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I was wondering, how is equality (==) established for STL iterators? Is it a simple pointer comparison (and thus based on addresses) or something more fancy?
If I have two iterators from two different list objects and I compare them, will the result always be false?
What about if I compare a valid value with one that's out of range? Is that always false?
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An iterator is used to point to the memory address of the STL container classes. For better understanding, you can relate them with a pointer, to some extent. Iterators act as a bridge that connects algorithms to STL containers and allows the modifications of the data present inside the container.
end(): The end() function returns an iterator pointing to the past-the-last element of the container.
The iterator is implemented as a pointer to a node, and contains operator overloads for the four usual iterator operations of dereference, increment, comparison, and assignment. in the list class that can be used to insert new data items at arbitrary locations in the list.
Most standard STL containers at least support forward iterators. Bidirectional Iterators: These are similar to forward iterators with the only difference that they are bidirectional. This means that we can use these bidirectional iterators to traverse through the range in the forward as well as backward direction.
Iterator classes can define overloaded == operators, if they want. So the result depends on the implementation of operator==.
You're not really supposed to compare iterators from different containers. I think some debug STL implementations will signal a warning if you do this, which will help you catch cases of this erroneous usage in your code.
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Daniel asked: I was wondering, how is equality (==) established for STL iterators? Is it a simple pointer comparison (and thus based on addresses) or something more fancy?
It depends on implementation. Right now, on Visual C++ 2008, I see the following code (for the list iterator):
bool operator==(const _Myt_iter& _Right) const
{ // test for iterator equality
#if _HAS_ITERATOR_DEBUGGING
_Compat(_Right);
#else
_SCL_SECURE_TRAITS_VALIDATE(this->_Has_container() && this->_Same_container(_Right));
#endif /* _HAS_ITERATOR_DEBUGGING */
return (_Ptr == _Right._Ptr);
}
You'll see above that there is both code for verification of iterator validity, and _Ptr being a pointer to a list node.
So I guess there is both verification, and simple, raw pointer comparison.
Daniel asked: If I have two iterators from two different list objects and I compare them, will the result always be false?
Until now, it appears the standard was somewhat unclear on the subject. Apparently, they will explicitly write that this kind of operation has undefined results:
Quoting: http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#446
The result of using any iterator operation (24.2.1 [input.iterators], 24.2.2 [output.iterators], 24.2.3 [forward.iterators], 24.2.4 [bidirectional.iterators], 24.2.5 [random.access.iterators]) that uses two iterator values as arguments (footnote) which were obtained from two different ranges r1 and r2 (including their past-the-end values) which are not subranges of one common range is undefined, unless explicitly described otherwise.
footnote) Among others these operations are ==, <, binary -, and copy assignment
So I guess it is evil to compare iterator from different containers... ^_^
Daniel asked: What about if I compare a valid value with one that's out of range? Is that always false?
Same as above.
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