class X
{
public:
    void x()
    {
        //...
    }
};

//for the first attempt
class A1 :public virtual X {};
class B1 :public virtual X {};
class C1 :public A1, public B1 {};

//for the second attempt
class A2 :public X {};
class B2 :public X {};
class C2 :public A2, public B2 {};

int main(int argc,char *argv[])
{
    //first attempt
    A1 *a1=new A1();
    B1 *b1=new B1();
    C1 *c1=new C1();

    a1->x();
    b1->x();
    c1->x();

    X *ax1=a1;
    X *bx1=b1;
    X *cx1=c1;

    ax1->x();
    bx1->x();
    cx1->x();

    a1=(A1 *)ax1;   //Error: cannot convert pointer to base class "X" to pointer to derived class "A1" -- base class is virtual
    b1=(B1 *)bx1;   //Error: cannot convert pointer to base class "X" to pointer to derived class "B1" -- base class is virtual
    c1=(C1 *)cx1;   //Error: cannot convert pointer to base class "X" to pointer to derived class "C1" -- base class is virtual

    //second attempt
    A2 *a2=new A2();
    B2 *b2=new B2();
    C2 *c2=new C2();

    a2->x();
    b2->x();
    c2->x();    //Error: "C2::x" is ambiguous

    X *ax2=a2;
    X *bx2=b2;
    X *cx2=c2;  //Error: base class "X" is ambiguous

    ax2->x();
    bx2->x();
    cx2->x();

    a2=(A2 *)ax2;
    b2=(B2 *)bx2;
    c2=(C2 *)cx2;   //Error: derived class "C2" contains more than one instance of class "X"

    return 0;
}