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?

Have you read the answer of the question from link mention by you? Accepted answer says it all. — Swapnil, Sep 22 '18 at 13:46
You have the answer here: https://stackoverflow.com/questions/10564222/error-no-match-for-operator-for-list-iterator — Amit G., Sep 22 '18 at 13:54

score 3 · Answer 1 · answered Sep 22 '18 at 13:40

3

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

answered Sep 22 '18 at 13:40

user7860670

score 3 · Accepted Answer · answered Sep 22 '18 at 13:45

3

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);

answered Sep 22 '18 at 13:45

Hatted Rooster

2 Answers2