iterator error

[=?gb2312?B?wNbA1rTzzOzKpg==?=]

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

[Victor Bazarov]

ÀÖÀÖ´óÌìʦ wrote:

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

The iterator type in 'map' is implementation-defined. What
it means to initialise it with (int)0 is implementation-defined.
Why operator != (int)0 doesn't work is (you guessed it!)
implementation-defined. You need to either look at the code
in the debugger to see what's going on or ask in the newsgroup
dedicated to your implementation (microsoft.publ ic.vc.* family
of newsgroups come to mind).

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Ian Collins]

ÀÖÀÖ´óÌìʦ wrote:

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

That all depends on what the type of a std::map<int, int>::iterator is
and whether it cam be initialised by or compared to an int. If it lacks
a public constructor, the assignment is illegal and if it lacks an
operator==(int) , the comparison is illegal.

--
Ian Collins.

[Victor Bazarov]

Ian Collins wrote:

ÀÖÀÖ´óÌìʦ wrote:

>windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

That all depends on what the type of a std::map<int, int>::iterator is
and whether it cam be initialised by or compared to an int. If it
lacks a public constructor, the assignment is illegal and if it lacks
an operator==(int) , the comparison is illegal.

It's actually operator!=(int) we're discussing here... Unless you think
they are related somehow...

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Ian Collins]

Victor Bazarov wrote:

Ian Collins wrote:

>>
That all depends on what the type of a std::map<int, int>::iterator is
and whether it cam be initialised by or compared to an int. If it
lacks a public constructor, the assignment is illegal and if it lacks
an operator==(int) , the comparison is illegal.

It's actually operator!=(int) we're discussing here... Unless you think
they are related somehow...

Typo.

--
Ian Collins.

[Jim Langston]

"ÀÖÀÖ´óÌìʦ " <ch*********@gm ail.comwrote in message
news:11******** *************@b 75g2000hsg.goog legroups.com...

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

Read the other's comments. You are trying to compare an iterator and an
integer. On some (most?) implementations it = 0 would probably be invalid
also. The correct usage is to initialize them with .begin() or .rbegin() or
some iterator value. For example:

int main()
{
std::map<int, intMyMap;
std::map<int, int>::iterator it = MyMap.begin();
// code
}

persontally, I normally use iterators mostly in for loops to iterate through
a container (vector or map) or to get a pointer to a newly inserted member.
Such as:

typedef std::map<int, intIntMap;
IntMap MyMap;
// fill MyMap
for ( IntMap::iterato r it = MyMap.begin(); it != MyMap.end(); ++it )
std::cout << it->first << "-" << it->second << "\n";

[red floyd]

ÀÖÀÖ´óÌìʦ wrote:

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

Repeat after me. Iterators are NOT POINTERS. Again. Iterators are NOT
POINTERS.

There is no such thing as a "0" or NULL iterator. Any valid iterator
must refer to a container.

[James Kanze]

On Apr 6, 3:02 am, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

ÀÖÀÖ´óÌìʦ wrote:

windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

The iterator type in 'map' is implementation-defined. What
it means to initialise it with (int)0 is implementation-defined.
Why operator != (int)0 doesn't work is (you guessed it!)
implementation-defined.

It's not even implementation-defined, it's undefined.

Most good implementations will not compile either of the above
lines.

You need to either look at the code
in the debugger to see what's going on or ask in the newsgroup
dedicated to your implementation (microsoft.publ ic.vc.* family
of newsgroups come to mind).

Actually, he needs to change his code. It might help if he
explained what he is trying to accomplish.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[bnonaj]

James Kanze wrote:

On Apr 6, 3:02 am, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>ÀÖÀÖ´óÌìʦ wrote:

>>windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;
int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!" ;
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

>The iterator type in 'map' is implementation-defined. What
it means to initialise it with (int)0 is implementation-defined.
Why operator != (int)0 doesn't work is (you guessed it!)
implementati on-defined.

It's not even implementation-defined, it's undefined.

Strictly speaking without an implementation it wouldn't even exist and
therefore would not compile

>
Most good implementations will not compile either of the above
lines.

>You need to either look at the code
in the debugger to see what's going on or ask in the newsgroup
dedicated to your implementation (microsoft.publ ic.vc.* family
of newsgroups come to mind).

Actually, he needs to change his code. It might help if he
explained what he is trying to accomplish.

--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

I agree the code is somewhat dodgy and probably not portable

JB

[bnonaj]

red floyd wrote:

ÀÖÀÖ´óÌìʦ wrote:

>windows xp, visual studio 2005
----------------------------------------------------------------------------------------------------------------------------------
#include <iostream>
#include <map>
using namespace std;

int main()
{
map<int, int>::iterator it = 0;
if( it != 0 ) //break point,
this is an run time error?
cout<<"ok!";
}
----------------------------------------------------------------------------------------------------------------------------------
why it can be assign "0", but can't compare with "0"?

Repeat after me. Iterators are NOT POINTERS. Again. Iterators are NOT
POINTERS.

There is no such thing as a "0" or NULL iterator. Any valid iterator
must refer to a container.

So what does the default constructor for an iterator refer to? Just
curious as a valid container may not yet exist

JB