How to flatten a Vector of Vectors or 2D Vector in C++

// C++ program to flatten a 
// Vector of Vectors or 2D Vector 

#include <bits/stdc++.h> 
using namespace std; 

// Class to flatten the 2d vector 
class FlattenVector { 

public: 
    int n; 

    vector<vector<int>::iterator> iStart; 
    vector<vector<int>::iterator> iEnd; 
    int currIndex; 

    // Store ending and starting iterators. 
    FlattenVector(vector<vector<int> >& v) 
    { 

        // Get the number 
        // of rows in 2d vector 
        n = v.size(); 
        currIndex = 0; 
        iStart.resize(n); 
        iEnd.resize(n); 

        for (int i = 0; i < n; i++) { 
            iStart[i] = v[i].begin(); 
            iEnd[i] = v[i].end(); 
        } 
    } 

    // Returns true if any element is left. 
    bool hasNext() 
    { 
        for (int i = 0; i < n; i++) { 
            if (iStart[i] != iEnd[i]) 
                return true; 
        } 
        return false; 
    } 

    int next() 
    { 
        // Vector at currIndex is printed, 
        // increment currIndex. 
        if (iStart[currIndex] 
            == iEnd[currIndex]) { 
            currIndex++; 
            return next(); 
        } 

        // Increment iterator 
        // and return the value. 
        else
            return *iStart[currIndex]++; 
    } 
}; 

// Driver code 
int main() 
{ 
    vector<vector<int> > 
        v{ { 1, 2 }, 
        { 3 }, 
        { 4, 5, 6 }, 
        { 7, 8, 9, 10 } }; 
    FlattenVector iter(v); 

    while (iter.hasNext()) 
        cout << iter.next() << " "; 

    return 0; 
}