Error: "tuple_size" has already been declared in the current scope

[gregha]

I added the following lines of code to my current project:

#include <nlohmann/json.hpp>
using json = nlohmann::json;
// ...
void Model::initJSONData(json::json &json_data, std::string path_to_file)
{
  std::ifstream json_file_stream(path_to_file.c_str());
  if(!json_file_stream.is_open())
    throw std::runtime_error("JSON file DTModelDatabase.json could not be opened.");
  json_file_stream >> json_data;
  json_file_stream.close();
}

However, I get the following error:

".../include/json/include/nlohmann/detail/iterators/iteration_proxy.hpp", line 151: error: 
          "tuple_size" has already been declared in the current scope
  class tuple_size<::nlohmann::detail::iteration_proxy_value<IteratorType>>
        ^

As can be seen, the error originates from the header-file iteration_proxy.hpp. Is it possible that there is a conflict with the already defined tuple_size in std? I run a Red Hat Enterprise Linux with GCC 6.3.0 (c++11 is set). Can anybody help me out with this error?

[nickaein]

I've tested your snippet on my machine (GCC 7.3) and it worked. I just had to remove json:: prefix from json_data argument.

I've also tested it on GCC 6.3 and GCC 6.2 on godbolt and it also works. You can verify the godbolt instance here: https://godbolt.org/z/oM7Vs8.

Maybe there is something else that are interfering which is not present in this code snippet?

[gregha]

Thanks for testing my code snippet. I tested the project I am working on now for different versions of this JSON library, producing even other errors like:

"/home/ceres_ns_scratch5/nxf36874/TAG180628_AHB/ss_solver_new_json/include/json/include/nlohmann/detail/conversions/to_json.hpp", line 301: error: 
          too few arguments for class template "std::pair"
  void to_json(BasicJsonType& j, const std::pair<Args...>& p)
                                                        ^

Thus, as you stated as well, the error is most probably not due to the library or due to the way I use it, but somewhere else in the project.