Accessing a Protected Member of a Base Class in Another Subclass

accessing a protected member of a base class in another subclass

When foo receives a FooBase reference, the compiler doesn't know whether the argument is a descendant of Foo, so it has to assume it's not. Foo has access to inherited protected members of other Foo objects, not all other sibling classes.

Consider this code:

class FooSibling: public FooBase { };

FooSibling sib;
Foo f;
f.foo(sib); // calls sib.fooBase()!?

If Foo::foo can call protected members of arbitrary FooBase descendants, then it can call the protected method of FooSibling, which has no direct relationship to Foo. That's not how protected access is supposed to work.

If Foo needs access to protected members of all FooBase objects, not just those that are also known to be Foo descendants, then Foo needs to be a friend of FooBase:

class FooBase
{
protected:
  void fooBase(void);
  friend class Foo;
};

How can I access base class's protected members through derived class?

There is not more to it than you already discovered. Derived instances may acces their protected members and those of other derived instances but not those of base class instances. Why? Because thats how protected works by definition.

For more details I refer you to cppreference (emphasize mine):

A protected member of a class Base can only be accessed

1) by the members and friends of Base

2) by the members and friends (until
C++17) of any class derived from Base, but only when operating on an
object of a type that is derived from Base (including this)

Cannot access protected member of another instance from derived type's scope

The rule in [class.access.base] is:

A member m is accessible at the point R when named in class N if [...]

• m as a member of N is protected, and R occurs in a member or friend of class N, or in a member of a class P derived from N, where m as a member of P is public, private, or protected

There's a lot of letters in there. But there are basically two conditions:

1. R is in a member or friend of the class. This handles the d.prot example - we are in a member of Derived while accessing a protected member of Derived.
2. R is in a member of a derived class and the member being accessed is a member of the derived class instance. This handles the b.prot example - we are in a member of a derived class, but prot is not a member of the derived class.

In other words, Derived can access Base's protected members - but only in the case that it is accessing its own subobject's protected members. It cannot access other Base object's protected members. This makes sense when you consider that this other Base could easily be SomeOtherDerived, in which case that's just another unrelated object to us that we have no special access privileges to.

Why can't I access a protected member from an instance of a base class?

You can access the protected members of a base class only through a pointer or reference to an object of the derived type.

If you change

void copy_a_from_bar(bar& o){
  a = o.a;
}

to

void copy_a_from_bar(bar& o){
  foo& foo_ref = o;
  a = o.a;        // OK. Accessing it through `bar&`
  a = foo_ref.a;  // Not OK. Accessing it through `foo&`
}

you will see the same error.

This SO answer gives an indication of why allowing access to the protected members of the base class will be a potential breach of the protected status of the base class members.

Say you have:

class baz : public foo
{
   void update_a(foo& f)
   {
      f.a = 20;
   }
};

and use:

bar x;
baz z;
z.update_a(x);

If this were allowed, baz will be able to change the values of members of bar. That is not good.

Why can't a subclass of a subclass access protected member of its ancestor?

When you write

class Dog : Animal { … };

what you're really writing is

class Dog : private Animal { … };

because the default access specifier for classes defined with class-key class is private [class.access.base]/2 (fancy way of saying: if you have a class that class will inherit everything to be private unless you explicitly say otherwise). Private inheritance means all protected and public members of the base class will be private in the derived class. Since Dog inherited all the Animal stuff privately, it's not accessible to BigDog anymore (which, by the way, also inherits all the Dog stuff privately). What you most likely wanted to write is

class Dog : public Animal { … };

and

class BigDog : public Dog { … };

Note: if you had a struct, the default would be public.

Access protected members of base class in grandchild class

Your problem is that you're inheriting from you base classes privately, so public and protected members of the base class get the same access control as private members of the derived class. While possible, private inheritance is a very specific tool and used rarely. In the vast majority of cases, you want public inheritance:

class SmallBox: public Box {
   protected:
      double height;
};

class TooSmall: public SmallBox {
    public:
        void setSmallWidth( double wid );
        void setHeight(double hei);
        double getSmallWidth( void );
        double getHeight(void);
};

Done this way, protected members will be visible to all descendants (not just direct children) normally.

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If, for some reason, you want to stick with private inheritance, you will have to "promote" the privately-inherited protected members back to protected:

class SmallBox:Box {
   protected:
      double height;
      using Box::width; // make it protected again
};

class TooSmall:SmallBox {
    public:
        void setSmallWidth( double wid );
        void setHeight(double hei);
        double getSmallWidth( void );
        double getHeight(void);
};

Accessing protected members in a derived class

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.

Why can't I access a protected member variable of a base class passed into a function as an argument?

In this answer I'll assume that you used public inheritance in your code (which was missing from the question).

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[C++11: 11.2/1]: If a class is declared to be a base class (Clause 10) for another class using the public access specifier, the public members of the base class are accessible as public members of the derived class and protected members of the base class are accessible as protected members of the derived class. If a class is declared to be a base class for another class using the protected access specifier, the public and protected members of the base class are accessible as protected members of the derived class. If a class is declared to be a base class for another class using the private access specifier, the public and protected members of the base class are accessible as private members of the derived class.

This covers the case where you're accessing a member of the same object.

However, it's a little curiosity of protected member access that in order to access a protected member of another object, it has to be located within the definition of the same type or a more derived type; in your case, it is in a less-derived type (i.e. a base):

[C++11: 11.4/1]: An additional access check beyond those described earlier in Clause 11 is applied when a non-static data member or non-static member function is a protected member of its naming class (11.2) As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C. If the access is to form a pointer to member (5.3.1), the nested-name-specifier shall denote C or a class derived from C. All other accesses involve a (possibly implicit) object expression (5.2.5). In this case, the class of the object expression shall be C or a class derived from C.

That is, you'd have to run this code from within a Class1 member function.

Bjarne mentions this in his book The C++ Programming Language (Sp. Ed.) on page 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.

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