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Protected member inaccessible from derived class - why

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  Protected member inaccessible from derived class - why avasilev <alxvasilev@gmail.com> - 2011-11-21 03:10 -0800
    Re: Protected member inaccessible from derived class - why Victor Bazarov <v.bazarov@comcast.invalid> - 2011-11-21 07:51 -0500
      Re: Protected member inaccessible from derived class - why avasilev <alxvasilev@gmail.com> - 2011-11-21 05:10 -0800
        Re: Protected member inaccessible from derived class - why "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2011-11-21 16:41 +0100
      Re: Protected member inaccessible from derived class - why Leigh Johnston <leigh@i42.co.uk> - 2011-11-21 18:40 +0000
        Re: Protected member inaccessible from derived class - why "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-11-21 20:36 +0000

#11658 — Protected member inaccessible from derived class - why

Hi all,
Quick question: why doesn't this code compile:

class Base
{
protected:
 int a;
};

class Derived: public Base
{
public:
	void test(Base* b)
	{
	  b->a = 5;
	}
};

int main()
{
	Derived d;
	Derived d1;
	d.test(&d1);
}
The error is that Base::a is protected in the test() method of Derived
Thanks in advance
Best regards
Alex

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#11660

On 11/21/2011 6:10 AM, avasilev wrote:
> Quick question: why doesn't this code compile:
>
> class Base
> {
> protected:
>   int a;
> };
>
> class Derived: public Base
> {
> public:
> 	void test(Base* b)
> 	{
> 	  b->a = 5;
> 	}
> };
>
> int main()
> {
> 	Derived d;
> 	Derived d1;
> 	d.test(&d1);
> }
> The error is that Base::a is protected in the test() method of Derived

The access to protected members is allowed only for the '*this' object, 
and not for any other object.  IOW, in 'Derived::test' you should be 
able to access 'this->a', but not 'anyotherBase.a'.  That's just the 
rule of the language.

V
-- 
I do not respond to top-posted replies, please don't ask

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#11661

On Nov 21, 2:51 pm, Victor Bazarov <v.baza...@comcast.invalid> wrote:
> On 11/21/2011 6:10 AM, avasilev wrote:
>
>
>
>
>
>
>
>
>
> > Quick question: why doesn't this code compile:
>
> > class Base
> > {
> > protected:
> >   int a;
> > };
>
> > class Derived: public Base
> > {
> > public:
> >    void test(Base* b)
> >    {
> >      b->a = 5;
> >    }
> > };
>
> > int main()
> > {
> >    Derived d;
> >    Derived d1;
> >    d.test(&d1);
> > }
> > The error is that Base::a is protected in the test() method of Derived
>
> The access to protected members is allowed only for the '*this' object,
> and not for any other object.  IOW, in 'Derived::test' you should be
> able to access 'this->a', but not 'anyotherBase.a'.  That's just the
> rule of the language.
>
> V
> --
> I do not respond to top-posted replies, please don't ask

Thanks for the response, Victor

I thought about this, but I think I remember having written operator
methods, that contain something like:


  MyClass& operator=(MyClass& other)
  {
     mMember = other.mMember;
     ...
  }

 where mMember was internal protected stuff. IS there any exception
for operators, or I am just mistaken?

Thanks

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#11663

On 21.11.2011 14:10, avasilev wrote:
>>>     void test(Base* b)
>
>    MyClass&  operator=(MyClass&  other)
>    {
>       mMember = other.mMember;
>       ...
>    }
>
>   where mMember was internal protected stuff. IS there any exception
> for operators, or I am just mistaken?

Can you see any obvious difference above?

Cheers & hth.,

- Alf

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#11667

On 21/11/2011 12:51, Victor Bazarov wrote:
> On 11/21/2011 6:10 AM, avasilev wrote:
>> Quick question: why doesn't this code compile:
>>
>> class Base
>> {
>> protected:
>> int a;
>> };
>>
>> class Derived: public Base
>> {
>> public:
>> void test(Base* b)
>> {
>> b->a = 5;
>> }
>> };
>>
>> int main()
>> {
>> Derived d;
>> Derived d1;
>> d.test(&d1);
>> }
>> The error is that Base::a is protected in the test() method of Derived
>
> The access to protected members is allowed only for the '*this' object,

Wrong.

class Base
{
protected:
  int a;
};

class Derived: public Base
{
public:
	void test(Derived* d)
	{
	  d->a = 5;
	}
};

int main()
{
	Derived d;
	Derived d1;
	d.test(&d1);
}

HTH.

/Leigh

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#11669

"Leigh Johnston" <leigh@i42.co.uk> wrote in message 
news:LYudnaCQn4GFBlfTnZ2dnUVZ8nudnZ2d@giganews.com...
> On 21/11/2011 12:51, Victor Bazarov wrote:
>> On 11/21/2011 6:10 AM, avasilev wrote:
>>> Quick question: why doesn't this code compile:
>>>
>>> class Base
>>> {
>>> protected:
>>> int a;
>>> };
>>>
>>> class Derived: public Base
>>> {
>>> public:
>>> void test(Base* b)
>>> {
>>> b->a = 5;
>>> }
>>> };
>>>
>>> int main()
>>> {
>>> Derived d;
>>> Derived d1;
>>> d.test(&d1);
>>> }
>>> The error is that Base::a is protected in the test() method of Derived
>>
>> The access to protected members is allowed only for the '*this' object,
>
> Wrong.
>
> class Base
> {
> protected:
>  int a;
> };
>
> class Derived: public Base
> {
> public:
> void test(Derived* d)
> {
>   d->a = 5;
> }
> };
>
> int main()
> {
> Derived d;
> Derived d1;
> d.test(&d1);
> }
>
lol
Well done Leigh! :)
You are gaining experience young Jedi. 



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