c – 在类的begin()方法上编译错误

g++ -std=c++11 map2.cc 
map2.cc:20:2: error: invalid use of template-name ‘std::iterator’ without an argument list
  iterator begin() { return l.begin(); }

这是小班：

#include <iostream>
#include <string>
#include <list>
#include <unordered_map>

using namespace std;
class cEvents {
public:
    list<pair<double,int>> l;
    int add(double ts,int pktNum,double maxTime) {
        l.push_back(make_pair(ts,pktNum));
        pair<double,int> tmp;
        while (1) {
            tmp = l.front();
            if ((ts - tmp.first) < maxTime) break;
            l.pop_front();
        }
        return l.size();
    }
    iterator begin() { return l.begin(); }
};

int main () {
    cEvents e;
    cout << e.add(0,1,2) << endl;
    cout << e.add(0.1,2,2) << endl;
    cout << e.add(0.2,3,2) << endl;
    cout << e.add(0.5,4,2) << endl;
    cout << e.add(1.2,5,2) << endl;
    cout << e.add(1.7,6,2) << endl;
    for (auto x : e) {
        //cout << x.first << " " << x.second << endl;
    }
    cout << e.add(2.2,7,2) << endl;
    cout << e.add(3.2,8,2) << endl;
    return 0;
}