关于front_insert_iterator的问题

一角钱

#include <iostream>
#include <string>
#include <iterator>
#include <vector>
#include <algorithm>

void output(const std::string& s)
{
    std::cout << s << " ";
}

int main()
{
    using namespace std;

    string s1[4] = {"fine", "fish", "fashion", "fate"};
    string s2[2] = {"busy", "bats"};
    string s3[2] = {"silly", "singers"};
    vector<string> words(4);

    copy(s1, s1+4, words.begin());
    for_each(words.begin(), words.end(), output);
    cout << endl;

    copy(s2, s2+2, back_insert_iterator< vector<string> >(words));
    for_each(words.begin(), words.end(), output);
    cout << endl;

    copy(s3, s3+2, front_insert_iterator< vector<string> >(words));   //错误
    for_each(words.begin(), words.end(), output);
    cout << endl;

    copy(s3, s3+2, insert_iterator< vector<string> >(words, words.begin()));
    for_each(words.begin(), words.end(), output);
    cout << endl;
    return 0;
}