Unable to assign derived class object to base class pointer

Question

4.00/5 (1 vote)

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Hi Experts,

I am inheriting class B from class A in protected mode. But I am unable to assign the derived class object to the base class pointer as

MIDL

A *ptr;
    B b(30,50);
    ptr=&b;

The same issue is happened in private mode inheritance.But if we inherit it in public mode then everything is ok. What I am missing.? Any idea.? Please help me on this.

My code

XML

#include <stdafx.h>
#include <iostream>
#include <fstream>
#include <assert.h>
#include <cassert> // for assert()
using namespace std;

class A
{
public:
int a;
    A(int t)
    {
        a=t;
    }
     void Fun_A()
    {
    cout<<"Class A Fun()"<<a;
    }

};

class B: protected A // Private mode has the same problem But in public mode it is ok
{
    public:
    int b;
    B(int t,int x):b(t),A(x)
    {

    }

};

void main(void)
{
    A *ptr;
    B b(30,50);
    ptr=&b;

}

Thanks
Arun

Posted 24-Apr-11 21:09pm

Arun India

Updated 24-Apr-11 21:38pm

Sergey Alexandrovich Kryukov

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6 solutions

Solution 1

I think you are getting error C2243.
Since you are inherited A in protected mode you can not access the Class A part from Class B.Check the links for more info.

Msdn(Error info):
http://msdn.microsoft.com/en-us/library/s5480d1f.aspx[^]

Stackoverflow:
http://stackoverflow.com/questions/1471800/getting-rid-of-error-c2243[^]

Posted 24-Apr-11 21:32pm

వేంకటనారాయణ(venkatmakam)

Comments

Arun India 25-Apr-11 4:00am

But why it is happening.? I can create the object of the derived class. No problem. But when I assign it to the base pointer it making problem. Strange..?

Albert Holguin 25-Apr-11 11:26am

your statement "you can not access the Class A part from Class B" is incorrect

Solution 2

Error no error C2243 comes for lots of Typecasting issues
You may try like follows

A *ptr;
B *b(30,50);
ptr=(A*)b;

Posted 25-Apr-11 0:02am

santosh dhanawade

Comments

Arun India 25-Apr-11 6:55am

Yes Santhosh, if I use the following code it is working fine.
A *ptr;
B b(30,50);
ptr=(A*)&b;

But still my doubt is there. Why I am not able to assign the address of the class B to base class pointer like
A *ptr;
B b(30,50);
ptr=&b;

Could you explain..?

Himansu sekhar bal 25-Apr-11 7:52am

The problem is that because of protected inheritance, the default constructor is not accessible

Albert Holguin 25-Apr-11 11:25am

see my solution

Solution 3

The problem is that because of protected inheritance, the default constructor is not accessible. Although, I don't understand why this is be an issue considering that protected members are supposed to be accessible by derived classes. I'll admit, however, I'm not too familar with inheritance other than Public inheritance because I've never needed to use any kind of inheritance other than Public. Why do you want your class to have protected inheritance?

Posted 25-Apr-11 1:40am

Himansu sekhar bal

Solution 4

You need an explicit cast opertor:

class B: protected A // Private mode has the same problem But in public mode it is ok
{
public:
  A*  operator & (){ return this; }
  // other members
};

and

MIDL

A *ptr;
B b(30,50);
ptr=&b;

will work as expected.
Regards.

Posted 25-Apr-11 1:40am

mbue

Comments

Albert Holguin 25-Apr-11 11:24am

this is true but makes little sense to derive protected or private simply to override the protection with a typecast

mbue 25-Apr-11 12:57pm

Its not my decision.
Usually virtual classes should be full proteced except the virtual members. Thats the background of COM or interfaces generally too. The pure virtual members are public and everyting else is protected. So the user can only call the virtual members.
Regards.

Albert Holguin 25-Apr-11 13:00pm

this is true, but you should probably inform the OP of what they're overriding... a good habit I've picked up from SA...

mbue 25-Apr-11 13:58pm

I think the op has to learn more about the fundamentals of the c++ concept, before asking such questions. therefore i'm with you ;-)
Regards.

Solution 5

Hi,

If you F1 on the error itself you will get the explanation that the conversion is possible but is inaccessible.

Access protection prevented the conversion from happening, hence it is possible in public & not in private or protected.

Posted 25-Apr-11 1:59am

ganeshkawade2003

Comments

Himansu sekhar bal 25-Apr-11 8:01am

u r correct

Arun India 25-Apr-11 8:25am

That means, we can conclude that- if we are using base pointer to derived objects, then we should publicly inherit the base class to derived class(Not in private or protected mode). Correct..?

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Albert Holguin · Accepted Answer · 2011-04-25T05:20:00

Solution 6

The reason for this error is because the base class should not be accessible by whomever instantiates the object when derived as protected or private. This behavior can be overriden by typecasting as others have suggested, but it is not recommended and is expected behavior.

See a good explanation here:
http://vapvarun.com/study/softE/john%20wiley%20and%20sons%20-%20programming%20with%20object-oriented%20programming/5399final/lib0164.html[^]

If you want to access the base pointer, then you want a public derivation.

Posted 25-Apr-11 5:20am

Albert Holguin

Updated 25-Apr-11 5:22am

v2

Comments

Nish Nishant 25-Apr-11 12:20pm

Good answer, voted 5.

Albert Holguin 25-Apr-11 12:57pm

thanks :)

Arun India 25-Apr-11 23:41pm

Thanks Albert Holguin, This is the answer which I expect. My 5

Albert Holguin 26-Apr-11 0:02am

thanks and happy coding :)