Intersection

#include <iostream>
#include <fstream>
#include <set>
#include <algorithm>

std::vector<std::string> readUsers(const std::string& fname)
{
    std::vector<std::string> users;
    std::ifstream infile(fname);
    std::string line;
    while(std::getline(infile, line))
    {
        users.push_back(line);
    }
    return users;
}

void saveUsers(const std::vector<std::string>& users, const std::string& fname)
{
    std::ofstream file(fname);
    for(auto& user: users)
        file << user << '\n';
}

int main()
{
    auto users1 = readUsers("onimille_followers_180.txt");
    auto users2 = readUsers("centauri_followers.txt");

    std::sort(users1.begin(), users1.end());
    std::sort(users2.begin(), users2.end());

    decltype(users1) intersection(users1.size() + users2.size());
    auto it = std::set_intersection(users1.begin(), users1.end(), users2.begin(), users2.end(), intersection.begin());
    intersection.resize(it - intersection.begin());

    for(auto& user: intersection)
        std::cout << user << '\n';

    saveUsers(intersection, "intersection.txt");

    return 0;
}