I have been using advance
on some iterators
, but I am afraid of a possible leapfrog above end()
. I would like to make sure my iterators stay between the bounds, I thought of the distance
but it seems it does not return what I would be expecting (non-positive values when iterators overpass end()
). How would you make sure there is no leapfrog?
#include <iostream>
#include <iterator>
#include <list>
using namespace std;
int main () {
list<int> mylist;
for (int i=0; i<10; i++) mylist.push_back (i*10);
list<int>::const_iterator first = mylist.begin();
const list<int>::const_iterator last = mylist.end();
cout << "The distance is: " << distance(first,last) << endl; // 10
advance(first, 10);
cout << "The distance is: " << distance(first,last) << endl; // 0
advance(first, 1);
cout << "The distance is: " << distance(first,last) << endl; // 10
advance(first, 10);
cout << "The distance is: " << distance(first,last) << endl; // 0
return 0;
}
Here is the output:
The distance is: 10
The distance is: 0
The distance is: 10
The distance is: 0
advance() past end() results in undefined behaviour. You are going to have to test as you go per this snippet:
template <class Iter, class Incr>
void safe_advance(Iter& curr, const Iter& end, Incr n)
{
size_t remaining(std::distance(curr, end));
if (remaining < n)
{
n = remaining;
}
std::advance(curr, n);
}
You need to think about what happens when you don't move the full amount (curr
returned as end()
.