Can I display the default value and help message for each arguments?

[DapengFeng]

I find a similar library at https://github.com/morrisfranken/argparse?tab=readme-ov-file

It provides a feasible way to define default values and help messages.

[DapengFeng]

Similar issues: #16 #17

[DapengFeng]

Inspired by morrisfranken/argparse, I develop the similar function using template struct,

template <typename T>
struct arg
{
  T value_;
  std::string msg_;

  arg(const T& value = T(), const std::string& msg = "") : value_(value), msg_(msg) {}

  operator T&() { value_.msg_ = msg_; return value_; }

  friend std::istringstream& operator >>(std::istringstream& ss, arg& obj) { ss >> obj.value_; return ss;}
  
};

struct Options {
  struct Init : public structopt::sub_command
  {
    Init() {}

    friend std::istringstream& operator >> (std::istringstream& ss, Init& obj) {
      ss >> obj.path.value();
      return ss;
    }

    std::optional<arg<std::filesystem::path>> path = arg<std::filesystem::path>("init", "Path to the scene file");
    std::optional<arg<float>> scale = arg<float>(1.0f, "Scale of the scene");
    arg<float> density = arg<float>(float(), "Density of the scene");
  };

  Init init = arg(Init(), "Initialize a new scene");
};
STRUCTOPT(Options, init);
STRUCTOPT(Options::Init, path, scale, density);

the examples are shown as follow:

./main -h
USAGE: main [FLAGS] [SUBCOMMANDS] 

FLAGS:
    -h, --help Prints help information

    -v, --version Prints version information


SUBCOMMANDS:
    init : Initialize a new scene

and the subcommand mode

./main init -h
USAGE: init [FLAGS] [OPTIONS] density 

FLAGS:
    -h, --help Prints help information

    -v, --version Prints version information


OPTIONS:
    -p, --path Path to the scene file (default: "init")

    -s, --scale Scale of the scene (default: 1)


ARGS:
    density : Density of the scene