Accessing protected members in a derived class

I ran into an error yesterday and, while it's easy to get around, I wanted to make sure that I'm understanding C++ right.

I have a base class with a protected member:

class Base
{
  protected:
    int b;
  public:
    void DoSomething(const Base& that)
    {
      b+=that.b;
    }
};

This compiles and works just fine. Now I extend Base but still want to use b:

class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething(const Base& that)
    {
      b+=that.b;
      d=0;
    }
};

Note that in this case DoSomething is still taking a reference to a Base , not Derived . I would expect that I can still have access to that.b inside of Derived , but I get a cannot access protected member error (MSVC 8.0 - haven't tried gcc yet).

Obviously, adding a public getter on b solved the problem, but I was wondering why I couldn't have access directly to b . I though that when you use public inheritance the protected variables are still visible to the derived class.

---

You can only access protected members in instances of your type (or derived from your type).
You cannot access protected members of an instance of a parent or cousin type.

In your case, the Derived class can only access the b member of a Derived instance, not of a different Base instance.

Changing the constructor to take a Derived instance will also solve the problem.

---

您可以访问Derived的受保护成员，但不能访问Base那些成员（即使它是Derived的受保护成员的唯一原因是因为它是从Base继承的）

---

As mentioned, it's just the way the language works.

Another solution is to exploit the inheritance and pass to the parent method:

class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething(const Base& that)
    {
      Base::DoSomething(that);
      d=0;
    }
};

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