protected Members

As we’ve seen, a class uses protected for those members that it is willing to share with its derived classes but wants to protect from general access. The protected specifier can be thought of as a blend of private and public:

• Like private, protected members are inaccessible to users of the class.

• Like public, protected members are accessible to members and friends of classes derived from this class.

In addition, protected has another important property:

• A derived class member or friend may access the protected members of the base class only through a derived object. The derived class has no special access to the protected members of base-class objects.

To understand this last rule, consider the following example:

class Base {
protected:
    int prot_mem;     // protected member
};
class Sneaky : public Base  {
    friend void clobber(Sneaky&);  // can access Sneaky::prot_mem
    friend void clobber(Base&);    // can't access Base::prot_mem
    int j;                          // j is private by default
};
// ok: clobber can access the private and protected members in Sneaky objects
void clobber(Sneaky &s) { s.j = s.prot_mem = 0; }
// error: clobber can't access the protected members in Base
void clobber(Base &b) { b.prot_mem = 0; }

If derived classes (and friends) could access protected members in a base-class object, then our second version of clobber (that takes a Base&) would be legal. That function is not a friend of Base, yet it would be allowed to change an object of type Base; we could circumvent the protection provided by protected for any class simply by defining a new class along the lines of Sneaky.

To prevent such usage, members and friends of a derived class can access the protected members only in base-class objects that are embedded inside a derived type object; they have no special access to ordinary objects of the base type.