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

class A
{
protected:
  int data;
};

class B : public A
{
  public:

  B(A &a)
  {
    data = a.data;
  }
};

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

Error Message:

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86
Copyright (C) Microsoft Corporation.  All rights reserved.

demoFail.cpp
demoFail.cpp(12) : error C2248: 'A::data' : cannot access protected member declared in class 'A'
        demoFail.cpp(4) : see declaration of 'A::data'
        demoFail.cpp(2) : see declaration of 'A'

What am I doing wrong?

Answer 1

According to TC++PL, pg 404:

A derived class can access a base class’ protected members only for objects of its own type.... This prevents subtle errors that would otherwise occur when one derived class corrupts data belonging to other derived classes.

Of course, here's an easy way to fix this for your case:

class A
{
protected:
    int data;
};

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

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