Why "operator<" for std::vector<int>::iterator seems to compare absolute values when out of range (before .begin())?

Question

cppreference on std::vector doesn't seem to have any links for std::vector<T>::iterator type. Since it is a bidirectional iterator on a contigious memory, I supposed that it can be "negative".
std::distance(vec.begin(), std::prev(vec.begin()) yields -1 , and I ended up using std::distance to compare iterators for a loop.
But I was expecting while(iterLeft < iterRight) to be a valid check, and it is shorter than while(std::distance(iterLeft, iterRight) > 0) .

This code, however, demonstrates that operator< for some reason seems to compare absolute values:
(This code surved to recreate the problem. The actual code had a variable-sized vector as an input)

#include <vector>
#include <iostream>

int main()
{
    std::vector<int> vec{};
    auto iterLeft = vec.begin(),
        iterRight = std::prev(vec.end());

    std::cout << "Left = " << std::distance(vec.begin(), iterLeft) 
        << ", Right = " << std::distance(vec.begin(), iterRight) 
        << ", Left < Right: " << std::boolalpha << bool(iterLeft < iterRight)
        << std::endl;

    return 0;
}

Program returned: 0
Left = 0, Right = -1, Left < Right: true

https://godbolt.org/z/PErY35Ynj

---

So basically the UB is caused Not by std::prev(vec.end()) , but by an iterator goind out of range (before .begin() ).

---

The code example is just to recreate the problem. Initially the code is to traverse an array with 2 iterators and the intent was to make the code declarative (meaning to handle empty arrays without an explicit check).

for (auto leftIter = vec.begin(), rightIter = std::prev(vec.end()); leftIter < rightIter;)
{
 // do stuff
 ++leftIter;
 --rightIter;
}

The code worked fine for most cases, but empty arrays. Hence the question.

Answer 1

std::distance can yield negative values if applied to valid iterators to the same range and operator< can be used to compare the ordering of valid iterators into the same range. And it does this comparison in the expected way. iterLeft < iterRight will be equivalent to std::distance(iterLeft, iterRight) > 0 .

However the only valid iterators are those in the range from begin() to end() . Trying to obtain an iterator "before" begin() or "after" end() causes undefined behavior.

So std::distance(vec.begin(), std::prev(vec.begin()) does not result in -1 . It has undefined behavior.

Similarly in your code block std::prev(vec.end()) has undefined behavior because the vector is empty and so vec.begin() == vec.end() .