如何使用 boost：：json：：value 调用无参数函数

我正在修改here代码:

#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/json.hpp>
#include <boost/type_traits.hpp>
#include <boost/utility/string_view.hpp>
#include <stdexcept>
#include <string>
#include<iostream>
template<class C1, class C2, class R, class... A, std::size_t... I>
boost::json::value
call_impl_(C1& c1, R(C2::* pmf)(A...), boost::json::array const& args,
  std::index_sequence<I...>)
{
  return boost::json::value_from(
    (c1.*pmf)(
      boost::json::value_to< boost::remove_cv_ref_t<A> >(args[I])...));
}

template<class C1, class C2, class R, class... A>
boost::json::value
call_impl(C1& c1, R(C2::* pmf)(A...), boost::json::array const& args)
{
  if (args.size() != sizeof...(A))
  {
    throw std::invalid_argument("Invalid number of arguments");
  }

  return call_impl_(c1, pmf, args, std::index_sequence_for<A...>());
}

template<class C>
boost::json::value
call(C& c, boost::string_view method, boost::json::value const& args)
{
  using Fd = boost::describe::describe_members<C,
    boost::describe::mod_public | boost::describe::mod_function>;

  bool found = false;
  boost::json::value result;

  boost::mp11::mp_for_each<Fd>([&](auto D) {

    if (!found && method == D.name)
    {
      result = call_impl(c, D.pointer, args.as_array());
      found = true;
    }

    });

  if (!found)
  {
    throw std::invalid_argument("Invalid method name");
  }

  return result;
}

struct Object
{
  std::string greet(std::string const& who)
  {
    return "Hello, " + who + "!";
  }

  int add(int x, int y)
  {
    return x + y;
  }
  int foobar()
  {
    std::cout << "I'm stupid!" << std::endl;
    return 1;
  }
};

BOOST_DESCRIBE_STRUCT(Object, (), (greet, add, foobar))

int main()
{
  Object obj;
  std::cout << call(obj, "greet", { "world" }) << std::endl;
  std::cout << call(obj, "add", { 1, 2 }) << std::endl;
  boost::json::value sc{};
  std::cout << call(obj, "add", sc) << std::endl;
}

我添加的部分是

int foobar()
{
    std::cout << "I'm stupid!" << std::endl;
    return 1;
}

和

boost::json::value sc{};
std::cout << call(obj, "foobar", sc) << std::endl;

输出:

error C2672: 'call_impl': no matching overloaded function found

I know I can add an overload of call_impl 和 modify call to:

  boost::mp11::mp_for_each<Fd>([&](auto D) {
    if (!found && method == D.name)
    {
      auto temp = args.as_array();
      std::cout << typeid(temp).name() << std::endl;
      if (!temp.empty())
      result = call_impl(c, D.pointer,temp );
      else
        result = call_impl(c, D.pointer);
      found = true;
    }});

我的问题是:

1. 如何构造包含空值的boost::json::value，以便我可以使用它调用该函数.

I think I've described my problem clearly, so many of my attempts 和 manipulations are so wrong that they don't need to be written, but it still warns me "It looks like your post is mostly code; please add some more details." ...

OS:windows11
IDE:visual studio2019