typeid(const xhost &))
dynamic_cast<const xhost="">(other).handle_recv(*this, message);

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