关于for循环中 != 和 < 的问题

#include <iostream>
#include <vector>
using namespace std;
int main()
{
    vector<int> ivec;
    int ival;
//读入数据到 vector 对象
    cout << "Enter numbers(Ctrl+Z to end):" << endl;
    while (cin>>ival)
        ivec.push_back(ival);
//计算相邻元素的和并输出
    if (ivec.size() == 0)
    {
        cout << "No element?!" << endl;
        return -1;
    }
    cout << "Sum of each pair of adjacent elementsin the vector:"
         << endl;
    vector<int>::size_type cnt = 0;

    for (vector<int>::iterator iter = ivec.begin();
            iter < ivec.end()-1;                              //此处 < 换成 != 后就出问题
            iter = iter + 2)
    {
        cout << *iter + *(iter+1) << "\t";
        ++cnt;
        if ( cnt % 6== 0) //每行输出 6 个和
            cout << endl;
    }
    if (ivec.size() % 2 != 0) //提示最后一个元素没有求和
        cout << endl
             << "The last element is not been summed "
             << "and its value is "
             << *(ivec.end()-1) << endl;
    return 0;
}