C++ embarassing question on passing iterators to functions

Question

So, given this function definition:

void xx(list<int> my_list, list<int>::iterator start){
   
    list<int>::iterator _start = start;
    distance(my_list.begin(),_start);
}

Why does the following cause distance() inside the function to hang?

list<int> L;
L.push_back(0);
L.push_back(1);

xx(L, L.begin());

I thought it would just be zero. Yes, I know I suck.

question from:https://stackoverflow.com/questions/66057952/c-embarassing-question-on-passing-iterators-to-functions

Answer 1

my_list is declared as pass-by-value, it's a copy from the argument. That means, given distance(my_list.begin(),_start);, my_list.begin() and _start point to two different std::lists, the behavior is undefined.

If you change it to pass-by-reference, then both the iterators point to the same std::list and the code would be fine. e.g.

void xx(list<int>& my_list, list<int>::iterator start){
   
    list<int>::iterator _start = start;
    distance(my_list.begin(),_start);
}