Debug assertion failed with list

Question

Hello everybody can you help me? the problem is that when a person enters a element from list he can insert before this element elements from another list, but I constantly knock out mistakes. Maybe someone will help? It looks like: 3 6 7 9 user enters 6 he can create another list like 8 7 5 and output is 3 8 7 5 6 7 9 The error

#include <iostream>
#include <list>
#include <algorithm>
#include <iterator>
int main()
{
int size
int size1;
int el1;
int znach=0;
    cout << "Enter size: " << endl;
        cin >> size;
         list<int>lis;
            list<int>lis1;
            list <int> ::iterator it;
            int s = lis.size() / 2;
            auto it1 = lis.begin();
            advance(it1, s);
            for (int i = 0; i <size; i++)
            {
                cout << "Enter " << i << " element: ";
                cin >> t;
                lis.push_back(t);
            }
            cout << "Enter element: " << endl;
            cin >> el1;
            for (it = lis.begin(); it != lis.end(); it++)
            {

                if (*it = el1)
                {
                    znach++;
                }
            }
                 if (znach == 0)
                    {
                        cout << "There is no element;" << endl;
                    }
        else
                {
                    cout << "Enter size of new vector: " << endl;
                    cin >> size1;
                    for (int i = 0; i < size1; i++)
                    {
                        int t1;
                        cout << "Enter " << i << " element: ";
                        cin >> t1;
                        lis1.push_back(t1);
                    }
                    auto it2 = lis.begin();
                    while (*it2 != el1)
                    {
                        it2++;
                    }
                    --it2;
                    lis.splice(it2, lis1);
                }


            for (it = lis.begin(); it != lis.end(); it++)
            {
                cout << *it<<" ";
            }
        }

Answer 1

I don't know what your code is doing, and maybe there are other problems. However, the runtime error you get is caused by

auto it2 = lis.begin();
while (*it2 != el1)
{
    it2++;
}
--it2;

When *lis.begin() == el1 then you never increment it2 and then decrement it, but you cannot decrement the begin iterator. Usually thats just undefined, but as you compiled a debug build you got an assertion fired up that tells you what went wrong.