Thread: I upgraded to g++ 4.2.2 and...

now some code doesn't work. I upgraded from g++ 4.0.3.

Here's the offending line of code

Code:

some_container::iterator it = 0;

I'm assuming that it doesn't conform to the standard and that's why it's now giving me an error message.

So how do you do for iterators what you do for pointers? Like

Code:

some_type* T = 0;

What is the exact error message?

[And your code-tag at the end is missing the / to mark an end-tag]

--
Mats

There's no such thing as a NULL iterator, unlike for pointers. For a given container c, you can use the c.end() iterator for the same purpose.

Edit: From Stroustrup, TC++PL, page 550:

An iterator is not a general pointer. Rather, it is an abstraction of the notion of a pointer into an array. There is no concept of a "null iterator." The test to determine whether an iterator points to an element or not is conventionally done by comparing it against the end of its sequence (rather than comparing it against a null element). This notion simplifies many algorithms by removing the need for a special end case and generalizes nicely to sequences of arbitrary types.

But what if my procedure was designed like this:

Code:

some_container c1, c2, c3;
some_container::iterator it;

a_loop {
  if either c1 or c2 or c3 fits some criteria {
    "it" points to an element in either c1 or c2 or c3
  }
  else if none of these fit the criteria {
    leave "it" alone
  }
}

if "it" hasn't been touched {
  do something else
}
if "it" has been touched {
   do something to "it"
}

I don't even know what container instance "it" has been linked to. Much less compare ".end()" with "it".

Of course, I could keep a separate boolean variable to track the change, but I'd like to know if this can be done without creating it.

A workaround: use a boolean flag.

Originally Posted by brewbuck

Save a copy of "it" before you start. Then compare to see if it changed at the end.

The only problem is that it needs to be initialized to some special value so it can be determined whether it changed, and with iterators any valid value must be associated with a specific container. One workaround would be to define a dummy container c of zero size, and set it to c.end().

Edit: Ignore what I just said. Comparing iterators to different containers is undefined behavior, so it won't work (not reliably, anyway). You need to know in advance which container (if any) it points to before you do anything with it. Use either the boolean flag or the index method mentioned above to implement this.

with iterators any valid value must be associated with a specific container

Not entirely true, e.g., the stream iterators have a "end of stream" default value not associated with a specific container, unless you consider the stream a container.