Storing Iterators of std::deque

Question

I am trying to store iterators of a deque in a vector and want to preserve them inside the vector even when I have erased or inserted some elements from or into the deque. Is this possible?

I have the following code:

typedef struct {

int id;
int seedId;
double similarity;

} NODE_SEED_SIM;

typedef std::deque<NODE_SEED_SIM> NodesQueue;
typedef std::deque<NODE_SEED_SIM>::iterator ITRTR;
typedef std::vector<const ITRTR> PointerVec;  

void growSegments (CSG_PointCloud *pResult, IntMatrix *WdIndices, NodesQueue *NodesList, IntMatrix *Segments) {

    ITRTR nodeslistStart = (*NodesList).begin();
    int pointCount = (*WdIndices).size();
    int nodeslistSize = (*NodesList).size();

    IntVector book(pointCount);
    PointerVec pointerList (pointCount); // Vector of ITRTRs

    for (int i = 0; i < nodeslistSize; i++) {

         book [ (*NodesList)[i].id ] = 1;
         pointerList [ (*NodesList)[i].id ] = nodeslistStart + i; // REF: 2

    }

    while (nodeslistSize > 0) {

    int i = 0;
    int returnCode = 0;
    int nodeId = (*NodesList)[i].id;
    int seedId = (*NodesList)[i].seedId;
    int n_nbrOfNode = (*WdIndices)[ nodeId ].size();

    (*Segments)[ seedId ].push_back ( nodeId );
    (*NodesList).erase ( (*NodesList).begin() ); // REF: 3; This erase DOES NOT mess the pointerList
    nodeslistSize --;

    Point node;
    /*
               GET ATTRIBUTES OF NODE
            */

    for (int j = 0; j < n_nbrOfNode; j++) {

        int nborId = (*WdIndices)[nodeId][j];

        if (nborId == seedId)
            continue;

        Point neighbor;
        /*
                        GET ATTRIBUTES OF NEIGHBOUR
                    */

        double node_nbor_sim = computeSimilarity (neighbor, node);

        if (book[nborId] == 1) {

            ITRTR curr_node = pointerList[nborId]; // REF: 1

            if ( curr_node -> similarity < node_nbor_sim) {

                curr_node -> similarity = node_nbor_sim;
                NODE_SEED_SIM temp = *curr_node;
                (*NodesList).erase (curr_node); // REF: 4; This erase completely messes up the pointerList
                returnCode = insertSortNodesList (&temp, NodesList, -1);


            }

        }

    }

}

}

The nodes in the NodesList hold a global ID inside them. However they are stored in NodesList, not according to this global ID but in descending order of their "similarity". So later when I want to get the node from NodesList corresponding to a global ID (nborID in code)[REF: 1] I do it via the "pointerList" where I have previously stored the iterators of the deque but according to the global IDs of the nodes [REF: 2]. My pointerList stays true after the first erase command [REF: 3], but gets messed up in the next erase [REF: 4].

What is wrong here? Thanks

Answer 1

I am trying to store iterators of a deque in a vector and want to preserve them inside the vector even when I have erased or inserted some elements from or into the deque. Is this possible?

As from the documentation it says

_{Sorry I'm posting this as an image here, but the formatting is too tedious to replicate in markup!}

So the short answer is: NO! I'm afraid you cannot safely store iterators pointing to certain elements stored in a std::deque , while it's changed elsewhere.

Some other relevant Q&A:

• Problem with invalidation of STL iterators when calling erase
• C++ deque: when iterators are invalidated

Answer 2

If you want to create a vector of iterators then this is what you want to do:

#include<iostream>
#include <deque>
#include <vector>
using namespace std;
int main()
{
    deque<int> dq { 1, 2, 3, 4, 5, 6, 7, 8 };
    deque<int>::iterator it;

    vector<deque<int>::iterator> vit;
    for (it = dq.begin(); it != dq.end(); it++)
    {
        vit.push_back(it);
        cout << *it;
    }

    //note that `it` is a pointer so if you modify the `block` it points in your deque then the value will change.
    //If you delete it then you will have a segmenant fault.
    //--- Don't Do: while (!dq.empty()) { dq.pop_back(); } ---//

    for (size_t i = 0; i < vit.size(); i++)
    {
        cout << *vit[i];
    }

    system("pause");
    return 0;
}

However, if you want to preserve that value of that iterator after it has been changed/deleted you may want to create a copy of each iterator and store the copy rather than the actual iterator