I see that casting changes the value of a pointer in case of multiple
inheritance.In single inheritance also it is the same know?Isn't it?
Vinodh Kumar P 5 2263
"Vinodh Kumar" <th*************@yahoo.com> wrote in message
news:bg**********@news.mch.sbs.de... I see that casting changes the value of a pointer in case of multiple inheritance.In single inheritance also it is the same know?Isn't it?
Vinodh Kumar P
It can happen, for instance if the derived class has virtual functions and
the base class does not.
#include <iostream>
using namespace std;
class A
{
int a;
};
class B : public A
{
virtual ~B() {}
int b;
};
int main()
{
B* b = new B;
A* a = b;
cout << a << ' ' << b<< '\n';
}
Basically this is an area where you shouldn't mind if a cast or conversion
changes the pointer value. Do you think its a problem, or are you just
trying to learn more about C++?
john
John Harrison wrote:
It can happen, for instance if the derived class has virtual functions and the base class does not.
But having a base class with no virtual destructor is asking for trouble. #include <iostream> using namespace std;
class A { int a; };
Note no virtual destructor. class B : public A { virtual ~B() {} int b; };
int main() { B* b = new B; A* a = b; cout << a << ' ' << b<< '\n';
Assuming you want to prevent memory leaks, you'd obviously need to free
the allocated object. This:
delete a;
Creates undefined behavior. In this particular case it's easy to work
around. You just do this instead:
delete b;
But in general you may have a pointer of type A* which may point to some
derived class, which is not known. The only safe way to delete it (in
the absence of a virtual destructor) is to first cast it to the exact
correct pointer type. This can obviously be a major hassle.
}
Basically this is an area where you shouldn't mind if a cast or conversion changes the pointer value. Do you think its a problem, or are you just trying to learn more about C++?
I agree - usually, the exact value of a pointer should not matter.
-Kevin
----- Original Message -----
From: "Kevin Goodsell" <us*********************@neverbox.com>
Newsgroups: comp.lang.c++
Sent: Thursday, July 31, 2003 12:22 PM
Subject: Re: Casting changes value of ptr John Harrison wrote:
It can happen, for instance if the derived class has virtual functions
and the base class does not. But having a base class with no virtual destructor is asking for trouble.
#include <iostream> using namespace std;
class A { int a; };
Note no virtual destructor.
class B : public A { virtual ~B() {} int b; };
int main() { B* b = new B; A* a = b; cout << a << ' ' << b<< '\n';
Assuming you want to prevent memory leaks, you'd obviously need to free the allocated object. This:
delete a;
Creates undefined behavior. In this particular case it's easy to work around. You just do this instead:
delete b;
But in general you may have a pointer of type A* which may point to some derived class, which is not known. The only safe way to delete it (in the absence of a virtual destructor) is to first cast it to the exact correct pointer type. This can obviously be a major hassle.
}
Basically this is an area where you shouldn't mind if a cast or
conversion changes the pointer value. Do you think its a problem, or are you just trying to learn more about C++?
I agree - usually, the exact value of a pointer should not matter.
But the type of the object that is being pointed to difers in case of a
casting in multiple inheritance.Thats the same case when we cast between
objects of single inheritance hiearchies.So I conclude that both in single
and multiple inheritance hierarchies, casting changes the value of the
pointer.I agree that the exact value of the point doesn't matter.
"Kevin Goodsell" <us*********************@neverbox.com> wrote in message
news:3f28bc98@shknews01... John Harrison wrote:
It can happen, for instance if the derived class has virtual functions
and the base class does not.
But having a base class with no virtual destructor is asking for trouble.
#include <iostream> using namespace std;
class A { int a; };
Note no virtual destructor.
class B : public A { virtual ~B() {} int b; };
int main() { B* b = new B; A* a = b; cout << a << ' ' << b<< '\n';
Assuming you want to prevent memory leaks, you'd obviously need to free the allocated object. This:
delete a;
Creates undefined behavior. In this particular case it's easy to work around. You just do this instead:
delete b;
But in general you may have a pointer of type A* which may point to some derived class, which is not known. The only safe way to delete it (in the absence of a virtual destructor) is to first cast it to the exact correct pointer type. This can obviously be a major hassle.
}
Basically this is an area where you shouldn't mind if a cast or
conversion changes the pointer value. Do you think its a problem, or are you just trying to learn more about C++?
I agree - usually, the exact value of a pointer should not matter.
-Kevin
"tom_usenet" <to********@hotmail.com> wrote in message
news:3f***************@news.easynet.co.uk... On Thu, 31 Jul 2003 12:46:35 +0530, "Vinodh Kumar" <th*************@yahoo.com> wrote:
But the type of the object that is being pointed to difers in case of a casting in multiple inheritance.Thats the same case when we cast between objects of single inheritance hiearchies.So I conclude that both in
singleand multiple inheritance hierarchies, casting changes the value of the pointer.I agree that the exact value of the point doesn't matter.
In single inheritence, in common implementations the cast shouldn't change the value of the pointer. Imagine:
class A { int i; virtual ~A(); };
class B: public A { int j; };
B* b = new B; A* a = b;
Now, the layout of a B might look like this:
vtable ptr (4 bytes) A part: int i; (4 bytes) B part: int j; (4 bytes)
Now, if the B object is created at the address 10, say, then the object will run from address 10 though to 22.
The value of b above will be 10. The value of a above will also be 10!
IOW, the value of the pointer will not change in single inheritence cases.
class A
{
int i;
virtual ~A();
};
class B: public A
{
int j;
};
class C: public B
{
int k;
}
Now, the layout of a C might look like this:
vtable ptr (4 bytes)
A part: int i; (4 bytes)
B part: int j; (4 bytes)
C part: int k;(4 bytes)
Now, if the B object is created at the address 10, say, then the
object will run from address 10 though to 24.
C* c = new C;
A* a = C;
The value of c above will be 10.
The value of a above will also be 10.
Fine.
B* b = c;
Now the value of b will be different.
So in single inheritence casting changes the value of the pointer if we are
not casting to top most base class or bottom most derived class.
On Fri, 1 Aug 2003 12:06:37 +0530, "Vinodh Kumar"
<th*************@yahoo.com> wrote: class A { int i; virtual ~A(); };
class B: public A { int j; };
class C: public B { int k; }
Now, the layout of a C might look like this:
vtable ptr (4 bytes) A part: int i; (4 bytes) B part: int j; (4 bytes) C part: int k;(4 bytes)
Ok, there are certainly implementations that will use that layout. Now, if the B object is created at the address 10, say, then the object will run from address 10 though to 24.
?? You don't create a B object on the line below, but a C object.
C* c = new C; A* a = C;
The value of c above will be 10.
Ok.
The value of a above will also be 10. Fine.
Ok. B* b = c; Now the value of b will be different.
Why? It will still be 10 on normal implementations (I don't know of an
implementation where it will change, and there may well not be one,
although the standard allows it). The B object doesn't care whether
it's a subobject of a C object, a D object or whether it isn't a sub
object at all, but the whole object.
So in single inheritence casting changes the value of the pointer if we are not casting to top most base class or bottom most derived class.
Why do you think that the value will change in the above case? It
wouldn't make sense for it to change. Try this on your favourite
compilers:
class A
{
public:
int i;
virtual ~A(){}
};
class B: public A
{
public:
int j;
};
class C: public B
{
public:
int k;
};
#include <iostream>
int main()
{
C* c = new C;
B* b = c;
A* a = c;
std::cout << (void*)a << '\n';
std::cout << (void*)b << '\n';
std::cout << (void*)c << '\n';
}
Tom This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: Jacob Jensen |
last post by:
This question has probably been asked a million time, but here it comes
again. I want to learn the difference between the three type cast operators:
static_cast, reinterpret_cast, dynamic_cast. A...
|
by: Brian Blais |
last post by:
Hello,
I saw on a couple of recent posts people saying that casting the return
value of malloc is bad, like:
d=(double *) malloc(50*sizeof(double));
why is this bad? I had always thought...
|
by: Visual Systems AB \(Martin Arvidsson\) |
last post by:
Hi!
I have created an enum list like this:
enum myEnum : int
{
This = 2,
That,
NewVal = 10,
LastItm
|
by: Ken Allen |
last post by:
I am relatively new to .Net, but have been using VB and C/C++ for years.
One of the drawbacks with VB6 and earlier was the difficulty in casting a
'record' to a different 'shape' so one could...
|
by: René Nordby |
last post by:
Hi there,
Is there anyone that knows how to do the following?
I have a class A and a class B, that 100% inherits from class A (this means
that I don't have other code in class B, than...
| |
by: S. Lorétan |
last post by:
Hi guys,
Sorry for this stupid question, but I don't know why it isn't working.
Here is my (example) code:
namespace Test {
class A {
public string Label1;
}
|
by: Neha |
last post by:
Hi all
Its seems bit silly que but pleasee explan me why this error is coming
? and what will be the solution if i want void * to be intilaize by
struct * and this code is puerly in C.
--...
|
by: brekehan |
last post by:
I've always been a little sketchy on the differences between static,
dynamic, and reinterpret casting. I am looking to clean up the
following block by using C++ casting instead of the C style...
|
by: Tinkertim |
last post by:
Hi,
I have often wondered if casting the return value of malloc() (or
friends) actually helps anything, recent threads here suggest that it
does not .. so I hope to find out.
For instance :
...
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However,...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can...
| |
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers,...
|
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The...
|
by: adsilva |
last post by:
A Windows Forms form does not have the event Unload, like VB6. What one acts like?
| |
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated ...
| |