Private inheritance and returning references from functions

Now I have code below:

class Env
{
public:
   int ra(){ return a;}
   int rb(){ return b;}
private:
   int a;
   int b;
};


class CEnv: private Env
{
public:
    static Env* Instance()
    {
        CEnv* pEnv = new CEnv;
        return pEnv;
    }

};



int _tmain(int argc, _TCHAR* argv[])
{

    Env* pEnv = CEnv::Instance();
    pEnv->ra();

    return 0;
}

And it works well.Later I add some code.

class Env
{
public:
   int ra(){ return a;}
   int rb(){ return b;}
private:
   int a;
   int b;
};


class CEnv: private Env
{
public:
    static Env* Instance()
    {
        CEnv* pEnv = new CEnv;
        return pEnv;
    }

};

Env* test()
{
    CEnv *p = new CEnv;
    return p;
}

int _tmain(int argc, _TCHAR* argv[])
{

    Env* pEnv = CEnv::Instance();
    pEnv->ra();

    return 0;
}

Then VS2010 will tell out compiler error : error C2243: 'type cast' : conversion from 'CEnv *' to 'Env &' exists, but is inaccessible.

In my opinion, it's right to show the error ,because it's not as-a relationship if using private herit. But the first code pattern works well. And I wonder why ?

If you replace private inheritance with a private member you will get the exact same result:

class Env
{ /* whatever */
};

class CEnv{
private: 
    Env m;

public:
    static Env *Instance()
    {
        CEnv *pEnv = new CEnv; /* memory leak */
        return &pEnv->m;
    }
};

(Forget about the memory leak as this is just for illustrative purpose.)

Here the member is accessible because Instance() is a member of the class.

With a non-member function:

Env *test()
{
    CEnv *p = new CEnv;
    return &p->m;
}

Function test() isn't a member or friend; try it :

prog.cpp: In function 'Env* test()':
prog.cpp:7:13: error: 'Env CEnv::m' is private
         Env m;
             ^
prog.cpp:20:20: error: within this context
         return &p->m;
                    ^

Make it a friend if you must:

class CEnv2 {
private: 
    Env m;
    friend Env *::test2();
};

Env *test2()
{
    CEnv2 *p = new CEnv2;
    return &p->m;
}

Now it compiles.