Doubt regarding Virtual Inheritance

[Anarki]

##include <iostream>
using namespace std;

class A
{
};
class B:virtual public A
{
};
class C:virtual public A
{
};
class D:public B, public C
{
};

int main()
{

cout << "Sizeof(A) " << sizeof(A) << endl;
cout << "Sizeof(B) " << sizeof(B) << endl;
cout << "Sizeof(C) " << sizeof(C) << endl;
cout << "Sizeof(D) " << sizeof(D) << endl;
return 0;
}
usually virtual inheritance comes into action to resolve the ambiguty
of data access of grandparent class when there exist more than 1 path
between grandparent and grandchild.

Ok i take it as granted VI resolves data access ambiguity.

But please take look at the output of the program i gave

1
4 //Any virtual pointer to a virtual function table? Note i havent
specified and virtual functions
4 //Any virtual pointer to a virtual function table? Note i havent
specified and virtual functions
8 //Two vptrs of B and C??

if there are vptrs what is their role in avoiding the data access
ambiguity?

i will also give another analysis of memory structure with and without
virtual inheritance.

[Anarki]

ok this is an upgraded prog of the above one

#include <iostream>
using namespace std;

class A
{
public:
int a;
A():a(10){}
};
class B:public A
{
public:
int b;
B():b(20){}
};
class C:public A
{
public:
int c;
C():c(30){}
};
class D:public B, public C
{
public:
int d;
D():d(40){}
};

int main()
{
int size[4];

size[0] = sizeof(A);
size[1] = sizeof(B);
size[2] = sizeof(C);
size[3] = sizeof(D);

cout << "Sizeof(A) " << size[0] << endl;
cout << "Sizeof(B) " << size[1] << endl;
cout << "Sizeof(C) " << size[2] << endl;
cout << "Sizeof(D) " << size[3] << endl << endl;

/*
* The following operations are done for
* the sake for anatomy of structure of
* memory alignment in an object.
*/

int i = 0;
int count = 0;
int *p = NULL;
cout << "Analysis of object of A" << endl;
A a;
p = (int*)&a;
for(i = 0, count = 0; i < size[0]; i += 4, ++count)
cout << "*(pa + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of B" << endl;
B b;
p = (int*)&b;
for(i = 0, count = 0; i < size[1]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of C" << endl;
C c;
p = (int*)&c;
for(i = 0, count = 0; i < size[2]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of D" << endl;
D d;
p = (int*)&d;
for(i = 0, count = 0; i < size[3]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

return 0;
}

if you people got time check the program with and without virtual
keyword. You can see virtual pointers are there in action. whats their
role in virtual inheritance even though i didnt declare any virtual
functions?.

I even checked the virtual address table its empty (since no virtual
function). Whats is purpose of a virtual table/pointer in virtual
inheritance even though there is no virtual function?

moderators please pardon for making a huge post...

[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-07-06 12:25, Anarki wrote:

ok this is an upgraded prog of the above one

#include <iostream>
using namespace std;

class A
{
public:
int a;
A():a(10){}
};
class B:public A
{
public:
int b;
B():b(20){}
};
class C:public A
{
public:
int c;
C():c(30){}
};
class D:public B, public C
{
public:
int d;
D():d(40){}
};

int main()
{
int size[4];

size[0] = sizeof(A);
size[1] = sizeof(B);
size[2] = sizeof(C);
size[3] = sizeof(D);

cout << "Sizeof(A) " << size[0] << endl;
cout << "Sizeof(B) " << size[1] << endl;
cout << "Sizeof(C) " << size[2] << endl;
cout << "Sizeof(D) " << size[3] << endl << endl;

/*
* The following operations are done for
* the sake for anatomy of structure of
* memory alignment in an object.
*/

int i = 0;
int count = 0;
int *p = NULL;
cout << "Analysis of object of A" << endl;
A a;
p = (int*)&a;
for(i = 0, count = 0; i < size[0]; i += 4, ++count)
cout << "*(pa + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of B" << endl;
B b;
p = (int*)&b;
for(i = 0, count = 0; i < size[1]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of C" << endl;
C c;
p = (int*)&c;
for(i = 0, count = 0; i < size[2]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

cout << "Analysis of object of D" << endl;
D d;
p = (int*)&d;
for(i = 0, count = 0; i < size[3]; i += 4, ++count)
cout << "*(p + " << count << ") = " << *(p + count) << endl;
cout << endl;

return 0;
}

if you people got time check the program with and without virtual
keyword. You can see virtual pointers are there in action. whats their
role in virtual inheritance even though i didnt declare any virtual
functions?.

I even checked the virtual address table its empty (since no virtual
function). Whats is purpose of a virtual table/pointer in virtual
inheritance even though there is no virtual function?

Compiler vendors are free to implement the standard as they choose,
trying to infer things about the language from an implementation is
doomed to fail. The reason you see this behaviour (whatever it is) is
probably because your vendor thought that this implementation was the
best way to implement things.

moderators please pardon for making a huge post...

This is an unmoderated group, and the post is not very large compared to
some others.

--
Erik Wikström

[Pete Becker]

Anarki wrote:

>
Ok i take it as granted VI resolves data access ambiguity.

Virtual inheritance is an implementation technique. Data access
ambiguity is either a design error or a coding error. If it's the result
of a design error, changing to a single base class (using virtual
inheritance) may be the right solution. But it should never be an
automatic reaction to ambiguity.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

[gpuchtel]

On Jul 6, 6:25 am, Anarki <[e-mail]:

Whats is purpose of a virtual table/pointer in virtual
inheritance even though there is no virtual function?

I'm not a compiler writer, nor am I a guru; however I did read Stanley
Lippman's book: "Inside the C++ Object Module". To paraphrase, he
describes the presence of a virtual table (in this case) as a general
implementation solution to support a form of shared subobject
inheritance when a class has one or more virtual base classes. The
virtual table is used to store the location of invariant and shared
regions of the object. Therefore, its presence (in your case) is the
result of having a virtual base class, not virtual functions.