Why protected superclass member cannot be accessed in a subclass function when passed as an argument?

Question

I get a compile error, which I'm slightly confused about. This is on VS2003.

error C2248: 'A::y' : cannot access protected member declared in class 'A'

class A
{
public:
  A() : x(0), y(0) {}
protected:
  int x;
  int y;
};

class B : public A
{
public:
  B() : A(), z(0) {}
  B(const A& item) : A(), z(1) { x = item.y;}
private:
  int z;
};

The problem is with x = item.y;

The access is specified as protected. Why doesn't the constructor of class B have access to A::y?

Answer 1

It's because of this:

class base_class
{
protected:
    virtual void foo() { std::cout << "base::foo()" << std::endl; }
};

class A : public base_class
{
protected:
    virtual void foo() { std::cout << "A::foo()" << std::endl; }
};

class B : public base_class
{
protected:
    virtual void foo() { std::cout << "B::foo()" << std::endl; }

public:
    void bar(base_class *b) { b->foo(); }
};

If that were legal, you could do this:

A a;
B b;
b.bar(&a);

And you'd be calling a protected member of A from B, which isn't allowed.