std::map.find() returns const_iterator instead of iterator

Hi i am reading an example of class implementation but getting some errors. First i am writing the code(with corresponding file names) and then i will write the errors and then my doubts. Below are the different files.

1. MAINFILE.cpp

#include "textquery.h"
#include "queryresult.h"
#include<iostream>
#include<string>
#include<fstream>
void runQueries(std::ifstream &infile){
    
    TextQuery tq(infile);
    
    while(true){
        std::cout<<"enter word to look for, or q to quit:";
        std::string s;
        
        if(!(std::cin>>s)|| s=="q")break;
        
        print(std::cout, tq.query(s))<<std::endl;
    }
}

int main(){
    return 0;
}

2. textquery.h

#ifndef TEXTQUERY_H
#define TEXTQUERY_H

#include<iostream>
#include<memory>
#include<string>
#include<fstream>
#include<iterator>
#include<vector>
#include<map>
#include<set>
#include<sstream>

class QueryResult;
class TextQuery {
    public:
        using line_no = std::vector<std::string>::size_type;
        TextQuery(std::ifstream&);
        QueryResult query(const std::string&) const;
    private:
        std::shared_ptr<std::vector<std::string>> file;//input file
        std::map<std::string, std::shared_ptr<std::set<line_no>>> wm; 


};

#endif

3. textquery.cpp

#include "textquery.h"
#include "queryresult.h"
TextQuery::TextQuery(std::ifstream& is): file(new std::vector<std::string>){
std::string text;
    while(std::getline(is, text)){
        file->push_back(text);

        int n = file->size() -1;
        std::istringstream line(text);
        std::string word;
        while (line >> word){
            auto &lines = wm[word];
            if(!lines){
                lines.reset(new std::set<line_no>);
            }
            lines->insert(n);
        }
   }
}
QueryResult TextQuery::query(const std::string &sought) const{
    
    static std::shared_ptr<std::set<TextQuery::line_no>> nodata(new std::set<line_no>);
    std::map<std::string,std::shared_ptr<std::set<TextQuery::line_no>>>::const_iterator loc = wm.find(sought);
    if(loc == wm.end()){
        return QueryResult(sought, nodata, file);//not found
    }
    else {
        return QueryResult(sought, loc->second, file);
    }
}

4. queryresult.h

#ifndef QUERYRESULT_H
#define QUERYRESULT_H
#include<iostream>
#include<memory>
#include<string>
#include<fstream>
#include<iterator>
#include<vector>
#include<map>
#include<set>


class QueryResult {
    friend std::ostream& print(std::ostream&, const QueryResult&);
    public:
        using line_no = std::vector<std::string>::size_type;
        QueryResult(std::string s, std::shared_ptr<std::set<line_no>> p, std::shared_ptr<std::vector<std::string>> f)
            :sought(s), lines(p), file(f) {

        }
        
    private:
        std::string sought;
        std::shared_ptr<std::set<line_no>> lines;
        std::shared_ptr<std::vector<std::string>> file;
};

std::ostream& print(std::ostream&, const QueryResult&);
#endif

5. queryresult.cpp

#include "queryresult.h"
std::ostream& print(std::ostream& os, const QueryResult& qr){
    
    os << qr.sought << " occurs " << qr.lines->size() << " "<< make_plural(qr.lines->size(), "time", "s")<< std::endl;
for(auto num: *qr.lines)//for every element in the set
       { 
           os<< "\t(line"<<num+1<<")"<<*(qr.file->begin() + num)<<std::endl;
        }
return os;
}
std::string make_plural(std::size_t ctr, const std::string &word, const std::string &ending){
    return (ctr >1) ? word + ending : word;
}

Now the error i am getting are:

/tmp/ccpkNtoM.o: In function runQueries(std::basic_ifstream<char, std::char_traits<char> >&)': MAINFILE.cpp:(.text+0xdd): undefined reference to TextQuery::TextQuery(std::basic_ifstream<char, std::char_traits >&)' MAINFILE.cpp:(.text+0x170): undefined reference to TextQuery::query(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const' MAINFILE.cpp:(.text+0x183): undefined reference to print(std::ostream&, QueryResult const&)' collect2: error: ld returned 1 exit status

My questions(doubts) are as follows :

1. How can i resolve these erros and why i am getting those even though i have defined the corresponding functions.
2. As you can see in the textquery.cpp file i have defined a variable loc of type std::map<std::string,std::shared_ptr<std::set<TextQuery::line_no>>>::const_iterator using wm.find(sought);. But when i try to change the const_iterator to iterator in the above line it gives error. Why can't i use std::map<std::string,std::shared_ptr<std::set<TextQuery::line_no>>>::iterator=wm.find(sought). I know that find() function returns either an iterator or a const_iterator but why in our program it is returning const_iterator instead of iterator?