Virtual function dynamic binding issue

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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:

C++

#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

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

C++

#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);
        }
    }
};

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?

Posted 14-May-15 6:14am

Member 11212276

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Comments

Philippe Mori 14-May-15 12:50pm

It has to be protected so that derived class can access it.

Member 11212276 14-May-15 13:07pm

Do you mean I have to modify the base class in host.h? But it's not allowed. Is there a way to call handle_recv defined in xhost without modifying host?

Philippe Mori 14-May-15 13:13pm

You cannot. Either you are not supposed to call the method and it was made private for that reason or it was made private by error and you need to fix it.

Member 11212276 14-May-15 13:22pm

I think there might be some misunderstanding... I never intended to access handle_recv in host class. What I wanted to do is to access handle_recv in xhost. I thought that being a virtual function this can be done by dynamic binding. But obviously my attempt was incorrect but I don't know where I made my mistake.

To be more specific I think I did exactly the same thing as done here:
http://stackoverflow.com/questions/3610936/why-can-i-access-a-derived-private-member-function-via-a-base-class-pointer-to-a
But it seems that it worked for him but not me. Weird...

Philippe Mori 14-May-15 13:58pm

The example on Stack Overflow has a public member in the base class. It is a totally different case.

In you case, you are calling other.handle_recv. Since other is a host (and not an xhost), you need public access to call it.

A private member can only be accessed from the class itself and not derived classes. It can access field in itself or other object of the same type.

As I have already mentionned, either, you are not using the xhost class as intended and don't need public access to handle_recv or the class was not properly designed (wrong access specifier).

Each languages has its own rules for accessibility. You simply have to conform to those rule. In C#, for example, it is illegal to declare a virtual function to be private. In either case, public access is required to access a function defined in another class.

Protected member allows derived class to access the field but only on this. Acces to other objects require public access.

Philippe Mori 14-May-15 14:11pm

By the way, how the function handle_recv is intended to be called in original code? Maybe there is a public function that cause this function to be called...

Member 11212276 15-May-15 3:19am

Thanks for your patience! You were correct from the beginning, but it turned out I was not asking the right question because I misunderstood the requirements of the project. I wrote the details below but I guess you would only facefalm reading it:P

Sergey Alexandrovich Kryukov 14-May-15 14:44pm

Mentally depict a vtable and the indirect call through its entry, and that will give you a key to what's going on.
—SA

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Member 11212276 · Accepted Answer · 2015-05-14T21:14:00

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....)

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:

C++

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