Accessing protected members in a derived class

Question

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.

Answer 1

A class can only access protected members of instances of this class or a derived class. It cannot access protected members of instances of a parent class or cousin class.

In your case, the Derived class can only access the b protected member of Derived instances, not that of Base instances.

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

Answer 2

protected members can be accessed:

• through this pointer
• or to the same type protected members even if declared in base
• or from friend classes, functions

To solve your case you can use one of last two options.

Accept Derived in Derived::DoSomething or declare Derived friend to Base:

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

You may also consider public getters in some cases.