When using boost::program_options, how does one set the name of the argument?

Question

When using boost::program_options, how do I set the name of an argument for boost::program_options::value<>()?

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

int main()
{
  boost::program_options::options_description desc;

  desc.add_options()
    ("width", boost::program_options::value<int>(),
     "Give width");

  std::cout << desc << std::endl;

  return 0;
}

The above code gives:

  --width arg           Give width

What I want is to replace the arg name with something more descriptive like NUM:

  --width NUM           Give width

Answer 1

One can replace arg with something different via the global variable boost::program_options::arg:

boost::program_options::arg = "NUM";

But as that is a global variable, it doesn't help much to fix the problem when multiple option might require different arguments.

Answer 2

The program_options::value_semantic class doesn't parameterize the argument name, so I think you will have to define your own class. Something like this:

struct my_arg_type
    : public boost::program_options::typed_value<int>
{
    my_arg_type(std::string const& name)
        : boost::program_options::typed_value<int>(&my_value)
        , my_name(name)
        , my_value(0)
    {
    }
    std::string name() const { return my_name; }
    std::string my_name;
    int my_value;
};

boost::program_options::options_description desc;

my_arg_type arg("foo");
desc.add_options()
    ("width", &arg, "give width");

This should give something like:

--witdh foo    give width

Answer 3

The approach given by Codebender is the only one you can use. This is actually intentional -- using "NUM" for name of argument appears to be micro-optimization that is not worth doing. A good description of the option should also say what kind of argument is expected.