How to add a description to boost::program_options' positional options? - c++

Question

I would like to make a positional, list program option with boost_program_options that do not allow named program options (like --files).
I have the following snippet of code:
#include <boost/program_options.hpp>
#include <iostream>
#include <string>
#include <vector>
namespace po = boost::program_options;
int main(int argc, const char* argv[]) {
po::options_description desc("Allowed options");
desc.add_options()("help", "produce help message")
( "files", po::value<std::vector<std::string>>()->required(), "list of files");
po::positional_options_description pos;
pos.add("files", -1);
po::variables_map vm;
try {
po::store(po::command_line_parser(argc, argv).options(desc).positional(pos).run(), vm);
po::notify(vm);
} catch(const po::error& e) {
std::cerr << "Couldn't parse command line arguments properly:\n";
std::cerr << e.what() << '\n' << '\n';
std::cerr << desc << '\n';
return 1;
}
if(vm.count("help") || !vm.count("files")) {
std::cout << desc << "\n";
return 1;
}
}
The problem is that I can read files list as positional arguments lists as follows:
./a.out file1 file2 file3
but unfortunately like this as well ( which I would like to disable )
./a.out --files file1 file2 file3
The problem is also with the help which yields:
./a.out
Couldn't parse command line arguments properly:
the option '--files' is required but missing
Allowed options:
--help produce help message
--files arg list of files
So my desired scenario would be more like (os similar):
./a.out
Couldn't parse command line arguments properly:
[FILES ...] is required but missing
Allowed options:
--help produce help message
--optionx some random option used in future
[FILE ...] list of files
After I remove files options from desc.add_option()(...) it stop working so I believe I need it there.

Answer 1

As to the question posed in the title, "How to add a description to boost::program_options' positional options?", there's no functionality provided for this in the library. You need to handle that part yourself.
As for the body of the question... it's possible, but in a slightly round-about way.
The positional options map each position to a name, and the names need to exist. From what I can tell in the code (cmdline.cpp), the unregistered flag won't be set for arguments that are positional. [1], [2]
So, to do what you want, we can do the following:
Hide the --files option from showing up in the help. You will need to display appropriate help for the positional options yourself, but this is no different than before.
Add our own validation between parsing and storing of the parsed options to the variables_map.
Hiding --files from help
Here we take advantage of the the fact that we can create composite options_description using the add(...) member function:
po::options_description desc_1;
// ...
po::options_description desc_2;
// ...
po::options_description desc_composite;
desc_composite.add(desc_1).add(desc_2);
We can therefore place our files option into a hidden options_description, and create a composite that we will use only for the parsing stage. (see code below)
Preventing explicit --files
We need to intercept the list of options between parsing and storing them into the variables_map.
The run() method of command_line_parser returns an instance of basic_parsed_options, whose member options holds a vector of basic_options. There is an element for each parsed argument, and any positional options are enumerated starting from 0, any non-positional options have position -1. We can use this to perform our own validation and raise an error when we see --files as an explicit (non-positional) argument.
Example Source Code
See on Coliru
#include <boost/program_options.hpp>
#include <iostream>
#include <string>
#include <vector>
namespace po = boost::program_options;
int main(int argc, const char* argv[])
{
std::vector<std::string> file_names;
po::options_description desc("Allowed options");
desc.add_options()
("help", "produce help message")
("test", "test option");
std::string const FILES_KEY("files");
// Hide the `files` options in a separate description
po::options_description desc_hidden("Hidden options");
desc_hidden.add_options()
(FILES_KEY.c_str(), po::value(&file_names)->required(), "list of files");
// This description is used for parsing and validation
po::options_description cmdline_options;
cmdline_options.add(desc).add(desc_hidden);
// And this one to display help
po::options_description visible_options;
visible_options.add(desc);
po::positional_options_description pos;
pos.add(FILES_KEY.c_str(), -1);
po::variables_map vm;
try {
// Only parse the options, so we can catch the explicit `--files`
auto parsed = po::command_line_parser(argc, argv)
.options(cmdline_options)
.positional(pos)
.run();
// Make sure there were no non-positional `files` options
for (auto const& opt : parsed.options) {
if ((opt.position_key == -1) && (opt.string_key == FILES_KEY)) {
throw po::unknown_option(FILES_KEY);
}
}
po::store(parsed, vm);
po::notify(vm);
} catch(const po::error& e) {
std::cerr << "Couldn't parse command line arguments properly:\n";
std::cerr << e.what() << '\n' << '\n';
std::cerr << visible_options << '\n';
return 1;
}
if (vm.count("help") || !vm.count("files")) {
std::cout << desc << "\n";
return 1;
}
if (!file_names.empty()) {
std::cout << "Files: \n";
for (auto const& file_name : file_names) {
std::cout << " * " << file_name << "\n";
}
}
}
Test Output
Valid options:
>example a b c --test d e
Files:
* a
* b
* c
* d
* e
Invalid options:
>example a b c --files d e
Couldn't parse command line arguments properly:
unrecognised option 'files'
Allowed options:
--help produce help message
--test test option