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casting pointer to static

Posted on 2001-06-11
13
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Last Modified: 2011-09-20
i m not sure if i should post this question here or on the MFC board, but lets try :-)
i have the following:
Class A
{
....
};
class B : public A
{
...
};

void main (voif)
{
  A* a = new A;
  B  b = static_cast<B>a;
}

i was wondering if i can do that ? ofcourse it doesn't work, but is there's a way to do that ?

Shay
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Comment
Question by:Shay050799
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13 Comments
 
LVL 31

Expert Comment

by:Zoppo
ID: 6177755
What should this code do? Cast a pointer to an instance of a base class to an instance of a
derived class ... what should 'b' be after that cast?
0
 
LVL 42

Expert Comment

by:sedgwick
ID: 6177761
u r trying to cast a pointer to class A to an instance of class b, of course it won't work, check this code below:

      A* pa = new A;

      B* pb = static_cast<B*>(pa);

now its gonna work...

good luck
0
 
LVL 3

Author Comment

by:Shay050799
ID: 6177774
that works but pb is a pointer
i need to refer to it using -> operator
i want a static instance of B

Shay
0
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LVL 30

Expert Comment

by:Axter
ID: 6177788
sedgwick,
I think he's trying to make b equal to a.
0
 
LVL 30

Expert Comment

by:Axter
ID: 6177799
A* a = new A;
 B  b = static_cast<B>*a;
0
 
LVL 30

Expert Comment

by:Axter
ID: 6177806
If you want b equal to the value of a, then you need to use the asteric.
*a
0
 
LVL 42

Expert Comment

by:sedgwick
ID: 6177955
shay:
>>i want a static instance of B
so why dont u declare a static instance of B as follow:

static B b;

what is the purpose of the casting from A* ???
0
 
LVL 31

Expert Comment

by:Zoppo
ID: 6177983
you could write a B::operator = (A& a) and then call

static B b;
b = *a;

or write a ctor B::(A& a) and then call

static B b(a);
//or
static B b = a;

ZOPPO
0
 
LVL 7

Accepted Solution

by:
KangaRoo earned 50 total points
ID: 6178728
You can not cast a pointer to A to a pointer to B, using static_cast.
static_cast verifies at compile time wheter a cast is legal or not. Casting a pointer to a base class object into a pointer to a derived class object is /never/ legal. And even if it were, it would be very, very unsafe:
class A {
   int i;
   // whatever
};

class B : public A
{
  int j;
  public: void set(int jj) { j = jj; }  
};

A* a = new A;
B* b = magic_cast<B*>(a);
b->set(42); // Now you've done it, deamons flying all over the place.

But of course, C++ wouldn't be C ++ if there wouldn't be ways to cast some magic and get them daemons out flying of yer nose.

First, there is
   reinterpret_cast<>
which more or less allows you to cast anything into anything else (well, more or less, it won't let you cast functionpointer into memberfunctionpointers and the likes).

Then there is an old C union trick, something like (I might have the syntax wrong, I never use unions):
union{
   A* a;
   B* b;
};

The following code demonstates them:

struct A {};
struct B : A {int i;};

void f()
{
   A* a = new A;
   B* b = reinterpret_cast<B*>(a);

   union{
      A* a;
      B* b;
   } gotcha;

   gotcha.a = a;
   gotcha.b->i = 42; // Welcome deamons! And we didn't even use magic :)

}
0
 
LVL 7

Expert Comment

by:KangaRoo
ID: 6179041
Like I said, this is 'dangerous' to do and will easily lead to 'undefined' behaviour of your program, or crashes (undefined behaviour is what the Standard calls it).

What do you need it for?
0
 
LVL 1

Expert Comment

by:Moondancer
ID: 6799465
Shay----->  You have asked 95 questions at this site, and only awarded 53 of them....  this is totally against the Guidelines of this site.  Please return to them and update/finalize them this week.  The list is below.  Of these 53 awarded, there are 7 questions that are -1 user id problems which we will need to manually correct.  Please accept the first correct response in each of them and award the experts who helped you with the deserved "A" grade, or comments if less than an A as to why.  If you need our help to special handle any of these, let us know that as well via comment here.  If these are not finalized this week, I will contact Administration for further action.  Please click the HELP DESK link on the left for the rules of engagement here (Guidelines, Member Agreement and Question/Answer process).

I also noticed that a number of your questions are less than 50 points, which is the recommended value for EASY questions, with 300 points the maximum.  To draw additional attention, you can always increase their values without our assistance.

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http://www.experts-exchange.com/jsp/qShow.jsp?ta=wince&qid=12019739 November, 2000, zero comments added, 5 points.
http://www.experts-exchange.com/jsp/qShow.jsp?ta=winprog&qid=11169491 November, 2000 ZERO points, comments added
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http://www.experts-exchange.com/jsp/qShow.jsp?ta=cplusprog&qid=11536619 October, 2000 - 15 points
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http://www.experts-exchange.com/jsp/qShow.jsp?ta=mfc&qid=20149200  July, 2001 - 5 points
http://www.experts-exchange.com/jsp/qShow.jsp?ta=mfc&qid=20148518    "      - 10 points
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http://www.experts-exchange.com/jsp/qShow.jsp?ta=mfc&qid=20121466  May, 2001 0 points
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I have asked Administration to Email you in these regards.  I also noticed several other things I'd like to bring to you attention.  There are some that you did award with "C" grades, and no comment as to why.  Also, some of your questions have had zero response, and that is likely for two reasons:  Poor closure on open items and poor grades.  I think you'll agree, epecially given the number of questions you've asked here, that experts here take great pride in their work, and deserve "A" grades and responses to all help they've given you.

EXPERTS -->  In the event any of these items remain open at the end of this week, I will finalize them and look to you for guidance on the fair results.  As always, we very much appreciate your willingness to help and the excellent support you all give.

Thanks,

Moondancer
Community Support Moderator @ Experts Exchange
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LVL 11

Expert Comment

by:griessh
ID: 6814535
Losts of true comments, but no clarification of the problem ... I suggest to PAQ & refund.

Please do not accept this comment as an answer!
======
Werner
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Expert Comment

by:Netminder
ID: 6822832
Admin notified of user neglect. Force/accepted by

Netminder
Community Support Moderator
Experts Exchange
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