Json: Serializing a vector of classes?

[DimosthenisK]

I have a vector* of class* (vector doctors) and I need to serialize it. The class Doctor has some other vector of class* that have to be serialized too. The question is, do I need to create to_json and from_json methods for vector*, and for any other vectors of classes inside Doctor? I have already created the methods for the class doctor and for every other class that needs it, yet I keep getting the "could not find to_json() method in T's namespace".

I am using MSVS 2017. Thank you in advance for your help!

[nlohmann]

Are the methods in the right name space? Can you try without any pointers just to make sure this works?

[DimosthenisK]

It does work when the vector is for the class Doctor and not for a pointer.

[nlohmann]

Maybe @theodelrieu has an idea.

[theodelrieu]

Pointer to user-defined types conversion is not supported by the library.

But you can do it, you have to specialize adl_serizalizer on pointer types (I would recommend supporting Doctor* to begin with though).

[nlohmann]

Thanks @theodelrieu ! We should put this into the README at some point...

[theodelrieu]

In fact it's way simpler, for a single type at least:

namespace my_ns {
  void to_json(json& j, const Doctor* doc) {}
}

Never thought about that before... So if you want to support both Doctor* and Doctor&, you have to write two to_json overloads.

[DimosthenisK]

But I have more vector to class pointers inside the Doctor class that also need serialization. I have to create a to_json and from_json overload for each of them?

Thank you for the great help btw!

[theodelrieu]

I have to create a to_json and from_json overload for each of them?

Absolutely, that's a bit tedious though. I don't know what is your exact use-case for having vectors of pointers, but I would advise not doing that if you can avoid it.

Unless you're using base classes and inheritance, storing Doctor instead of Doctor* gives you back a real copy (value semantics, instead of reference semantics).

[DimosthenisK]

I am using inheritance and I am sadly required to have pointers instead of values. I will try to create the methods for class pointers and will report back.

[nlohmann]

@voidblaze Any news on this?

[DimosthenisK]

Not yet, I will have more soon.

[DimosthenisK]

Sadly same error. I created the to_json methods for every class* and I still get that the to_json method doesnt exist.

It would be very nice if there was a way for me to check where the program hangs instead of the not-so-useful "T's namespace" that I get.

[nlohmann]

Unfortunately, the current error is already an improvement, because it is very difficult to indicate errors there. Can you share some code so we can better understand what's going wrong?

[DimosthenisK]

For sure. I uploaded the project here https://github.com/voidblaze/CPP-Appointment-Scheduler.

All the methods lie inside the DataHandler class.

[theodelrieu]

I could compile the project with latest GCC on Linux (with -fpermissive though, there are some errors unrelated to json).

[DimosthenisK]

Yeah sorry, I forgot to uncomment the line that was causing the error. Its on data handler, line 191. I believe the errors have something to do with the file opening on line 188, on a path that obviously doesn't exist on your machine.

I pushed the "fixed" project.

[theodelrieu]

You're trying to serialize a std::vector<...>**, I would advise passing such argument by reference in the first place.

Changing the problematic line to json jDoctors = *doctors makes the project compile.

Personally, I would keep pointers for virtual classes, and use references everywhere else.

[DimosthenisK]

Ah im dumb! Thanks a lot. Was that the problem from the start or were the class* json methods required?

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[theodelrieu]

It was only a part of the issue, you had to write the methods as well.

[DimosthenisK]

Okay then. Be sure to add that to the documentation if possible, someone may need it someday. Thanks again for your great help!

[nlohmann]

Thanks @theodelrieu for helping out!

@voidblaze What would you add to the documentation?