Decrement of end of std::map

Question

Here is my code:

#include <iostream> #include <map> using namespace std;  int main() {     map<int , int > myMap;      map<int , int>::iterator it;      myMap.insert(pair<int , int>(1,2));     myMap.insert(pair<int , int>(671,223));     myMap.insert(pair<int , int>(353,245352));      it = myMap.end() - 1;      cout << it->first << it->second << endl;      return 0; } 

Compiling this code produces the following compilation error:

error: no match for ‘operator-’ (operand types are ‘std::map<int, int>::iterator {aka std::_Rb_tree_iterator<std::pair<const int, int> >}’ and ‘int’)   it = myMap.end() - 1; 

I don't know why I am getting this error as I think arithmetic operations are allowed in all types of iterators.

Answer 1

Not all iterator categories support arithmetic operations, that's a misconception. If you aim to write more generic code, you can use std::prev:

it = std::prev(myMap.end()); 

It expects a bidirectional iterator, which std::map's iterator is. If you want to move the iterator more than a single step, it can also accept a second parameter that designated how far to move the iterator.

In addition, when you pass it a random access iterator, it will be as fast as the arithmetic.