downcasting in c++ I couldnt success

[eMRe]

class A {
..
};
class B : public A
{
..
};

int main( )
{
stack<Amystack;
A a = new B( );
mystack.push(a);

A newA = mystack.top( );
B b = (B) newA; //doesnt work here
return 0;
}

how could I do downcast???

[Keith Thompson]

eMRe <em********@gmail.comwrites:

class A {

[snip]

You're looking for comp.lang.c++.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Antoninus Twink]

On 4 Oct 2008 at 21:10, eMRe wrote:

class B : public A

[snip]

A newA = mystack.top( );
B b = (B) newA; //doesnt work here

how could I do downcast???

Since B is derived from A, in general it doesn't make sense to cast from
A to B.

If you happen to know that the object you've popped is actually a B,
then you can convert (a pointer to the object) with dynamic_cast. (If
the cast isn't valid, then dynamic_cast will return NULL.)

[Ian Collins]

eMRe wrote:

Please don't multi-post on Usenet.

>
how could I do downcast???

What was wrong with the answer you got in the appropriate group?

--
Ian Collins.

[Ian Collins]

Antoninus Twink wrote:

On 4 Oct 2008 at 21:10, eMRe wrote:

>class B : public A

[snip]

> A newA = mystack.top( );
B b = (B) newA; //doesnt work here

how could I do downcast???

Since B is derived from A, in general it doesn't make sense to cast from
A to B.

If you happen to know that the object you've popped is actually a B,
then you can convert (a pointer to the object) with dynamic_cast. (If
the cast isn't valid, then dynamic_cast will return NULL.)

Both off topic and plain wrong.

You know the post is OT here and you're being an even bigger wanker than
usual by posing a misleading response.

--
Ian Collins.

[Flash Gordon]

Antoninus Twink wrote, On 04/10/08 22:37:

On 4 Oct 2008 at 21:10, eMRe wrote:

>class B : public A

[snip]

> A newA = mystack.top( );
B b = (B) newA; //doesnt work here

how could I do downcast???

Since B is derived from A, in general it doesn't make sense to cast from

<snip>

The OP needs to be aware that whilst it is possible that what Antoninus
Twink says is correct it is also entirely possible that it is compete
rubbish but that the people here (who are C experts and not necessarily
C++ experts) won't be able to correct it. However, if the question is
asked in the correct group, i.e. comp.lang.c++ there will definitely be
people who can correct any miss-information unlike here.

Antoninus Twink is doing you (the OP) a dis-service by failing to tell
you where the experts in C++ are and doing this group a dis-service by
trying to encourage you and others to ask about C++ in this group which
is about C.
--
Flash Gordon
If spamming me sent it to sm**@spam.causeway.com
If emailing me use my reply-to address
See the comp.lang.c Wiki hosted by me at http://clc-wiki.net/

[Antoninus Twink]

On 4 Oct 2008 at 22:11, Ian Collins wrote:

Antoninus Twink wrote:

>On 4 Oct 2008 at 21:10, eMRe wrote:

>>class B : public A

[snip]

>> A newA = mystack.top( );
B b = (B) newA; //doesnt work here

how could I do downcast???

Since B is derived from A, in general it doesn't make sense to cast from
A to B.

If you happen to know that the object you've popped is actually a B,
then you can convert (a pointer to the object) with dynamic_cast. (If
the cast isn't valid, then dynamic_cast will return NULL.)

Both off topic and plain wrong.

Whether it's off topic is a matter of opinion.

If it's wrong, why not point out the error instead of hurling mud?

I freely admit that I'm not a C++ expert, but I know enough to get by,
and given that the people here who *are* C++ experts will refuse to help
because they'd rather play topicality games, I'll give it my best shot.

I think the following is true:
1) What I've said above is correct (assuming A is polymorphic)
2) Using a dynamic_cast instead of a static_cast is safer in the
situation above, where error-checking that what comes off mystack really
is of type B could be valuable.

[Joachim Schmitz]

Antoninus Twink wrote:

On 4 Oct 2008 at 22:11, Ian Collins wrote:

>Antoninus Twink wrote:

>>On 4 Oct 2008 at 21:10, eMRe wrote:
class B : public A
[snip]
A newA = mystack.top( );
B b = (B) newA; //doesnt work here

how could I do downcast???

Since B is derived from A, in general it doesn't make sense to cast
from A to B.

If you happen to know that the object you've popped is actually a B,
then you can convert (a pointer to the object) with dynamic_cast.
(If the cast isn't valid, then dynamic_cast will return NULL.)

Both off topic and plain wrong.

Whether it's off topic is a matter of opinion.

No, it is not. C is not C++ and C++ is not topical at all in comp.lang.c
This is not a matter of opinions, just a fact.

Bye, Jojo

[Malcolm McLean]

"Antoninus Twink" <no****@nospam.invalidwrote in message

Ian Collins

>>
Both off topic and plain wrong.

Whether it's off topic is a matter of opinion.

If it's wrong, why not point out the error instead of hurling mud?

Some regulars have a rather narrow view of topicality.

That doesn't mean that there's no such thing. Libertarians have a very
narrow view of what governemnts should do. However unless you are some sort
of fanatical Marxist, you will agree that there are some things that
governments shouldn't do.

The C++ casting rules are off-topic here and belong in comp.lang.c++. Claims
that replies are wrong are also off-topic, though by replying with off-topic
information you've created an impossible situation, because no-one wants to
see incorrect information go unchallenged.

In my understanding of C++ you are in fact right, but it's a long time since
I used C++. Intricate casting rules were one of the things that put me off.

Incidentally certain words are also inappropriate for the newsgroup. Some
hobby programmers are children, and we should be careful what we expose
young people to.

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Default User]

Joachim Schmitz wrote:

Antoninus Twink wrote:

Whether it's off topic is a matter of opinion.

No, it is not. C is not C++ and C++ is not topical at all in
comp.lang.c This is not a matter of opinions, just a fact.

Fortunately, the OP proved to be smarter than Twink (admittedly not
much of a benchmark) and reposted the question in proper newsgroup.

Brian

[Ian Collins]

Antoninus Twink wrote:

On 4 Oct 2008 at 22:11, Ian Collins wrote:

>Antoninus Twink wrote:

[wrong C++ advice]

>Both off topic and plain wrong.

Whether it's off topic is a matter of opinion.

Can you read? Where's the "C++" in this groups name?

If it's wrong, why not point out the error instead of hurling mud?

If you were to follow the example of the OP an ask in an appropriate
place, I might.

--
Ian Collins.

[Antoninus Twink]

On 5 Oct 2008 at 17:09, Malcolm McLean wrote:

Some regulars have a rather narrow view of topicality.

Is that the famous British understatement?

Incidentally certain words are also inappropriate for the newsgroup.
Some hobby programmers are children, and we should be careful what we
expose young people to.

I'm not sure exactly what I said that was offensive... I rarely use
strong language, though it's possible that CBF's bloody-minded ignorance
has pushed me to it once or twice.

Although it's hard to believe they'll hear anything in clc remotely
comparable to the unmitigated filth that flies around most
schoolyards...

[Eric Sosman]

Ian Collins wrote:

Antoninus Twink wrote:

>>
If it's wrong, why not point out the error instead of hurling mud?

If you were to follow the example of the OP an ask in an appropriate
place, I might.

"Never mud-wrestle with a pig. You both get dirty,
and the pig enjoys it."

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Antoninus Twink]

On 5 Oct 2008 at 21:07, Eric Sosman wrote:

Ian Collins wrote:

>Antoninus Twink wrote:

>>If it's wrong, why not point out the error instead of hurling mud?

If you were to follow the example of the OP an ask in an appropriate
place, I might.

"Never mud-wrestle with a pig. You both get dirty,
and the pig enjoys it."

Don't worry, I have no intention of playing Collins' games.

[Malcolm McLean]

"Antoninus Twink" <no****@nospam.invalidwrote in message

On 5 Oct 2008 at 17:09, Malcolm McLean wrote:

>Some regulars have a rather narrow view of topicality.

Is that the famous British understatement?

>Incidentally certain words are also inappropriate for the newsgroup.
Some hobby programmers are children, and we should be careful what we
expose young people to.

I'm not sure exactly what I said that was offensive... I rarely use
strong language, though it's possible that CBF's bloody-minded ignorance
has pushed me to it once or twice.

Although it's hard to believe they'll hear anything in clc remotely
comparable to the unmitigated filth that flies around most
schoolyards...

You quoted a rude word instead of asterisking it out.

Whilst children certainly know these words, sometimes know what they mean,
and use them amongst themselves, they shouldn't be exposed to adults using
them in their presence, nor should adults accept swear words from children.
It's a sort of inter-generational compact. On clc, we should remember that
everyone can access the ng.

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Malcolm McLean]

"Antoninus Twink" <no****@nospam.invalidwrote in message

On 5 Oct 2008 at 17:09, Malcolm McLean wrote:
I'm not sure exactly what I said that was offensive...

Actually I checked back in the thread and you snipped. I got mixed up as to
who quoted whom.

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Kenny McCormack]

In article <sl*******************@nospam.invalid>,
Antoninus Twink <no****@nospam.invalidwrote:

>On 5 Oct 2008 at 17:09, Malcolm McLean wrote:

>Some regulars have a rather narrow view of topicality.

Is that the famous British understatement?

>Incidentally certain words are also inappropriate for the newsgroup.
Some hobby programmers are children, and we should be careful what we
expose young people to.

I'm not sure exactly what I said that was offensive... I rarely use
strong language, though it's possible that CBF's bloody-minded ignorance
has pushed me to it once or twice.

Although it's hard to believe they'll hear anything in clc remotely
comparable to the unmitigated filth that flies around most
schoolyards...

Well, I know that we are not supposed to use "the S word" in this
newsgroup. I assume that we all know what the S word is; I don't think
I have to explain further.

I assume that's what Malcolm was referring to.

[CBFalconer]

Malcolm McLean wrote:

>

.... snip ...

>
Incidentally certain words are also inappropriate for the
newsgroup. Some hobby programmers are children, and we should
be careful what we expose young people to.

Examples include "Antoninus", "Twink", "Kenny McCormack".

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.

[lawrence.jones]

Kenny McCormack <ga*****@shell.xmission.comwrote:

>
Well, I know that we are not supposed to use "the S word" in this
newsgroup. I assume that we all know what the S word is; I don't think
I have to explain further.

SN*BOL? S*mula? Sm*lltalk? Sch*me? ;-)
--
Larry Jones

My dreams are getting way too literal. -- Calvin

[Richard Bos]

la************@siemens.com wrote:

Kenny McCormack <ga*****@shell.xmission.comwrote:

Well, I know that we are not supposed to use "the S word" in this
newsgroup. I assume that we all know what the S word is; I don't think
I have to explain further.

SN*BOL? S*mula? Sm*lltalk? Sch*me? ;-)

St*nd*rd :-(

Richard