Parse nested json to TMap in Unreal Engine 4 - c++

Question

I have a nested json with the following structure within UE4:
{
"boundary_points__per__cell": {
"0": [{
"x": 16.451330042628662,
"y": 2.0813326861577965
},
{
"x": 16.755262971791506,
"y": 2.0406535171257136
}
],
"1": [{
"x": -1.6378002918100634,
"y": 4.9689057223409412
},
{
"x": 0.9452331911724825,
"y": 6.1469903490163311
}
]
}
}
I tried using proposed techniques from other threads that suggest using the JsonObjectStringToUStruct. In particular I have the following structs to parse the json logic:
USTRUCT()
struct FBoundaryPoint
{
GENERATED_USTRUCT_BODY()
UPROPERTY()
int32 x;
UPROPERTY()
int32 y;
};
USTRUCT()
struct FBoundaryPoints
{
GENERATED_USTRUCT_BODY()
UPROPERTY()
TArray<FBoundaryPoint> BoundaryPoints;
};
USTRUCT()
struct FVoronoiStruct
{
GENERATED_USTRUCT_BODY()
UPROPERTY()
TMap<FString, FBoundaryPoints> boundary_points__per__cell;
};
However, although I can receive the key strings (e.g "0", "1") I am unable to extract the values of the json array. The boundary points dictionary contains only 2 keys ("0", "1") for the sake of the question. In a real scenario it will contain a lot more than the presented case.
Any help will be appreciated.

Answer 1

Firstly, those aren't integers, so you would probably want to use float or double instead of int32.
On the topic of why it won't parse, based on a comment by nick.shin:
the use of TMap is not applicable to JSON -- the simpler TArray must be used.
and, to give you the closest "TMap-like" functionality -- your JSON will need to look like:
{
"boundary_points__per__cell": [
{ "key":"0", "value":[{ "x":16.451330042628662, "y":2.0813326861577965 }] },
{ "key":"1", "value":[{ "x":16.755262971791506, "y":2.0406535171257136 }] }
]
}
When using JsonObjectConverter -- the engine will NOT allow you to make up a "key" name "on-the-fly" -- you need to define it so the engine can make the properties accessible as standard C++ structs (which makes it possible to setup and be used with Blueprints).
having said that, it is possible to make your own key:value pairs -- but, you will need to use the more low level TJsonWriter and TJsonWriterFactory & TJsonReader and TJsonReaderFactory functions. you will need to hold the engine's hand when you fill your own USTRUCT that may be using TMap...
and then, your custom JSON parser and writer can read something like this:
{
"MyMap": [
{ "MyKey1", { "0", "bliz" } },
{ "MyKey2", { "1", "blaz" } }
]
}