View difference between Paste ID: FxUq8iRu and VBBnQPBJ

 class X
 {
 public:
 	void x()
 	{
 		//...
 	}
 };
 
 //for the first attempt
-class A1 :public X {};
+class A1 :public virtual X {};
-class B1 :public 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;
 }