Virtual inheritance

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Hi to all, I'm facing a problem in a particularly complex inheritance
hierarchy, and I'd like to know what the standard says about it and if my
compiler is correct in what it does.

I have two consecutive layers of diamond-shaped inheritance; the first layer
is declared as using virtual inheritance, while the second one is declared
as *not* using it.

This is what I'd like to have:

     Base Base
    / \ / \
D1Base D2Base D1Base D2Base
    \ / \ /
    Middle Middle
      || ||
   D1Middle D2Middle
          \ /
           \ /
            \ /
             Last

This is what the compiler actually does:

      Base
     / D1Base D2Base
     \ /
     Middle
     / D1Middle D2Middle
     \ /
      Last

If I remove all virtual inheritance, everything goes just fine; if I use it
in the first layer, there seems to be no way of disabling it afterwards.

Does the standard saying anything about this, or is it
implementation-dependant? My compiler is Visual Studio 2005.

Code sample attached.

Thanks

Massimo

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#include <iostream>

using namespace std;

class Base
{
private:
    int n;
public:
    void SetN(int n)
    {
        this->n = n;
    }
    int GetN(void)
    {
        return(n);
    }
};

class D1Base:
    public virtual Base
{};

class D2Base:
    public virtual Base
{};

class Middle:
    public D1Base,
    public D2Base
{
    // I want Middle to only have one copy of n;
    // This goes ok.
};

class D1Middle:
    public Middle
{
    // I want D1Middle to have its own Middle object
    // and its own n.
};

class D2Middle:
    public Middle
{
    // I want D2Middle to have its own Middle object
    // and its own n.
};

class Last:
    public D1Middle,
    public D2Middle
{
    // I want Last to access two different Middle objects
    // and, ultimately, two different n's.
    // This doesn't happen.
};

int main(int,char**)
{
    Last* l = new Last;

    l->D1Middle::SetN(42);
    l->D2Middle::SetN(43);

    cout << l->D1Middle::GetN() << endl;
    cout << l->D2Middle::GetN() << endl;

    cin.get();

    return(0);
}

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