Strange Iterator behavior

Question

I have encountered strange behavior of iterators on empty <vector> . In my code, even though std::cend(children) - 1 - std::cbegin(children) is clearly negative ( 0 -1 -0 ), their comparison std::cbegin(children) < (std::cend(children) - 1) result is true ( 0 < (0-1) ). Is this compiler bug or language specification or undefined behavior ?

#include <iostream>
#include <vector>

int main() {
    std::vector<int> children{};

    auto pointer_to_child_first = std::cbegin(children);
    auto pointer_to_child_last = std::cend(children) - 1;
    auto pointer_to_child_current = pointer_to_child_first;

    for (; pointer_to_child_current < pointer_to_child_last; pointer_to_child_current += 1) {
        std::cout << ">if less code\n";
    }

    if (pointer_to_child_current == pointer_to_child_last) {
        std::cout << ">if equals code\n";
    }
}

Output:

>if less code
>if less code
>if less code
>if less code
>if less code
^C

If I change std::cbegin and std::cend to std::crbegin and std::crend or to std::crend and std::crbegin , than I got all as intended.

Compiler version:

>clang-cl.exe -v
clang version 8.0.0 (tags/RELEASE_800/final)
Target: x86_64-pc-windows-msvc

Answer 1

What happens when you're looking at a leaf node (a node with no children)? Specifically, what will this line do:

auto&& pointer_to_child_last = std::cend(children) - 1;

Since std::cbegin(children) == std::cend(children) , this is illegal!

To remedy this, if the node is a leaf, just return your accumulator string after you've added the node number:

if(children.size() == 0) { return accumulator.str(); }

See it in action: http://ideone.com/1oh9yr