C++ Nlohman::json third-party support not compiling

Question

I'm using nlohmann::json to (de)serialize some C++ objects in JSON. I haven't yet understood how to set up the code to work with a third-party library (SoapySDR). My code is in the global namespace in this example, while the SoapySDR code is in its own namespace. I'm getting a ton of compile errors with this simplified example:

#include "json.hpp"

using nlohmann::json;

namespace SoapySDR
{

  class Range
  {
  public:
    double minimum(void) const;
    double maximum(void) const;
    double step(void) const;

  private:
    double _min, _max, _step;
  };

  class ArgInfo
  {
  public:
    Range range;
  };

}; // namespace SoapySDR

void to_json(json &j, const SoapySDR::Range &r)
{
  j += {"min",r.minimum()};
  j += {"max",r.maximum()};
  j += {"step",r.step()};
}

void from_json(const json &j, SoapySDR::Range &r)
{
//  r = SoapySDR::Range(j.at("min"), j.at("max"), j.at("step"));
}

void to_json(json &j, const SoapySDR::ArgInfo &ai)
{
  j = json{{"range", ai.range}};
}

void from_json(const json &j, SoapySDR::ArgInfo &ai)
{
  j.at("range").get_to(ai.range);
}

These are the error messages, excluding all the extra information about attempted deductions:

bob.cpp:41:31: error: no matching function for call to ‘nlohmann::basic_json<>::basic_json(<brace-enclosed initializer list>)’
bob.cpp:41:31: note:   couldn't deduce template parameter ‘JsonRef’
bob.cpp:41:31: note:   couldn't deduce template parameter ‘BasicJsonType’
bob.cpp:41:31: note:   couldn't deduce template parameter ‘CompatibleType’
   j = json{{"range", ai.range}};
bob.cpp:46:32: error: no matching function for call to ‘nlohmann::basic_json<>::get_to(SoapySDR::Range&) const’
   j.at("range").get_to(ai.range);
json.hpp:19588:28: error: no type named ‘type’ in ‘struct std::enable_if<false, int>’
                    int > = 0 >
json.hpp:19613:11: note:   template argument deduction/substitution failed:
bob.cpp:46:32: note:   mismatched types ‘T [N]’ and ‘SoapySDR::Range’
   j.at("range").get_to(ai.range);

Answer 1

The to_json and from_json overloads for your data types need to be in the type's namespace for the library to find them:

namespace SoapySDR {

void to_json(json &j, const SoapySDR::Range &r)
{
  j += {"min",r.minimum()};
  j += {"max",r.maximum()};
  j += {"step",r.step()};
}

/* ... */

}

After that, your code compiles: https://godbolt.org/z/5cafYx