运算符重载的问题，帮忙看一下

一个istream_iterator的模拟程序，想问一下为什么重载++运算符的时候要加int？
#include <iostream>
#include <string>

using namespace std;
template <class T>
class CMyistream_iterator
{
    public:
        T value;
        CMyistream_iterator(istream& i){
            i>>value;
        }
        T operator* (void){
            return value;
        }
        void operator ++ (int){
            cin>>value;
        }   
};

int main()  
{
    int t;
    cin >> t;
    while( t -- ) {
         CMyistream_iterator<int> inputInt(cin);
         int n1,n2,n3;
         n1 = * inputInt;
         int tmp = * inputInt;
         cout << tmp << endl;
         inputInt ++;  //如果上面红色语句不加（int），这一句会报错  
         n2 = * inputInt;
         inputInt ++;
         n3 = * inputInt;
         cout << n1 << " " << n2<< " " << n3 << " ";
         CMyistream_iterator<string> inputStr(cin);
         string s1,s2;
         s1 = * inputStr;
         inputStr ++;
         s2 = * inputStr;
         cout << s1 << " " << s2 << endl;
    }
     return 0;  
}