Cannot assign from ordered_json vector<CustomStruct> to value in not ordered json #2528

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1. kotori2

   added

   kind: bug

   on Dec 13, 2020
2. YarikTH commented on Dec 14, 2020

   

   YarikTH

   on Dec 14, 2020

   Last edited by YarikTH

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   It's a bug in your own code.
   https://godbolt.org/z/qvjdGW
   You defined to_json and from_json exactly for ordered_json& type, while try to use with json& type.
   j̶s̶o̶n̶ ̶a̶n̶d̶ ̶o̶r̶d̶e̶r̶e̶d̶_̶j̶s̶o̶n̶ ̶a̶r̶e̶ ̶n̶o̶t̶ ̶c̶o̶n̶v̶e̶r̶t̶i̶b̶l̶e̶ ̶t̶o̶ ̶e̶a̶c̶h̶ ̶o̶t̶h̶e̶r̶, so you don't write proper to_json overload to be used with json.
   Also, I don't get a part about "Actually it works if ..., or I removed the vector."
   https://godbolt.org/z/v8xrhM
   As you can see, vector or not, compilation failed, because you don't have to_json(nlohmann::json&, const releaseInfo&) overload.

   Actually, json and ordered_json are convertible to each other so you can construct ordered_json from your type and then use it as a field in json, But it's not scalable.
   https://godbolt.org/z/a8fhss

   int main() {
       std::vector<releaseInfo> test;
       ordered_json children = test;
       json parent;
       parent["test"] = children;
       return 0;
   }

   See a possible solution:
   https://godbolt.org/z/xMvaja

   #include <nlohmann/json.hpp>
   
   using nlohmann::json;
   using ordered_json = nlohmann::ordered_json;
   
   #define _NLOHMANN_DEFINE_TYPE_INTRUSIVE(JsonType, Type, ...)  \
       friend void to_json(JsonType& nlohmann_json_j, const Type& nlohmann_json_t){ \
           NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) \
       }
   
   
   class releaseInfo {
   public:
       uint64_t id = 0;
       std::string key;
       _NLOHMANN_DEFINE_TYPE_INTRUSIVE(nlohmann::json, releaseInfo, id, key);
       _NLOHMANN_DEFINE_TYPE_INTRUSIVE(nlohmann::ordered_json, releaseInfo, id, key);
   };
   
   int main() {
       std::vector<releaseInfo> test;
       json parent;
       parent["test"] = test;
       return 0;
   }

   As I can see NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE both don't work with ordered_json. And moreover, they don't work with any other basic_json overload.

   From the nlohmann/json point of view, it's kind of lack of feature.
   Ideally, NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE should work with any basic_json overload.
   Like here:
   https://godbolt.org/z/d16erK

   #define NLOHMANN_DEFINE_TYPE_INTRUSIVE_2(Type, ...)  \
       template<class JsonType> friend void to_json(JsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
       template<class JsonType> friend void from_json(const JsonType& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
   
   #define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_2(Type, ...)  \
       template<class JsonType> void to_json(JsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
       template<class JsonType> void from_json(const JsonType& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }

   Need to test new macroses more though. I wrote them in a few minutes. They may break ODR or something.

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4. kotori2 commented on Dec 15, 2020

   

   kotori2

   on Dec 15, 2020

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   @YarikTH Thanks for your kind and thorough explanation.

   Also, I don't get a part about "Actually it works if ..., or I removed the vector."

   I think my memory just corrupted...

   json and ordered_json are not convertible to each other
   Actually, json and ordered_json are convertible to each other so you can construct ordered_json from your type and then use it as a field in json, But it's not scalable.

   Actually it's kinda weird to me. Simply value assigning will work between json and ordered_json.

   json a;
   ordered_json b(a);
   json c(b);
   

   But when it comes to function calls, it doesn't work.
   Does it means there is explicit constructor for json -> ordered_json, and ordered_json -> json, but it doesn't apply for function calls?
   I tried to read the code but the tons of templates stopped me
5. YarikTH commented on Dec 15, 2020

   

   YarikTH

   on Dec 15, 2020

   Contributor

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   But when it comes to function calls, it doesn't work.
   Does it means there is explicit constructor for json -> ordered_json, and ordered_json -> json, but it doesn't apply for function calls?

   I'm not the author, but it seems it is.
   IMHO, in general, it's a good idea to declare to_json and from_json with templated BasicJsonType, as NLOHMANN_JSON_SERIALIZE_ENUM does. And not to mix json of different type, because it inconvenient at least and very likely does something unexprectable, because json values can't contain json values of another type. See example below:
   https://godbolt.org/z/e3PzYc

   #include <nlohmann/json.hpp>
   #include <iostream>
   
   using nlohmann::json;
   using ordered_json = nlohmann::ordered_json;
   
   #define _NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type, ...)  \
       friend void to_json(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 ordered_json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
   
   class releaseInfo {
   public:
       uint64_t id = 0;
       std::string key;
       std::string author;// field especialy intended to be alphabetically lower than others
       _NLOHMANN_DEFINE_TYPE_INTRUSIVE(releaseInfo, id, key, author);
   };
   
   int main() {
       std::vector<releaseInfo> test(1, releaseInfo{});
       ordered_json children = test;
       std::cout << "ordered children: \n" << children.dump(4);
       
       std::cout << "\n\n";
   
       json parent;
       parent["test"] = children;
       std::cout << "unordered parent: \n" << parent.dump(4);
       return 0;
   }

   Output:

   ordered children: 
   [
       {
           "id": 0,
           "key": "",
           "author": ""
       }
   ]
   
   unordered parent: 
   {
       "test": [
           {
               "author": "",
               "id": 0,
               "key": ""
           }
       ]
   }
   

   As you can see, values added in ordered_json being used in json become reordered.

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6. kotori2

   closed this as completedon Dec 16, 2020
7. nlohmann

   added

   solution: proposed fixa fix for the issue has been proposed and waits for confirmationa fix for the issue has been proposed and waits for confirmation

   on Jan 18, 2025
8. nlohmann

   linked a pull request that will close this issueGenerate template functions with NLOHMANN_DEFINE_TYPE macros #4597on Jan 18, 2025
9. nlohmann

   added this to the Release 3.11.4 milestone on Jan 18, 2025