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I'm trying to make use of the fact that iterators to lists remain valid after insertions and removals (except iterators to what you just removed). Is this also true of std::list<T>::end();

Suppose I try the following:

typedef std::list<int> list_int;

list_int myList;
list_int::iterator iter = myList.end();

myList.push_back(1);
myList.push_back(2);
myList.push_back(3);

if(iter == myList.end()) {
    /* do things here */
} else {
    /* do different things here */
    /* I don't expect this branch to ever execute */
}

This is important because elsewhere I might store a collection of iterators into this list, and I would test for validity by comparing against myList.end(). It's important that invalid iterators remain so even after insertions and removals.

Filipp
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  • I had a [similar question](http://stackoverflow.com/q/6230350/596781) once... – Kerrek SB Oct 26 '12 at 02:40
  • Similar, yes. Your question does point that `myList.end()` has a unique value regardless of insertions and removals. My question asks whether this is standard behavior that I can depend on across systems and compilers. – Filipp Oct 26 '12 at 02:45
  • *"iterators to lists remain valid after insertions and removals"* - and `std::list::end` returns an iterator to the list. So what? – Christian Rau Oct 26 '12 at 07:06
  • Here's a [related question](http://stackoverflow.com/questions/14647007/does-stdlistclear-invalidate-stdlistend-iterator). Its answers and comments cite relevant parts of the standard. – Craig M. Brandenburg Nov 22 '13 at 14:49

2 Answers2

19

The value of std::lists end iterator never changes during the lifetime of the list. It is always valid, always the same and always corresponds to the imaginary "past the end" element of the list. This means that the value of some_list.end() memorized at any point in the list's lifetime will always be the same as the value of some_list.end() at any other point of its lifetime.

The language specification doesn't state it explicitly. However, there's simply no valid operation on the list that would invalidate the end iterator or associate its value with some other location.

In your example the second branch of that if will never execute.

If I'm not missing anything, the same is true for std::map and std::set as well.

AnT stands with Russia
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0

Have you tried a simple test by placing a cout in each clause of the IF statement? By doing so, you will realize that the first clause (iter == myList.end()) is indeed true.

I am going to base my reasoning simply on this documentation of the end() function: http://www.cplusplus.com/reference/stl/list/end/. It says: "Returns an iterator referring to the past-the-end element in the list container." Hence, if the iterator pointing to past-the-end is returned, then shouldn't it always be pointing past-the-end? Elements inserted before this past-the-end point shouldn't affect where the iterator is pointing to (which is a spot after the last element of the list).

ecbrodie
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