How to store Iterators in a Map in C++

Question

Consider I have a list of integers. I am trying to store the iterator for each integer in the list on a map. This way to quickly find an element, I can use the iterator to get to the element.

But why can't increment the iterator and store it in my map?

list<int> myList;
unordered_map<int, list<int>::iterator> myMap;
int count = 0;

myList.push_back(1);
//myMap[1] = myList.begin() + count;    <--- Why can't I do this

// Alternatively I can do something like this
auto tmpItr1 = myList.begin();
advance(tmpItr1, count);
myMap[1] = tmpItr1;
count++;

myList.push_back(2);
//myMap[2] = myList.begin() + count;    <--- Why can't I do this
auto tmpItr2 = myList.begin();
advance(tmpItr2, count);
myMap[2] = tmpItr2;
count++;

myList.push_back(3);
//myMap[3] = myList.begin() + count;    <--- Why can't I do this
auto tmpItr3 = myList.begin();
advance(tmpItr3, count);
myMap[3] = tmpItr3;
count++;

// Delte 2 from the list
auto mapItr = myMap.find(2);
if (mapItr != myMap.end())
{
    cout << "Delte: " << *(mapItr->second) << endl;
    myList.erase(mapItr->second);
}

Is using std::advance the right approach for this?

@gsamaras I should have asked why there is not support for operator+ if the iterator can be incremented — SyncMaster, Dec 08 '16 at 19:53
Well now you can go to the question I linked you to and find out! — Lightness Races in Orbit, Dec 08 '16 at 19:54
Thanks for the link to the other post. That answers my question. — SyncMaster, Dec 08 '16 at 19:55
just do `auto it = myList.insert( myList.end(), value );` and you would not need that silly way at all. — Slava, Dec 08 '16 at 19:59
@Slava: Er, that only works if you want to insert at the end. — Lightness Races in Orbit, Dec 08 '16 at 20:01
@LightnessRacesinOrbit `push_back()` can do anything else? Btw for `std::list` it would work anywhere. — Slava, Dec 08 '16 at 20:02

How to store Iterators in a Map in C++

0 Answers0