Friend parent class cannot access private constructor in child

Question

I want to create a parent class Base which has factories for its subclasses Derived1, Derived2 etc. The constructor of Base and all its derived classes should be private. I try to do it this way:

base.h:

class Derived1;    

class Base
{
    protected:
        Base ();

    public:
        Base *createDerived1 ();
}

#include "derived1.h"

derived1.h:

#include "base.h"    

class Derived1 : public Base
{
    friend class Base;

    private:
        Derived1 ();
}

derived1.cpp:

Derived1::Derived1 ()
    : Base ()
{

}

base.cpp:

Base *Base::createDerived1 ()
{
    return new Derived1 (); // error C2248: 'Derived1::Derived1' : cannot access private member declared in class 'Derived1'
}

However, I cannot access the private constructor inside the factory, despite declaring Base a friend class of Derived1. Am I doing something wrong, or there is no possibility to have the parent class as friend and I should make constructors of Derived classes public?

Rakib · Accepted Answer · 2014-06-10T11:33:33.730

1

Following code works for me:

//B.h 
class D;

class B{

protected:
  B(){}

public:
static  B* f();

};

//D.h
class D: public B{
 friend class B;

  private:
  D():B(){}

};

//B.cpp
B*  B::f(){
 D *d=new D;
 return d;
}

Your one bug is identified by the other reply. Another is the factory methods should be static, since you will not be able to create Base objects due to the fact that it has private constructor.

edited Jun 10 '14 at 11:33

answered Jun 10 '14 at 11:14

Rakib

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Right, but actually the 'Base' constructor in my code is 'protected', I posted it incorrectly as 'private'. Fixed, thanks. – Anton Poznyakovskiy Jun 10 '14 at 11:16
But I declared Base as a friend of Derived, which should provide access to all elements of Derived to Base, shouldn't it? – Anton Poznyakovskiy Jun 10 '14 at 11:22
No, actually, the factory method does work without being static, but you are correct since it should be static by definition. – Anton Poznyakovskiy Jun 10 '14 at 11:40
@AntonPoznyakovskiy, I don't understand, if u cant create any object, how you will call a *non-static* method of the class? could u plz clarify? – Rakib Jun 10 '14 at 11:43
I meant that it does compile. – Anton Poznyakovskiy Jun 11 '14 at 09:25

score 0 · Answer 2 · answered Jun 10 '14 at 11:33

That was indeed a silly mistake on my part, I wrote:

Base *createDerived1 ()
{
    return new Derived1 (); // error C2248: 'Derived1::Derived1' : cannot access private member declared in class 'Derived1'
}

instead of

Base *Base::createDerived1 ()
{
    return new Derived1 (); // everything fine
}

Of course, the private constructor could not be accessed from the global scope.

Friend parent class cannot access private constructor in child

2 Answers2