Question

This answer seems to suggest it should work so why does my example kick up a compiler error:

class Class1
{
protected:
    long m_memberVar;
};

class SubClass1: public Class1
{
public:
    void PrintMember(Class1 memberToPrintFrom)
    {
        Console::Write("{0}", memberToPrintFrom.m_memberVar); // <-- Compiler error: error C2248: 'BaseClassMemberAccess::Class1::m_memberVar' : cannot access protected member declared in class 'BaseClassMemberAccess::Class1'
    }
};

[Edit] – changed subclass to a public inheritance on Need4Sleep’s suggestion but it makes no difference.

Answer 1

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

---

[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.