Re: iterator error

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 06 Apr 2007 08:40:26 -0400

Message-ID:

<IMidnTF2K6MmoovbnZ2dnUVZ_uzinZ2d@giganews.com>

bnonaj wrote:

James Kanze wrote:

On Apr 6, 3:02 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

?????????? 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.

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

Well, perhaps. But the issue underlying this subthread is the
application of the technical terms "undefined" and "implementation
defined" as defined by the C++ standard. "Undefined behavior" means that
the standard does not impose any requirements. "Implementation defined"
means that the standard requires the implementation to document its
behavior.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)