Question

I’m confused about the errors generated by the following code.
In Derived::doStuff, I can access Base::output directly by calling it.

Why can’t I create a pointer to output() in the same context that I can call output()?

(I thought protected / private governed whether you could use a name in a specific context, but apparently that is incomplete?)

Is my fix of writing callback(this, &Derived::output); instead of callback(this, Base::output) the correct solution?

#include <iostream>
using std::cout; using std::endl;

template <typename T, typename U>
void callback(T obj, U func)
{
  ((obj)->*(func))();
}

class Base
{
protected:
  void output() { cout << "Base::output" << endl; }
};

class Derived : public Base
{
public:
  void doStuff()
  {
// call it directly:
    output();
    Base::output();

// create a pointer to it:
//    void (Base::*basePointer)() = &Base::output;
// error: 'void Base::output()' is protected within this context
    void (Derived::*derivedPointer)() = &Derived::output;

// call a function passing the pointer:
//    callback(this, &Base::output);
// error: 'void Base::output()' is protected within this context
    callback(this, &Derived::output);
  }
};

int main()
{
  Derived d;
  d.doStuff();
}

Edit: I’d love to know where this is in the stardard, but mostly I’m just trying to wrap my head around the concept. I think my problem is that callback doesn’t have access to protected members of Derived, but it is able to call Derived::output if you pass it a pointer. How is a protected member of Derived that comes from Derived different from a protected member of Derived that comes from Base?

Answer 1

In short, it’s “because the standard says so.” Why? I don’t know, I’ve emailed a couple of the standards guys, but haven’t received a response, yet.

Specifically, 11.5.1 (from C++0x FCD):

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)114 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.

Edit:

Also, you’ll see that you change the code to the following, according to what the standard specifies, it will compile (and run) cleanly:

void (Base::*derivedPointer)() = &Derived::output;