C++ imperfect(五) -- Protected member access

11.4     Protected  member  access                                                                   [class.protected]

      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. ( ISO/IEC 14882:2011(E)  §11.4 )
[ Example:

// friend_protected_member_access.cpp
//
#include <iostream>

using namespace std;


class B
{
protected:
    int i;
    static int j;
};

class D1 : public B
{
};

class D2 : public B
{
    friend void fr(B*,D1*,D2*);
    void mem(B*,D1*);

public:
    void memWrapper(B* pb, D1* p1)
    {
        mem(pb, p1);
    }
};

void fr(B* pb, D1* p1, D2* p2)
{
    cout<<"[void fr(B* pb, D1* p1, D2* p2)] called"<<endl;
    //pb->i = 1; // ill-formed
    //p1->i = 2; // ill-formed
    p2->i = 3; // OK (access through a D2)
    cout<<"[p2->i = 3;] p2->i ="<<p2->i<<endl;

    p2->B::i = 4; // OK (access through a D2, even though
    cout<<"[p2->B::i = 4;] p2->B::i="<<p2->B::i<<endl;

    // naming class is B)
    //int B::* pmi_B = &B::i; // ill-formed
    int B::* pmi_B2 = &D2::i; // OK (type of &D2::i is int B::*)
    cout<<"[int B::* pmi_B2 = &D2::i;] int B::* pmi_B2 = "<<pmi_B2<<endl;

    B::j = 5; // OK (because refers to static member)
    cout<<"[B::j = 5;] B::j="<<B::j<<endl;

    D2::j = 6; // OK (because refers to static member)
    cout<<"[D2::j = 6;] D2::j = "<<D2::j<<endl;
}

void D2::mem(B* pb, D1* p1)
{
    cout<<"[void D2::mem(B* pb, D1* p1)] called"<<endl;
    //pb->i = 1; // ill-formed
    //p1->i = 2; // ill-formed
    i = 3; // OK (access through this)
    cout<<"[i = 3;] i="<<i<<endl;

    B::i = 4; // OK (access through this, quali?cation ignored)
    cout<<"[B::i = 4;] B::i="<<B::i<<endl;

    //int B::* pmi_B = &B::i; // ill-formed
    int B::* pmi_B2 = &D2::i; // OK
    cout<<"[int B::* pmi_B2 = &D2::i;] int B::* pmi_B2="<<pmi_B2<<endl;

    j = 5; // OK (because j refers to static member)
    cout<<"[j = 5;] j = "<<j<<endl;

    B::j = 6; // OK (because B::j refers to static member)
    cout<<"[B::j = 6;] B::j = "<<B::j<<endl;
}

void g(B* pb, D1* p1, D2* p2)
{
    cout<<"[void g(B* pb, D1* p1, D2* p2)] called"<<endl;
    //pb->i = 1; // ill-formed
    //p1->i = 2; // ill-formed
    //p2->i = 3; // ill-formed
}

int B::j = 0;

int main(int argc, char* argv[])
{
    D1 d1;
    D2 d2;
    D2 dPb;
    B* pb = &dPb;

    fr(pb, &d1, &d2);
    d2.memWrapper(pb, &d1);
    g(pb, &d1, &d2);

    return 0;
}

 — end example ]
[Result:

[void fr(B* pb, D1* p1, D2* p2)] called
[p2->i = 3;] p2->i =3
[p2->B::i = 4;] p2->B::i=4
[int B::* pmi_B2 = &D2::i;] int B::* pmi_B2 = 1
[B::j = 5;] B::j=5
[D2::j = 6;] D2::j = 6
[void D2::mem(B* pb, D1* p1)] called
[i = 3;] i=3
[B::i = 4;] B::i=4
[int B::* pmi_B2 = &D2::i;] int B::* pmi_B2=1
[j = 5;] j = 5
[B::j = 6;] B::j = 6
[void g(B* pb, D1* p1, D2* p2)] called

— end result]