List iterator not incrementable error message in C++

Question

Hi there,

As a newbie I'm trying to implement a sorting function in C++, using the list-class. However, running the code I get the error that the list iterator is not incrementable... However it seems very unlikely as it should be incrementable!

code:

void shuffle (list<int> &list1)
{
    list<int> smaller;
    list<int> larger;

    if (list1.size() > 1)
    {
     list<int>::iterator it;
     //int it;

     int x = list1.front();


     for (it = list1.begin(); it != list1.end(); it++)
     {          
      if(*it <= x)
      {
       smaller.push_front(*it);
       list1.pop_front();

      }
      else
      {
       larger.push_back(*it);
       list1.pop_front();
      }
      shuffle (smaller);
      shuffle (larger);
     }
    }
    else
    {
     print(smaller);
     print(larger);

     //cout << "No sorting needed! The list still looks like: ";
     //print(list1);
    }
    print(smaller);  
    print(larger);
}

I implemented this function just in de CPP file, under the main.

Does anyone has any suggestions?

Answer 1

I was also able to compile the posted code with VS2008 after I commented out the calls to print() and added the following to the beginning:

#include <list>
using namespace std;

Answer 2

This is my main:

> int _tmain(int argc, _TCHAR* argv[])
{
//DEFINE LIST
list <int> list1;
//FILL LIST
list1.push_front(5);
list1.push_front(2);
list1.push_front(1);
list1.push_front(9);
list1.push_front(12);
list1.push_front(3);
list1.push_front(4);
//PRINT LIST BEFORE SORTING
print(list1);
//SORT LIST

shuffle(list1);



//PRINT AFTER SORTING

system("pause");




return 0;

And the error message is just 1, namely if I debug it (press F5 in VC++ 2008) I get a pop-up that list iterator is not incrementable

Answer 3

Your call to list1.pop_front() removes the element which the iterator is pointing to initially, invalidating it. And an invalid iterator can not be incremented. :)

It took a few minutes to find with the debugger. Just keep an eye on the value of 'it' as you step through the program. I don't know if you know how to use the debugger yet, but if not, do yourself a favor and learn it. It's an invaluable tool.

(By the way, in the future, please be explicit about whether the error occurs at compile-time or while running the program. Your question stated the error occurred when "compiling the program". I just edited the question for you, hope you don't mind. But it's an important distinction, and makes it harder to answer your question accurately)