Efficiently subset many lines from a text file

select-lines.cpp

#include <algorithm>
#include <cstdlib>
#include <fstream>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

auto main(int argc, char** argv) -> int {
    if (argc < 2) {
        std::cerr << "Usage: " << argv[0] << " file line_numbers\n";
        return EXIT_FAILURE;
    }

    auto const filename = argv[1];
    auto const lines_filename = argv[2];

    auto lines = std::vector<int>{};
    auto lines_file = std::ifstream{lines_filename};
    std::copy(
            std::istream_iterator<int>{lines_file},
            std::istream_iterator<int>{},
            std::back_inserter(lines));

    std::sort(begin(lines), end(lines));

    auto file = std::ifstream{filename};
    auto line = std::string{};
    auto lineno = int{1};
    auto next_line = unsigned{0};

    while (std::getline(file, line)) {
        if (next_line == lines.size()) break;
        if (lineno++ == lines[next_line]) {
            std::cout << line << '\n';
            ++next_line;
        }
    }
}