C++: iterator - list.begin() throws an error

Question

I'm trying to get the index of an iterator of a list, I've read this stackoverflow question and tried the following code:

std::list<int> v;
std::list<int>::iterator iter = v.insert(v.begin(), 1);
int i = iter - v.begin();

Surprisingly it doesn't work, I got an error.

error: invalid operands to binary expression

What's the problem? How to make it work?

Answer 1

v.insert returns a list iterator, that list iterator only satisfies BiDirectionalIterator. That means that operator- isn't defined for it.

To get the distance you can use std::distance instead:

int diff = std::distance(v.begin(), iter);

Answer 2

List container iterators are not random access iterators and therefore don't provide substration. You can use std::distance to obtain index.