Issue with json_fwd.hpp ???

[arsenhakobyan]

Please advise what is wrong with my case that does not work.

I have a extracted this minimal example to show the issue:
in my project folder (name is 'ttt'):

ttt$ ls
config.cpp  include  main.cpp


ttt$ ls include/
config.h  nlohmann

ls include/nlohmann/
json.hpp  json_fwd.hpp <<<<<These are copied from the 3.11.2 https://github.com/nlohmann/json/releases/tag/v3.11.2


ttt$ cat main.cpp 
#include "config.h"
#include <iostream>

int main() {
  Config c("../global_config.json");
  std::cout << c.getParcelCount() << std::endl;
return 0;
}



ttt$ cat config.cpp 
#include <nlohmann/json.hpp>
#include "config.h"
#include <fstream>

Config::Config(std::string config_path) {
  std::ifstream f(config_path);
  config_ = json::parse(f);
}




ttt$ cat include/config.h 
#include <nlohmann/json_fwd.hpp>

using json = nlohmann::json;

class Config {
  private:
	  json config_;

  public:
    Config(std::string path);
    int getParcelCount() {
      return config_.value("parcel_count", 3);
    }

};

Running with following command:

g++ config.cpp main.cpp -std=c++17 -I include/

I am getting following errors:

In file included from main.cpp:1:0:
include/config.h:7:9: error: field ‘config_’ has incomplete type ‘json {aka nlohmann::json_abi_v3_11_2::basic_json<>}’
    json config_;
         ^~~~~~~
In file included from include/config.h:1:0,
                 from main.cpp:1:
include/nlohmann/json_fwd.hpp:151:7: note: declaration of ‘using json = using json = class nlohmann::json_abi_v3_11_2::basic_json<> {aka class nlohmann::json_abi_v3_11_2::basic_json<>}’
 class basic_json;
       ^~~~~~~~~~

I am using g++ 7.5.0 version on Jetson Xavier NX with JetPack 4.6.3

g++ (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04) 7.5.0

If more information is needed please let me know.

[gregmarr]

You can't use json_fwd.hpp in a header that is defining a class containing a member of type nlohmann::json. The full definition must be available for that, for various reasons. You can only use it for headers containing functions that take parameters of type nlohmann::json.

If you don't want to include json.hpp in the config.h, then you need to change json config_; to json *config_; and hold a pointer to an object instead of the actual object.