Setting required fields & optional fields in a type safe way

req_fields.cpp

#include <iostream>
#include <string>
#include <vector>

template <uint32_t N>
struct request;

template <>
struct request<0>
{
  constexpr static uint32_t OPT_FIELDS = 1 << 0;
  constexpr static uint32_t REQ_FIELD1 = 1 << 1;
  constexpr static uint32_t REQ_FIELD2 = 1 << 2;
  constexpr static uint32_t ALL_FIELDS = OPT_FIELDS | REQ_FIELD1 | REQ_FIELD2;

  std::string required1;
  std::string required2;
  std::vector<std::string> optionals;
};

template <uint32_t N>
struct request : request<N-1>
{
  using request<0>::REQ_FIELD1;
  using request<0>::REQ_FIELD2;

  request<N & ~REQ_FIELD1>& set_required_field1(const std::string& s)
  {
    this->required1 = s;
    return *this;
  }

  request<N & ~REQ_FIELD2>& set_required_field2(const std::string& s)
  {
    this->required2 = s;
    return *this;
  }

  request& set_optional_field(const std::string& s)
  {
    this->optionals.push_back(s);
    return *this;
  }
};

auto make_request()
{
  return request<request<0>::ALL_FIELDS>{};
}

template <uint32_t N>
auto send_request(const request<N>& req) = delete;

auto send_request(const request<request<0>::OPT_FIELDS>& req)
{
  std::cout << "Send request: "
     << "req1 = " << req.required1
     << ", req2 = " << req.required2
     << ", opts = [";
  for (const auto& o : req.optionals)
    std::cout << o << ',';
  std::cout << "]\n";
}

int main()
{
  auto req = make_request()
    .set_required_field1("foo")
    .set_required_field2("bar")
    .set_optional_field("baz")
    .set_optional_field("quux")
    .set_optional_field("wibble")
    ;

  send_request(req);
}