error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

Hi,
I have written this code, and at the end, I am trying to write a
vector of strings into a text file. However, my program is nor
compiling, and it gives me the following error when I try to write to
the file:

error C2679: binary '<<' : no operator found which takes a right-hand
operand of type 'std::string' (or there is no acceptable conversion)

I don't know what I am doing wrong. I have posted my entire program
here.
Thank you

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <vector>
#include <math.h>
#include "string.h"

using namespace std;

char n_str[2000];
char a_str[2000];

char n_char[2000];
char a_char[2000];
string achar, nchar;

int main(int argc, char* argv[])
{
        vector<string> n_word_list;
        vector<string> a_word_list;

        ifstream in_a("10_a.txt");
        if (!in_a)
        {
                cout << "Error opening abnormal file" << endl;
        }
        while (!in_a.eof())
        {
                in_a.getline(a_str,2000);
                a_word_list.push_back(a_str);
        }
        cout << a_word_list.size()<< endl;

        ifstream in_n("10_n.txt");
        if (!in_n)
        {
                cout << "Error opening normal file" << endl;
        }
        while (!in_n.eof())
        {
                in_n.getline(a_str,2000);
                n_word_list.push_back(a_str);
        }
        cout << n_word_list.size()<< endl;

        for (unsigned int i=0; i<a_word_list.size(); i++)
        {
                for (unsigned int j=0; j<n_word_list.size();j++)
                {
                        if (a_word_list[i].compare( n_word_list[j]))
                        {
                                a_word_list.assign(1,"aa");

                                n_word_list.assign(1,"aa");

                        }
                }
        }

        ofstream out_a("10_a_new.txt");
        if (!out_a)
        {
                cout << "Error opening new abnormal file" << endl;
        }
        for (unsigned int i=0; i<a_word_list.size(); i++)
        {
                achar = a_word_list.at(i);
                out_a << achar << endl; //ERROR
        }

        ofstream out_n("10_n_new.txt");
        if (!out_n)
        {
                cout << "Error opening new normal file" << endl;
        }
        for (unsigned int i=0; i<n_word_list.size(); i++)
        {
                out_n << n_word_list.at(i) << endl; //ERROR
        }
        return 0;

}