Generic json

[laurentcau]

Hi, I thought it was possible to use a generic way to load json (like a DOM) with your library. But it doesn't seem to be as simple as I thought. I tried something like that:

struct Value: public std::variant<bool, double, std::string> {};
struct GenericJSon : public std::map<std::string, std::variant<Value, std::vector<Value>, GenericJSon>> {};

std::string json_string(
"\
    {\
        \"toto\":\"titi\" \
        , \"float\":2.0 \
        , \"int\":2 \
        , \"intV\":[1,2,3]\
        , \"floatV\": [1.1, 2.2, 3.3] \
        , \"sturct\": {\"a\":1.1, \"b\":2.2} \
        , \"sturctV\": [{\"a\":1.1, \"b\":2.2}, {\"c\":true, \"d\":\"tutu\"}] \
        , \"sturctNestedV\": [{\"a\":1.1, \"b\":[{\"a\":1.1}, {\"b\":1.1}]}, {\"c\":true, \"d\":\"tutu\"}] \
    }\
"
        );
    auto generic = rfl::json::read<GenericJSon>(json_string).value();

It partially works without recursive definition (GenericJSon) but it fails to compile with it. Do you know how to fix this or do you have other idea to achieve this?

[liuzicheng1987]

@laurentcau , you are on the right track.

But that won't compile because of restrictions C++ imposes on you...

struct GenericJSon : public std::map<std::string, std::variant<Value, std::vector<Value>, GenericJSon>>

A struct cannot inherit from itself. However, a struct can contain a pointer to itself:

https://github.com/getml/reflect-cpp/blob/main/docs/rfl_ref.md

And then you could define a ReflectionType, like this:

https://github.com/getml/reflect-cpp/blob/main/docs/custom_classes.md

I have actually been thinking about whether we should support something like this out-of-the-box, but so far it really hasn't been a top priority. But now that you have asked for it, I will think about it.

[laurentcau]

struct GenericJSon : public std::map<std::string, std::variant<Value, std::vector<Value>, GenericJSon>>

is valid c++. I have no problem to define and instantiate such object on VS2022. The issue is more in the rfl::json::read call that tries to count the number of fields that is infinite:

1>E:\build\unreal\external\include\rfl\internal\num_fields.hpp(145,24): error C2607: static assertion failed
1>(compiling source file '../../../../repos/eon/UnrealSandbox/Public/SDK/Core/Json/JsonTest.cpp')
1>E:\build\unreal\external\include\rfl\internal\num_fields.hpp(150,62):
1>while evaluating consteval function 'rfl::internal::CountFieldsHelper<T>::get_nested_base_field_count'
1>        with
1>        [
1>            T=GenericJSon
1>        ]
1>E:\build\unreal\external\include\rfl\internal\num_fields.hpp(150,62):
1>while evaluating consteval function 'rfl::internal::CountFieldsHelper<T>::get_nested_base_field_count'
1>        with
1>        [
1>            T=GenericJSon
1>        ]
1>E:\build\unreal\external\include\rfl\internal\num_fields.hpp(150,62):
....

[liuzicheng1987]

@laurentcau , here is an untested first draft:

#ifndef RFL_GENERIC_HPP_
#define RFL_GENERIC_HPP_

#include <map>
#include <string>
#include <variant>
#include <vector>

#include "rfl.hpp"

class Generic {
 public:
  using ReflectionType = std::variant<bool, double, int, std::string,
                                      std::vector<rfl::Ref<Generic>>,
                                      std::map<std::string, rfl::Ref<Generic>>>;

  Generic(const ReflectionType& _value) : value_(_value) {}

  ~Generic() = default;

  ReflectionType reflection() const { return value_; };

 private:
  ReflectionType value_;
};

#endif

Feel free to improve it and then open a PR with this as a new library feature.

[laurentcau]

@liuzicheng1987 It works well. A small improvement: the extra rfl::Ref is in fact not needed. It works well without (and remove extra pointer dereference when you want to retrieve the data). (We can also use unordered_map.) So

    using ReflectionType = std::variant<bool, int, double, std::string,
        std::vector<Generic>,
        std::unordered_map<std::string, Generic>>;

is working fine on my side. Thanks!

[laurentcau]

also, you need to declare "int" before "double" to make "int" detection work.

[liuzicheng1987]

@laurentcau, these are both good points.

This is just a first draft that I hacked into my keyboard within a minute or so.

Again, if you want to turn this into something more user-friendly and add it as an actual library feature, I would love to have it! Absolutely feel free to open a PR.