Handling '-' with boost.program_options

Question

Before you say OVERKILL, I don't care.

How can I make Boost.program_options handle the required cat option -?

I have

// visible
po::options_description options("Options");
options.add_options()("-u", po::value<bool>(), "Write bytes from the input file to the standard output without delay as each is read.");

po::positional_options_description file_options;
file_options.add("file", -1);

po::variables_map vm;
po::store(po::command_line_parser(argc, argv).options(options).positional(file_options).run(), vm);
po::notify(vm);

bool immediate = false;
if(vm.count("-u"))
  immediate = true;
if(vm.count("file"))
  support::print(vm["file"].as<vector<string>>());

which throws an exception when I run cat - - -:

unrecognised option '-'

I want it to see - as a positional argument, and I need it in the correct order in the full file list. How could I achieve this?

UPDATE

I have a half fix. I needed

po::options_description options("Options");
options.add_options()("-u", po::value<bool>(), "Write bytes from the input file to the standard output without delay as each is read.")
                    ("file", po::value< vector<string> >(), "input file");

po::positional_options_description file_options;
file_options.add("file", -1);

Problem is, I seem to only get 2 of the three - when I output the arguments:

if(vm.count("file"))
  support::print(vm["file"].as<vector<string>>());

where support::print nicely handles the vector and stuff.

Answer 1

You need to define the named program option that is positional, in your case it is file

#include <boost/foreach.hpp>
#include <boost/program_options.hpp>

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

namespace po = boost::program_options;

int
main( int argc, char* argv[] )
{
    std::vector<std::string> input;
    po::options_description options("Options");
    options.add_options()
        ("-u", po::value<bool>(), "Write bytes from the input file to the standard output without delay as each is read.")
        ("file", po::value(&input), "input")
        ;

    po::positional_options_description file_options;
    file_options.add("file", -1);

    po::variables_map vm;
    po::store(po::command_line_parser(argc, argv).options(options).positional(file_options).run(), vm);
    po::notify(vm);

    bool immediate = false;
    if(vm.count("-u"))
        immediate = true;
    BOOST_FOREACH( const auto& i, input ) {
        std::cout << "file: " << i << std::endl;
    }
}

Here's a coliru demo and the output if you don't want to click through

$ g++ -std=c++11 -O2 -pthread main.cpp -lboost_program_options && ./a.out - - -
file: -
file: -
file: -

---

If you only see 2 of 3 positional arguments, it's likely because argv[0] is by convention the program name, and is therefore not considered for argument parsing. This can be seen in the source code for the basic_command_line_parser template

 37     template<class charT>
 38     basic_command_line_parser<charT>::
 39     basic_command_line_parser(int argc, const charT* const argv[])
 40     : detail::cmdline(
 41         // Explicit template arguments are required by gcc 3.3.1 
 42         // (at least mingw version), and do no harm on other compilers.
 43         to_internal(detail::make_vector<charT, const charT* const*>(argv+1, argv+argc+!argc)))
 44     {}