C++ minimal progressive disclosure implementation for API

Following another post I am trying to implement a minimal, working, C++ application to understand how to implement the progressive disclosure pattern. Here is what I have written so far:

#include <iostream>

using std::cout;
using std::endl;

struct IExtBase;

struct IBase
    {
    virtual void basicMethod()   = 0;
    virtual IExtBase& ext()      = 0;
    };

struct IExtBase
    {
    virtual void advancedMethod() = 0;
    };

struct BaseImpl : public IBase, IExtBase
    {
    static IBase& initialize(int p_attr)
        {
        return reinterpret_cast<IBase&>(BaseImpl(p_attr));
        }

    void basicMethod()
        {
        m_attr++;
        cout << "basic...\tattr: " << m_attr << endl;
        }

    IExtBase& ext()
        {
        return reinterpret_cast<IExtBase&>(*this);
        }

    void advancedMethod()
        {
        m_attr++;
        cout << "advanced...\tattr: " << m_attr << endl;
        }

    private:
        BaseImpl(int p_attr) : m_attr {p_attr} {}

        int m_attr;
    };

int main()
    {
    IBase& myBase = BaseImpl::initialize(0);

    myBase.basicMethod();           // Only IBase part of the BaseImpl object
    myBase.ext().advancedMethod();  // Fails...

    return 0;
    }

When I try to run this, I get an error:

Unhandled exception at 0x00EFF798 in ProgressiveDisclosure.exe:
0xC0000005: Access violation executing location 0x00EFF798.

I'm guessing it has to do with the casting I'm doing. Furthermore, I find that code rather complex and ugly for such a small example. I fell like there should be another way, by I fail to see it.

Could you please tell me what's wrong here and/or how this could be implemented in a better way?

Thanks.