std::map::iterator decrement on single element [duplicate]

Question

What should std::map iterator decrement return, if there's only single element in the map? Here's the sample code

#include <map>
#include <stdio.h>
int main()
{
    std::map<int, int> m;
    m.insert(std::make_pair(1, 1));
    //std::map<int, int>::iterator it = m.begin();
    std::map<int, int>::iterator it = m.upper_bound(0);
    printf("isbegin: %d\n", it == m.begin());
    --it;
    bool isend = it == m.end();
    printf("isend: %d\n", isend);
}

On Windows it will print isend: 1, on Linux with g++ 4.6 it will print isend: 0.

The question: is the decrement above really a case of UB? and if not then what result is correct - Windows or Linux one?

UPDATE: modified code to show that upper_bound is called

Answer 1

Decrementing something to the element before begin() doesn't make sense. This is undefined behavior and there is no correct or incorrect answer.

Answer 2

for an iterator r the operation --r is valid if before the operation is done there exists s such that r == ++s and after the operation is done r is dereferenceable. (§24.2.6 Bidirectional.iterators )

Since begin() returns an iterator to the first element of the container there is no element s which can be incremented to get to r so this is undefined.