Inheritance Related Question

**Bargi** · 02-07-2015

Hi all

I was thinking on the behavior of inheritance. So I have declared protected constructor in base class. In derived class, I inherited base class as private, I have created a function which return a base class object.

Below is the code.

Code:

#include<iostream>

using namespace std;


class A
{
 int a;


	void getData()
	{
	  cout << "\n Inside A getData()";
	}

protected:
	A(){}


};


class B : private A
{
  
  int b;
  
public:
	void getData()
	{
	  cout << "\n Inside B getData()";
	}

	A* getBase()
	{
	  A *a = new A();  // error line
	  return a;
	}

	B() {} 
};

int main()
{


  B b;
  b.getData();
  getchar();
  return 0;

}

At error line, I am getting below error:

cannot access protected member declared in class 'A'

I am not able to understand that when I create B b; it can access the A constructor ( as constructor is protected), then why not when I am creating A object?

Can any body help?

thanks
Bargi

**manasij7479** · 02-07-2015

As clang says,

protected constructor can only be used to construct a base class subobject

That is, you could do the following, nothing more.

Code:

B():A() {}

This is new for me too, thanks !

I will leave it to some other ninja to post the standard document location for this.

**laserlight** · 02-07-2015

This is not really about the constructor per se. Consider this example:

Code:

class A
{
protected:
    int n;
};

class B : private A
{
public:
    void foo()
    {
        A a;
        a.n = 123;
    }

    void bar()
    {
        n = 123;
    }
};

int main()
{
    B b;
    b.foo();
    b.bar();
    return 0;
}

You will find that there's a similiar error found in foo(), i.e., you cannot assign to a.n even though n is protected, not private, in A, because that n is not inherited by B: it is the n non-public member variable of an A object that has nothing to do with B. Yet, you can assign to n in bar(), because that n is inherited by B.

**Bargi** · 02-07-2015

Then how, A() constructor get called when B object was created. It was too protected in class A, and was called from B class constructor?

**laserlight** · 02-07-2015

Originally Posted by Bargi

Then how, A() constructor get called when B object was created. It was too protected in class A, and was called from B class constructor?

In this sense the constructor was inherited by B.

**Bargi** · 02-07-2015

But what I have read is that, constructor,destructor,assignment operator and copy constructor do not inherit to sub classes. Isn't it contradict?

**laserlight** · 02-07-2015

Originally Posted by Bargi

But what I have read is that, constructor,destructor,assignment operator and copy constructor do not inherit to sub classes. Isn't it contradict?

Yes, that is why I say "in this sense" and provided you with an example that does not involve a constructor. It is a matter of context, i.e., in one case the context is that of invoking something in a derived class that comes from the base class subobject; in the other case you're invoking something in an entirely separate base class object.

**Bargi** · 02-07-2015

So it means, when we inherit, protected constructor was by default get to be accessed from derived class constructor. But if tried to create a new object, in derived class, it is same as creating new object of class base, and since it is protected, we can't call it.

Correct?

Thanks
Bargi

**laserlight** · 02-07-2015

Yes.

**Bargi** · 02-07-2015

Thanks a lot Laserlight...

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