Change the metavars in boost::program_options - c++

Question

This is a purely aesthetic issue.
I have written a CLI program in C++ and use boost::program_options to pasrse its args.
Per default, boost names all meta variables arg. I'd like to change that.
Here's the code:
#include <iostream>
using std::cerr;
using std::cout;
#include <optional>
using std::optional;
#include <string>
using std::string;
#include <boost/program_options.hpp>
using boost::program_options::unknown_option;
#include "Net.h"
#include "GameServer.h"
using proto::GameServer;
namespace args = boost::program_options;
static auto parseArgs(int argc, const char *argv[])
{
args::options_description desc("Command line options");
desc.add_options()
("help,h", "Show this page")
("address,a", args::value<string>()->default_value(net::Defaults::HOST), "IP address to listen on")
("port,p", args::value<unsigned short>()->default_value(net::Defaults::PORT), "Port to listen on")
;
optional<args::variables_map> result;
args::variables_map varMap;
try {
args::store(args::parse_command_line(argc, argv, desc), varMap);
} catch (unknown_option const &error) {
cerr << "Invalid option: " << error.get_option_name() << "\n";
cerr << desc << "\n";
return result;
}
args::notify(varMap);
if (varMap.count("help")) {
cout << desc << "\n";
return result;
}
return result = varMap;
}
int main(int argc, const char *argv[])
{
auto parsedArgs = parseArgs(argc, argv);
if (!parsedArgs.has_value())
return 1;
auto args = parsedArgs.value();
auto address = args.at("address").as<string>();
auto port = args.at("port").as<unsigned short>();
GameServer server(address, port);
server.listen();
}
And the output generated by the help page:
Command line options:
-h [ --help ] Show this page
-a [ --address ] arg (=127.0.0.1) IP address to listen on
-p [ --port ] arg (=9000) Port to listen on
I'd like to rename arg for -a to ip_address and for -p to portnum respectively.

Answer 1

#prehistoricpenguin's answer brought me onto the right track. In boost, the typed_value class has a method value_name() to set the argument name, which works analogous to setting the defaults, i.e. it modifies the value object and returns it for subsequent operations:
static auto parseArgs(int argc, const char *argv[])
{
options_description desc("Command line options");
desc.add_options()
("help,h", "Show this page")
("address,a", value<string>()->default_value(HOST)->value_name("ip_address"),
"IP address to connect to")
("port,p", value<unsigned short>()->default_value(PORT)->value_name("portnum"),
"Port to connect to");
return parseArgDesc(argc, argv, desc);
}
Resulting in:
Command line options:
-h [ --help ] Show this page
-a [ --address ] ip_address (=127.0.0.1)
IP address to connect to
-p [ --port ] portnum (=9000) Port to connect to

Answer 2

Seems it's an undocumented part of boost::program_options, I have a quick look at the implementation of boost::program_options and find there is a global variable which is used to control the behavior, so we come up with one line code hack:
args::arg = "i_am_not_arg";
Modify a global variable is not an elegant way, but I haven't found any usable APIs to do it, you may do more research and try to find a better solution(Don't forget to notify me here!)
Insert this line in some place of your code, then we get the output for --help command line:
->./a.out --help
Command line options:
-h [ --help ] Show this page
-a [ --address ] i_am_not_arg (=1234) IP address to listen on
-p [ --port ] i_am_not_arg (=42) Port to listen on
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provide a minimal, reproducible program. Your code doesn't compile on my machine, because you used variables from files that are not provided.
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