Protected data in parent class not available in child class?

Question

I am confused: I thought protected data was read/writable by the children of a given class in C++.

The below snippet fails to compile in MS Compiler

Answer 1

The constructor of B is private. If you do not specify anything, in a class, the default modifier is private (for structs it is public). So in this example the problem is that you cannot construct B. When you add public to constructor B anoter problem arises:

B has the right to modify the part of A it derives from but not another A like in this case.

You could do following:

class A
{
public:
  A()
      : data(0)
  {
  }
  A(A &a)
  {
    data = a.data;
  }
protected:
  int data;
};

class B : public A
{
public:
  B(A &a)
      : A(a)
  {
  }
};

int main()
{
  A a;
  B b = a;
  return 0;
}