关于“该符号在main中已被引用”的问题

Question

《essential c++》第3章习题3.1中关于书上的解答我在输入后总会报错说改符号已在main中引用

#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<set>
#include<map>
#include<iterator>
#include<fstream>

using namespace std;

void initialize_exclusion_set(set<string>&);
void process_file(const set<string>&, map<string, int>&, ifstream&);
void user_query(const map<string, int>&);
void display_word_count(const map<string, int>&, ofstream&);

int main()
{
    ifstream ifile("w:\\c++practice\\essential c++\\column.txt");
    ofstream ofile("w:\\c++practice\\essential c++\\column.map");
    if (!ifile || !ofile)
    {
        cerr << "unable to open file--bailing out!\n";
        return -1;
    }

    set<string>exclude_set;
    initialize_exclusion_set(exclude_set);

    map<string, int>word_count;
    process_file(exclude_set,word_count, ifile);
    user_query(word_count);
    display_word_count(word_count, ofile);
}

void initialize_exclude_set(set<string>&exs)
{
    static string _exclude_set[25] = {
        "the","and","but","that","then","are","been","can","a",
        "could","did","for","of","had","have","him","his","her","its","is",
        "were","which","when","with","would"
    };
    exs.insert(_exclude_set, _exclude_set + 25);
}

void process_file(const set<string>&exclude_set, map<string, int>&word_count, ifstream& ifile)
{
    string word;
    while (ifile >> word)
    {
        if (exclude_set.count(word))
            continue;
        word_count[word]++;
    }
}

void user_query(const map<string, int>& word_map)
{
    string search_word;
    cout << "please enter a word to search:(q to quit)\n";
    cin >> search_word;
    
    while (search_word.size() && search_word != "q")
    {
        map<string, int>::const_iterator it;
        if ((it = word_map.find(search_word)) != word_map.end())
            cout << "found! " << it->first << " occurs" << it->second << "times.\n";
        else cout << search_word << " was not found.\n";
        cout << "\nanother word to find?(q to quit)\n";
        cin >> search_word;
    }
}

void display_word_count(const map<string, int>&word_map, ofstream& os)
{
    map<string, int>::const_iterator it = word_map.begin(),
        end_it = word_map.end();
    while (it != end_it)
    {
        os << it->first << " ("
            << it->second << ") "<< endl;
        ++it;
    }
    os << endl;
}

不知道是哪里出来问题，求解。

Answer 1

initialize_exclusion_set这个函数没实现。
下面实现的函数initialize_exclude_set是不是名字写错了？