How to serialize nested classes to semi-flat JSON object?

[jlconlin]

• Describe what you want to achieve.
  I'm trying to serialize my objects. They look like this (overly simplified):

struct A{
 double Aa;
 double Ab;
};

struct B{
  int Ba;
  int Bb;
};

struct super{
  A myA;
  B myB;
};

I'm trying to create a JSON object like this:

super = {
 "Aa": 1.0,
 "Ab": 2.0,
 "Ba": 3,
 "Bb": 4
}

• Describe what you tried.
  I have functions like this:

inline void to_json( nlohmann::json& JSON, const A& a ) {
  JSON = { "Aa", a.Aa, "Ab", a.Ab };
}
inline void to_json( nlohmann::json& JSON, const B& b ) {
  JSON = { "Ba", b.Ba, "Bb", b.Bb };
}
inline void to_json( nlohmann::json& JSON, const super& sup ) {
  JSON = {
   sup.myA, 
   sup.myB
  };
}

I'd like to have a single object with keys: "Aa", "Ab", "Ba", "Bb". Instead I get an array of two objects:

super = [
  { "Aa": 1.0, "Ab": 2.0 },
  { "Ba": 3, "Bb": 4 }
]

Please help me know what I'm doing wrong.

• Describe which system (OS, compiler) you are using.
  LLVM 3.8.1

• Describe which version of the library you are using (release version, develop branch).
  Fairly recent from master branch.

[nlohmann]

The braces are interpreted as an array (there is no real nice syntax for C++...). You need additional braces - a list of pairs is interpreted as object:

inline void to_json( nlohmann::json& JSON, const A& a ) {
  JSON = { {"Aa", a.Aa}, {"Ab", a.Ab} };
}
inline void to_json( nlohmann::json& JSON, const B& b ) {
  JSON = { {"Ba", b.Ba}, {"Bb", b.Bb} };
}
inline void to_json( nlohmann::json& JSON, const super& sup ) {
  JSON = sup.myA;
  JSON.update(sup.myB);
}

Function update is used to merge two objects.

[jlconlin]

update is exactly what I needed. Thanks!