Why is my vector empty?

I want to create a simple inverted index. I have a file with with docIds and keywords that are in each document. So the first step is to try and read the file and tokenize the text file. I found a tokenize function online that was supposed to work and changed it a little. I want to tokenize each word after a blank space. My text file doesn't have any commas or periods. After tokenizing the text file the tokens are stored in a vector. So after running the tokenize function I tried printing out the elements of the vector but nothing happened. Then I tried printing out the size of the vector and as a result I get 0. Here is my code:

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include "functions.h"
#include "vector"

using namespace std;

int main()
{
    string line;
    vector<string> v;
    ifstream myfile("test.txt");


    if(myfile.is_open()){
        while(getline(myfile,line)){
            //cout << line << '\n';
            tokenize(line, ' ', v);
         }

      myfile.close();
    }
    else cout << "Unable to open file";

    cout << v.size() << '\n';

    return 0;
}

and here is my tokenize function:

using namespace std;

void tokenize(string s, char c, vector<string> v) {
   string::size_type i = 0;
   string::size_type j = s.find(c);

   while (j != string::npos) {
      v.push_back(s.substr(i, j-i));
      i = ++j;
      j = s.find(c, j);

      if (j == string::npos)
         v.push_back(s.substr(i, s.length()));
   }
}

I can't use strtok because I will use threads later in the program and I've read in a forum that strtok doesn't work well with threads.