two bases with shared_from_this?

How to solve following problem:

  A1 A2 - two parents, they must have enable_shared_from_this
  | |
  | |
   \ /
     B - B class
     |
     C - C class

now in C I need to get pointer to this as shared_ptr<A1>
and later I need to dynamic cast it into shared_ptr<C>

I tried several times with but the code doesnt work.

Two examples with resuls below

============================================================

One of the attempts, introducing additional base class which is base of
A1 and A2.

#include <string>
#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>

using namespace std;
using namespace boost;

struct cWithSharedThis : enable_shared_from_this<cWithSharedThis> {
    cWithSharedThis() {
        cout << "cWithSharedThis() " << (void*)this << endl;
    }
};

struct cA1 : public virtual cWithSharedThis {
    string name;
    cA1(string n) : name(n) {
        cout << "cA1 I am this " << (void*)this << endl;
    }
    virtual ~cA1() {}
    virtual void Hello() { cout<<"I am " << name << " type " << Type()
<< endl; }
    virtual string Type() { return "cA1"; }
    virtual void foo() { cout<<"Foo 1 nothing"<<endl; }
} ;

struct cA2 : public virtual cWithSharedThis {
    string name;
    cA2(string n) : name(n) {
        cout << "cA2 I am this " << (void*)this << endl;
    }
    virtual ~cA2() {}
    virtual void Hello() { cout<<"I am " << name << " type " << Type()
<< endl; }
    virtual string Type() { return "cA2"; }
    virtual void foo() { cout<<"Foo 2 nothing"<<endl; }
} ;

struct cB : public cA1, public cA2 {
    cB(string n):cA1(n+"1"),cA2(n+"2"){
        cout << "cB I am this " << (void*)this << endl;
    }
    virtual string Type() { return "cB"; }
};

struct cC : public cB {
    cC(string n):cB(n) {
        cout << "cC I am this " << (void*)this << endl;
    }
    virtual string Type() { return "cC"; }
    virtual void foo() {
        cout << "foo() I am this " << (void*)this << endl;
        shared_ptr<cWithSharedThis> ptr_base_this =
cA1::shared_from_this(); // <------
        if (!ptr_base_this) { cout << "ptr_base_this is NULL! " << endl
<< flush; return; }
        cout << "We got ptr_base_this in " << (void*)ptr_base_this.get()
<< endl << flush;
        shared_ptr<cA1> ptr = dynamic_pointer_cast<cA1>(ptr);
        cout << "We got ptr in " << (void*)ptr.get() << endl << flush;
        if (!ptr) { cout << "ptr is NULL! " << endl << flush; return; }
        ptr->Hello();
    }
};

int main() {
    shared_ptr<cC> c( new cC("xxx") );
    c->foo();
}

g++ x.cpp -o x.elf -g3 -O0 -Wall
../x.elf
cWithSharedThis() 0x804c308
cA1 I am this 0x804c2f8
cA2 I am this 0x804c300
cB I am this 0x804c2f8
cC I am this 0x804c2f8
foo() I am this 0x804c2f8
We got ptr_base_this in 0x804c308
make: *** [run] Segmentation fault

============================================================

second example

#include <string>
#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>

using namespace std;
using namespace boost;

struct cA1 : enable_shared_from_this<cA1> {
    string name;
    cA1(string n) : name(n) {}
    virtual ~cA1() {}
    virtual void Hello() { cout<<"I am " << name << " type " << Type()
<< endl; }
    virtual string Type() { return "cA1"; }
    virtual void foo() { cout<<"Foo 1 nothing"<<endl; }
} ;

struct cA2 : enable_shared_from_this<cA2> {
    string name;
    cA2(string n) : name(n) {}
    virtual ~cA2() {}
    virtual void Hello() { cout<<"I am " << name << " type " << Type()
<< endl; }
    virtual string Type() { return "cA2"; }
    virtual void foo() { cout<<"Foo 2 nothing"<<endl; }
} ;

struct cB : public cA1, public cA2 {
    cB(string n):cA1(n+"1"),cA2(n+"2"){}
    virtual string Type() { return "cB"; }
};

struct cC : public cB {
    cC(string n):cB(n) {}
    virtual string Type() { return "cC"; }
    virtual void foo() {
        shared_ptr<cA1> ptr = cA1::shared_from_this(); // <------
        ptr->Hello();
    }
};

int main() {
    shared_ptr<cC> c( new cC("xxx") );
    c->foo();
}

terminate called after throwing an instance of 'boost::bad_weak_ptr'
  what(): boost::bad_weak_ptr
Aborted

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