Re: Why can derived member function not access protected member of a base class object?

Here is a sample program to show what I mean:

// Base.h
#ifndef Base_incl
#define Base_incl

class Derived; // forward declaration - required because Method2()
               // of the Base class references the Derived class
class Base
{
public:
                 Base(); // constructor

    virtual void Method1(Base &);
    virtual void Method2(Base &);
    virtual void Method3(Derived &); //it is unusual to have a
             // derived class object as a parameter for a base
             // class method, but as you see, it can be done.
protected:
    int mBase;
};

#endif

// Derived.h
#ifndef Derived_incl
#define Derived_incl

#include "Base.h"

class Derived: public Base
{
public:
                 Derived(); // constructor

    virtual void Method2(Base &); // this method
                  // overrides Base class Method2()
    virtual void Method4(Base &); // a new method
        // which is not defined for the Base class

private:
    int mDerived;
};

#endif

// Base.cpp
#include "Base.h"
#include "Derived.h"

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

Base::Base()
{
    this->mBase = 2;
}

void Base::Method1(Base & B_Param)
{
    cout << (B_Param.mBase) * (this->mBase) << endl;
}

void Base::Method2(Base & B_Param)
{
    cout << this->mBase << endl;
}

void Base::Method3(Derived & D_Param)
{
    Base & BRef = static_cast<Base &>(D_Param); // This cast creates
                  // a Base class reference to the D_Param object.
    cout << BRef.mBase << endl;
}

//Derived.cpp
#include "Derived.h"
#include <iostream>
using std::cout;
using std::endl;

Derived::Derived()
{
    this->mBase = 3;
    this->mDerived = 5;
}

void Derived::Method2(Base & B_Param)
{
    cout << this->mDerived << endl;
}

void Derived::Method4(Base & B_Param)
{
    Derived * DPtr = dynamic_cast <Derived *> (&B_Param); // this cast
          // creates a Derived class pointer to B_Param, if and only if,
          // B_Param actually is Derived class object, otherwise the
          // pointer will be set to 0

    if (DPtr != 0) // if B_Param is actually a Derived class object
        cout << DPtr->mDerived << endl;
    else
        cout << B_Param.mBase << endl;
}

If I compile Derived.cpp I get the following errors:

1>c:\documents and settings\blangela\desktop\polymorphismtest
\derived.cpp(27) : error C2248: 'Base::mBase' : cannot access
protected member declared in class 'Base'
1> c:\documents and settings\blangela\desktop\polymorphismtest
\base.h(17) : see declaration of 'Base::mBase'
1> c:\documents and settings\blangela\desktop\polymorphismtest
\base.h(7) : see declaration of 'Base'