Demonstration of how to do subcommand option processing with boost program_options

cmdoptions.cpp

#define BOOST_TEST_MODULE subcommand options
#include <boost/test/unit_test.hpp>
#include <boost/program_options.hpp>
#include <boost/variant/variant.hpp>
#include <boost/variant/get.hpp>

struct GenericOptions {
    bool debug_;
};

struct LsCommand : public GenericOptions {
    bool hidden_;
    std::string path_;
};

struct ChmodCommand : public GenericOptions {
    bool recurse_;
    std::string perms_;
    std::string path_;
};

typedef boost::variant<LsCommand, ChmodCommand> Command;

Command ParseOptions(int argc, const char *argv[])
{
    namespace po = boost::program_options;

    po::options_description global("Global options");
    global.add_options()
        ("debug", "Turn on debug output")
        ("command", po::value<std::string>(), "command to execute")
        ("subargs", po::value<std::vector<std::string> >(), "Arguments for command");

    po::positional_options_description pos;
    pos.add("command", 1).
        add("subargs", -1);

    po::variables_map vm;

    po::parsed_options parsed = po::command_line_parser(argc, argv).
        options(global).
        positional(pos).
        allow_unregistered().
        run();

    po::store(parsed, vm);
    
    std::string cmd = vm["command"].as<std::string>();

    if (cmd == "ls")
    {
        // ls command has the following options:
        po::options_description ls_desc("ls options");
        ls_desc.add_options()
            ("hidden", "Show hidden files")
            ("path", po::value<std::string>(), "Path to list");

        // Collect all the unrecognized options from the first pass. This will include the
        // (positional) command name, so we need to erase that.
        std::vector<std::string> opts = po::collect_unrecognized(parsed.options, po::include_positional);
        opts.erase(opts.begin());

        // Parse again...
        po::store(po::command_line_parser(opts).options(ls_desc).run(), vm);

        LsCommand ls;
        ls.debug_ = vm.count("debug");
        ls.hidden_ = vm.count("hidden");
        ls.path_ = vm["path"].as<std::string>();

        return ls;
    }
    else if (cmd == "chmod")
    {
        // Something similar
    }

    // unrecognised command
    throw po::invalid_option_value(cmd);
}

BOOST_AUTO_TEST_CASE(NoCommand)
{
    const int argc = 2;
    const char *argv[argc] = { "0", "nocommand" };

    BOOST_CHECK_THROW(
        ParseOptions(argc, argv),
        boost::program_options::invalid_option_value);
}

BOOST_AUTO_TEST_CASE(LsTest)
{
    const int argc = 5;
    const char *argv[argc] = { "0", "--debug", "ls", "--hidden", "--path=./" };

    Command c = ParseOptions(argc, argv);

    BOOST_REQUIRE(boost::get<LsCommand>(&c));
    const LsCommand& ls = boost::get<LsCommand>(c);
    BOOST_CHECK(ls.debug_);
    BOOST_CHECK(ls.hidden_);
    BOOST_CHECK_EQUAL(ls.path_, "./");
}

This is great! The only problem is that it doesn't allow a global --help and specific --help options for the subcommands. After adding --help to both options descriptions, a.out ls --help and a.out --help gives the same output.

I ended up preprocessing the arguments before handing them off to boost.

First create a vector of strings that have your command names in, then loop through argv and sort the arguments into "global" and "command" arguments. I actually am using a vector of string pairs so I can put the command description with the command.

std::vector<std::<std::string,std::string>> cmds = { {"cmd1","cmd1 desc"}
                                                   , {"cmd2", "cmd2 desc"}
                                                   /* etc */ };
std::vector<std::string> global_args;
std::vector<std::string> cmd_args;

bool cmd_found = false;
for( int i = 1; i < argc; i++ )
{
  std::string arg( argv[i] );
  if(!cmd_found)
    global_args.push_back( arg );
  if(cmd_found)
    cmd_args.push_back( arg );

  for( auto c : cmds )
  {
    if( c.first == arg )
    {
      cmd_found = true;
      break;
    }
  }
}

Now you can setup an option parser for the global arguments and pass it global_args. Note that this snippet puts the command name in the global_args vector, so you global argument parser can look for it as a positional parameter.

Then just put your commands in separate functions, pass the function cmd_args and use another option parser inside the function to parse it.

This is exactly what I was looking for! Thanks!

@markusdr

This is great! The only problem is that it doesn't allow a global --help and specific --help options for the subcommands. After adding --help to both options descriptions, a.out ls --help and a.out --help gives the same output.

I know this reply comes late but I just had the same issue...
You can fix this by checking if a command was provided.
If you just use if (vm.count("help")) std::cout << global << std::endl;, you get the same help output whenever a --help flag is set anywhere in your options.
You can use this instead and you will get your different outpus:

// ...
if (vm.count("help") && !vm.count("command")) {
    std::cout << bm_options << std::endl;
    return 0;
}
// ...
// within the command specific section:
if (vm.count("help")) {
  std::cout << ls_desc << std::endl;
  return 0;
}

@lfreist Nice one! I'll update the code above