class ClassWithPrivateMember
{
protected:
    int m_a;
};

class ClassNeedModifyPrivateMember
{
public:
    void Modify(ClassWithPrivateMember* a)
    {
        class Friend : public ClassWithPrivateMember
        {
            friend ClassNeedModifyPrivateMember;
        };

        ((Friend*)a)->m_a = 5;

        
    }
};


int main(int argc, char* argv[])
{
    ClassWithPrivateMember a;
    ClassNeedModifyPrivateMember b;

    b.Modify(&a);

    return 0;
}

class ClassNeedModifyPrivateMember
{
public:
    void Modify(ClassWithPrivateMember* a)
    {
        class Friend : public ClassWithPrivateMember
        {
        public:
            void modify_a(int n) { m_a = n;}
        };

        static_cast<Friend*>(a)->modify_a(5);//***
    }
};

B>        static_cast<Friend*>(a)->modify_a(5);//***
B>

#include <iostream>

class ClassWithPrivateMember
{
protected:
    int m_a;
};

struct GetProtected: public ClassWithPrivateMember
{  
  ClassWithPrivateMember &ref;
  GetProtected(ClassWithPrivateMember &ref): ref(ref) {}
  int &Get() { return this->m_a; }
};

int main()
{
  ClassWithPrivateMember old_fat_lady;

  GetProtected(old_fat_lady).Get() = 10;
}

class ClassWithPrivateMember
{
public:
    template < typename T >
    void foo( T & )
    {
        // do something innocent
    }
    
private:
    int m_a;
};


// in another translation unit

namespace
{
    class Y {};
}

template <> void ClassWithPrivateMember::foo( Y & )
{
    ++m_a;  // do whatever you want with private m_a
}

int main()
{
    ClassWithPrivateMember a;
    Y y;
    a.foo( y );
}