Why can't we call protected destructors from a derived class?

Question

I'm aware of the uses of private (and of course public) destructors.

I'm also aware of the uses of a protected destructor in a derived class:

Use a protected destructor to prevent the destruction of a derived object via a base-class pointer

But I've tried running the following code and it won't compile:

struct A{
    int i;
    A() { i = 0;}
    protected: ~A(){}
};

struct B: public A{
    A* a;
    B(){ a = new A();}
    void f(){ delete a; }
};


int main()
{
   B b= B();
   b.f();
   return 0;
}

I get:

void B::f()':
main.cpp:9:16: error: 'A::~A()' is protected

What am I missing?

If I called a protected method in A from inside f() it would work. So why is calling the d'tor different?

I guess this is a duplicate, you can't call protected member functions of base class objects other than `*this` in a derived class' member function. — dyp, Oct 19 '13 at 17:31
Related or duplicate: http://stackoverflow.com/q/13723217/420683 — dyp, Oct 19 '13 at 17:34
possible duplicate of [cannot call base class protected functions?](http://stackoverflow.com/questions/477829/cannot-call-base-class-protected-functions) — Mat, Oct 19 '13 at 17:34

Lightness Races in Orbit · Accepted Answer · 2013-10-19T18:00:53.997

14

protected doesn't mean that your B can access the members of any A; it only means that it can access those members of its own A base... and members of some other B's A base!

This is in contrast to private, whereby some object with type A can always invoke the private members of another object with type A.

edited Oct 19 '13 at 18:00

answered Oct 19 '13 at 17:33

Lightness Races in Orbit

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2

A member function of class `B` can also access the protected members declared in class `A` of another object, given that this object is of type `B` or derived from `B`. It's quite confusing. – dyp Oct 19 '13 at 17:36
1

I shall `public`ly upvote you in a `protected` fashion. Wanna meet in `private` later? – Oct 19 '13 at 17:42
@DyP: Oh, yes, that's true. – Lightness Races in Orbit Oct 19 '13 at 18:00
1

So, what happens in a polymorphic scenario where the `A*` is actually used to allocate a `B` instance during the runtime - `B` doesn't know whether the actual instance is of type A or B? A no go because of using a base pointer even though it is a B instance? – dtech Oct 19 '13 at 19:58
1

@ddriver: That's right! Virtual dispatch works for function calls but access restrictions are applied first. – Lightness Races in Orbit Oct 19 '13 at 20:14

Why can't we call protected destructors from a derived class?

1 Answers1