unruh@cadlab2.ecn.purdue.edu (Vance W Unruh) (04/27/91)
I am having some trouble with C++ and multiple inheritence. Given
the class structure shown below, I am encountering problems with
virtual functions. Here is the scenario.
_________
| a |
---------
/ \
/ \
------ ------
| b | | c |
------ ------
/ \ /
/ \ /
----- --------
| e | | d |
----- --------
\ /
\ /
---------
| f |
---------
First, I create an "f" and assign it to a variable of type "a".
Then I call a virtual function of "a" which is overridden in class "b".
The problem is that in the function of class "b", the "this" pointer
is not pointing to the correct data so its methods obviously fail.
Everything is declared as virtual base classes, so there should be
only one copy of the data.
I am clueless as to a solution. I have tried many variations, none
of which made any sense or solved the problem. Any help will be greatly
appreciated.
C++ source code follows.
#include <stdio.h>
#include <string.h>
/* Class Definitions */
class a {
protected:
char name[256];
public:
a();
virtual void Print();
};
class b : public virtual a {
protected:
char myName[256];
public:
b();
virtual void Print();
};
class c : public virtual a { };
class d : public virtual b, public virtual c { };
class e : public virtual b { };
class f : public e, public virtual d { };
/* Method Definitions */
a::a() { strcpy(name,"a");}
b::b() { strcpy(myName,"B");}
void a::Print() { printf("%s\n", name); }
void b::Print() { printf("%s\n",myName); }
/* Main program */
main() {
a *vari;
b *bvari;
f *fvari;
vari = bvari = fvari = new f();
vari->Print(); /* Calls b::Print() but this points to bad data */
bvari->Print(); /* Calls b::Print() with correct data */
}
Vance Unruh
unruh@ecn.purdue.edu
(317)494-5944