How to do average operation on stl iterator in c++

Question

I am trying to do merge algorithm based on stl vector and iterator, i know there is also std::merge, but I would like to make my own version, I want to average two iterator and gain the middle index as I did in normal integeriMiddle = (iEnd + iBegin) / 2. Nevertheless, it never work, then I try to add value after by getting the vector size and divided by 2

iMiddle = iBegin+A.size()/2;

, but this is kind of stubid method to do and also run into problem when split more than 1 time, does any one have better solution for this?

 void TopDownMergeSort(int n, vector<int> A, vector<int> B)
{
    TopDownSplitMerge(A, A.begin(), A.end(), B);
}

void TopDownSplitMerge(vector<int> A, vector<int>::iterator iBegin, vector<int>::iterator iEnd, vector<int> B)
{
    vector<int>::iterator iMiddle;
    // if run size == 1
    if((iEnd - iBegin) < 2){
        cout<<"merge finshed!"<<endl;
        return;    //   consider it sorted
    }                       

    iMiddle = iBegin+A.size()/2;
    TopDownSplitMerge(A, iBegin,  iMiddle, B);  // split / merge left  half
}

UPDATE

USE

iMiddle = iBegin+distance(iBegin,iEnd)/2;

Everything works fine!

Answer 1

middle = begin + std::distance(begin, end) / 2;