boost::program_options reports no error when unexpected positional argument is specified

Question

Here is a simple program using boost to parse options:

#include <iostream> #include <boost/program_options.hpp> namespace po = boost::program_options; int main(int argc, char **argv) { try { size_t param = std::numeric_limits<size_t>::max(); po::options_description desc("Syntax: [options] \"input binary file\".\nAllowed options:"); desc.add_options() ("help,h", "produce help message") ("param,p", po::value<size_t>(&param), "param"); po::variables_map vm; po::store(po::parse_command_line(argc, argv, desc), vm); po::store(po::command_line_parser(argc, argv).options(desc).run(), vm); po::notify(vm); if (vm.count("help")) { std::cout << desc << "\n"; } else { std::cout << "Running with param " << param << std::endl; return 0; } } catch(std::exception& e) { std::cerr << "error: " << e.what() << "\n"; } catch(...) { std::cerr << "Exception of unknown type!\n"; } return 1; }

Here are the outputs produced:

hex2txt.exe outputs Running with param 0
hex2txt.exe --param 3 outputs Running with param 3
hex2txt.exe --toto outputs error: unrecognised option '--toto'

All this is expected, however: hex2txt.exe foo outputs Running with param 0

I would have expected to get an error as "foo" is not expected. What am I doing wrong here?

Your code has two competing command line parse runs + store. This is redundant, and might create surprising effects (I didn't think of an example). — sehe
– sehe, Commented May 13, 2020 at 11:35

sehe · Accepted Answer · 2020-05-13 11:27:57Z

Positionals are accepted by default. You could either check that you received none, or you can specify that you don't allow any:

po::positional_options_description positionals; po::store(po::command_line_parser(args.size(), args.data()) .options(desc) .positional(positionals) .run(),

DEMO

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#include <boost/program_options.hpp> #include <iostream> #include <iomanip> namespace po = boost::program_options; int main() { std::vector<char const*> const cases[] = { { "hex2txt.exe" }, // outputs Running with param 0 { "hex2txt.exe", "--param", "3" }, // outputs Running with param 3 { "hex2txt.exe", "--toto" }, // outputs error: unrecognised option '--toto' // All this is expected, however: { "hex2txt.exe", "foo" }, // outputs Running with param 0 }; for (auto args : cases) { std::cout << "------"; for (auto arg : args) std::cout << " " << std::quoted(arg); std::cout << "\n"; try { size_t param = 0; // std::numeric_limits<size_t>::max(); po::options_description desc( "Syntax: [options] \"input binary file\".\nAllowed options:"); desc.add_options()("help,h", "produce help message")( "param,p", po::value<size_t>(&param), "param"); po::positional_options_description positionals; po::variables_map vm; po::store(po::command_line_parser(args.size(), args.data()) .options(desc) .positional(positionals) .run(), vm); po::notify(vm); if (vm.count("help")) { std::cout << desc << "\n"; } else { std::cout << "Running with param " << param << std::endl; //return 0; } } catch (std::exception const& e) { std::cerr << "error: " << e.what() << "\n"; } catch (...) { std::cerr << "Exception of unknown type!\n"; } } }

Prints

------ "hex2txt.exe" Running with param 0 ------ "hex2txt.exe" "--param" "3" Running with param 3 ------ "hex2txt.exe" "--toto" error: unrecognised option '--toto' ------ "hex2txt.exe" "foo" error: too many positional options have been specified on the command line

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