NLOHMANN_DEFINE_TYPE_* does not provide definitions for ordered_json

[MHebes]

What is the issue you have?

When using the macros suggested here to automatically create serialization methods for structs, to_json is only defined with a concrete json type.

Please describe the steps to reproduce the issue.

1. Create a struct.
2. Add to_json and from_json methods automatically using the macro.
3. Attempt to initialize a ordered_json with the struct.

Can you provide a small but working code example?

#include "json.hpp"

typedef struct struct_t {
  int c = 1;
  int b = 2;
  int a = 3;

  NLOHMANN_DEFINE_TYPE_INTRUSIVE(struct_t, c, b, a);
} struct_t;

int main() {
  struct_t str;

  nlohmann::json j = str; // fine
  // nlohmann::ordered_json j = str; // error: not assignable

  std::cout << j.dump(4) << std::endl;
}

What is the expected behavior?

The assignment to the ordered_json should behave in the same way as assignment to json.

And what is the actual behavior instead?

The code snippet does not compile when the commented line is used.

Workaround

Define to_json and from_json manually:

  friend void to_json(nlohmann::ordered_json &j, const struct_t &t) {
    j["c"] = t.c;
    j["b"] = t.b;
    j["a"] = t.a;
  }

Or use this macro instead:

#define AS_JSON(Type, ...)                                                     \
  friend void to_json(nlohmann::ordered_json &nlohmann_json_j,                 \
                      const Type &nlohmann_json_t) {                           \
    NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))   \
  }                                                                            \
  friend void from_json(const nlohmann::ordered_json &nlohmann_json_j,         \
                        Type &nlohmann_json_t) {                               \
    NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) \
  }

Suggested fix

Either add ordered_json overloads in the macro itself, or possibly do some template magic to use basic_json in the parameter list.

Less ideal fix

Add a new macro specific to ordered_json

Which compiler and operating system are you using?

• Compiler: Microsoft Visual C++ 2019
• Operating system: Windows

Which version of the library did you use?

• latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

P.S. Love the library :)

[ScottHutchinson]

I had the same issue, and adding this macro seems to fix it.

/*!
@brief macro added to work with ordered_json -- hutch
@def NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_UNORDERED
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_UNORDERED(Type, ...)  \
    inline void to_json(nlohmann::ordered_json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
    inline void from_json(const nlohmann::ordered_json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }

[nlohmann]

Thanks for sharing - this should also be fixed with #2532.