Setting a iterator's object to NULL

This is a discussion on Setting a iterator's object to NULL within the C++ Programming forums, part of the General Programming Boards category; I created some test code to debug one of my problems, and I think I narrowed it down. One list ...

  1. #1
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    Setting a iterator's object to NULL

    I created some test code to debug one of my problems, and I think I narrowed it down. One list has shared pointers, the other has addresses of the object pointed to by the shared pointer. When the shared pointer is deleted, it doesn't set the object to NULL so the list full of addresses has some bogus value. Here's some code to explain:

    Code:
    	list<shared_ptr<Enemy>> elist;
    	list<shared_ptr<Enemy>>::iterator it;
    	Enemy* ep;
    
    	{
    		shared_ptr<Enemy> e(new Enemy);
    		elist.push_back(e);
    		ep=e.get();
    	}
    	
    	it=elist.begin();
    	//THIS DOES NOT WORK, but indicates what I want: (*it)=NULL;
    	elist.erase(elist.begin());
            //this fails
    	assert(ep);
    How can I assign a new value to what an iterator is pointing to, or otherwise fix this?

  2. #2
    Cat without Hat CornedBee's Avatar
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    it->reset()

    But erasing should be enough. The destructor takes care of deleting the object.

    Code:
            //this fails
    	assert(ep);
    No, it won't. ep is a dangling but non-null pointer.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  3. #3
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    I'll give reset a shot when I get home, but yes, that assertion does fail (with the code I have posted now).

  4. #4
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by CornedBee View Post
    No, it won't. ep is a dangling but non-null pointer.
    QFT!
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

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  5. #5
    The larch
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    "This fails" or this "fails to fail"?
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  6. #6
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    I guess I said that wrong. I WANT ep to be a NULL pointer, or somehow be able to tell when whatever ep is pointing to has been deleted.

  7. #7
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    Just a guess, but wouldn't making ep a weak_ptr do help with that?

  8. #8
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    Quote Originally Posted by Daved View Post
    Just a guess, but wouldn't making ep a weak_ptr do help with that?
    Thanks a ton. That was exactly what I needed.

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