Question

I am trying to write a data structure that I can cycle round, sort of a circular list, using a vector. I resize which I am thinking should initialise the underlying array with ten elements. I don’t understand why I cannot advance the iterator. Can someone please help.

I cannot use push_back() because that will always append to the end which is not what I want.

// re-use start of vector when get to end
#include <vector>
#include <iostream>
#include <algorithm>

using std::cout;
using std::endl;
using std::vector;

class printme {
public:
   void operator() (int val) {cout << val << endl; }
};


//get a debug assertion - message says: vector iterators incompatible
//I assume this means that it is invalid after previous it++
int main(int argc, char* argv[])
{
   vector<int> myvec;
   myvec.resize(10);  //underlying array now has size=10 elements
   vector<int>::iterator it = myvec.begin();  //point to start of array
   for(int i = 0; i < 100; ++i) {
      if(it == myvec.end())           //on 2nd iteration crashes here - invalid iterator
         it = myvec.begin();

      myvec.insert(it++, i);
   }

   //print contents of vector - check 90-99 printed
   for_each(myvec.begin(), myvec.end(), printme());

    return 0;
}

EDIT
Changed loop to this:

for(int i = 0; i < 100; ++i) {
   if(it == myvec.end())
      it = myvec.begin();

   *it++ = i;
}

I didn’t properly understand insert.

Answer 1

From what you expect in output – I believe you misunderstood what insert is doing.
Implement your loop in this way (without insering – just replacing). std::vector<>::insert increments the size of your vector by one – I believe it is not what you expect.

Do not do this:

myvec.insert(it++, i);

But this:

 *it++ = i;

Then you’ll get your desired ouput:

   //print contents of vector - check 90-99 printed
   for_each(myvec.begin(), myvec.end(), printme());