Doubt regarding Virtual Inheritance

[Anarki]

##include <iostream>
using namespace std;

class A
{
};
class B:virtual public A
{
};
class C:virtual public A
{
};
class Dublic 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 Bublic A
{
public:
int b;
B():b(20){}
};
class Cublic A
{
public:
int c;
C():c(30){}
};
class Dublic 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?=]

ok this is an upgraded prog of the above one

#include <iostream>
using namespace std;

class A
{
public:
int a;
A():a(10){}
};
class Bublic A
{
public:
int b;
B():b(20){}
};
class Cublic A
{
public:
int c;
C():c(30){}
};
class Dublic 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.

 

[Pete Becker]

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]

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.

 

[James Kanze]

#include <iostream>
using namespace std;

class A
{};

class B:virtual public A
{};

class C:virtual public A
{};

class Dublic 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.

I don't seem much relationship between resolving ambiguity and
virtual inheritance. Virtual inheritance affects the form of
the inheritance lattice. Design determines which form you want.
If the resulting form results in ambiguities, you use scope
resolution operators to resolve them, possibly introducing
additional intermediate classes. You do not change the
inheritance lattice without modifying the overall design.

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'm afraid I don't even understand the question. Most
implementations today use a single vptr to a table with all RTTI
information. What the vtbl actually contains will vary a lot
from one implementation to the next, however.

One effect that virtual inhertance does have is that it means
that the compiler cannot staticly know where the base class is
situated compared to the derived class, so the compiler must use
dynamic RTTI to convert derived to base. Thus, in your example
above, if the complete object has type B, in a typical
implementation, the A subobject will immediately follow the B
specific data in the object. In a complete object of type D,
however, the A subobject of the B subobject will generally
follow both the B and the C subobjects, as well as any data
specific to D. Given a B*, the compiler doesn't know which case
holds, so it will use RTTI to determine where the A subobject
is, relative to the address it has. And in order to do this, it
will probably need a vptr in the object.

Note that such analysis is much simpler if you insert a
ptruint_t in each object, initializing it with a different value
each time. You can then dump the object as an array of
ptruint_t (undefined behavior, but in practice, it will work),
and see where each sub-object is situated. Also, cast a pointer
to the object to each of the sub-object types, and output that
(as a void*). Then create objects of all the types involved,
and see how the layout of a B as most derived class is different
from that of a B sub-object in a D.