C++: Boost program_options: Multiple lists of arguments

Question

I'm currently working with boost::program_options. My program is supposed to take as arguments (amongst other things...) an arbitrary number of 'lists' of arbitrary length. For example, the user should be able to call

./myprogram -list item1 item2 item3 -list item1 item2 -list item1 item2

Obviously, I don't want to get one list/vector with all the items one after the other as a result, but (in this case) three lists/vectors (or, for example, one vector of vectors containing the elements) with two or three items per list (each item is supposed to be a string, but I guess this doesn't matter). As I said before, the number of lists (as well as the number of items per list!) should be arbitrary. How can I do that with boost::program_options?

Answer 1

This can be done without a whole lot of extra code. The secret is to separate the parsing step from the storage step, as also done in this answer.

The parser will return a container of key/value structs as the options are presented from the user. If an option is submitted multiple times then the container will have a separate entry for each option submission. It is quite straightforward to scan for a particular option and organize its values however we want.

Here's an example that prints out each input multi-token option on a separate line:

#include <iostream>
#include <string>
#include <vector>

#include <boost/program_options.hpp>

namespace po = boost::program_options;

int main(int argc, char *argv[]) {
   // Define a multi-token option.
   po::options_description desc("Allowed options");
   desc.add_options()
      ("list", po::value<std::vector<std::string>>()->multitoken(), "multiple values");

   // Just parse the options without storing them in a map.
   po::parsed_options parsed_options = po::command_line_parser(argc, argv)
      .options(desc)
      .run();

   // Build list of multi-valued option instances. We iterate through
   // each command-line option, whether it is repeated or not. We
   // accumulate the values for our multi-valued option in a
   // container.
   std::vector<std::vector<std::string>> lists;
   for (const po::option& o : parsed_options.options) {
      if (o.string_key == "list")
         lists.push_back(o.value);
   }

   // If we had other normal options, we would store them in a map
   // here. In this demo program it isn't really necessary because
   // we are only interested in our special multi-valued option.
   po::variables_map vm;
   po::store(parsed_options, vm);

   // Print out the multi-valued option, each separate instance on its
   // own line.
   for (size_t i = 0; i < lists.size(); ++i) {
      for (size_t j = 0; j < lists[i].size(); ++j)
         std::cout << lists[i][j] << ' ';
      std::cout << '\n';
   }

   return 0;
}

And here's a sample invocation (live at coliru):

$ ./po --list 1 2 3 --list foo bar --list how now brown cow
1 2 3 
foo bar 
how now brown cow