Serializing a Unreal Engine USTRUCT to JSON or deserializing JSON back to a USTRUCT is very painless thanks to the build-in FJsonObjectConverter class. The process is performed recursively without any extra effort as well as having the ability to include/exclude specific properties from the serialization. Below we’ll describe the requirements for using FJsonObjectConverter as well as a practical example.

Requirements

To use FJsonObjectConverter you will need to ensure that both Json and JsonUtilities are included within PublicDependencyModuleNames in your projects Build.cs, e.g.:

PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "HeadMountedDisplay", "NavigationSystem", "AIModule", "Json", "JsonUtilities" });

You’ll also need to include the JsonObjectConverter.h header in source files where you make calls to FJsonObjectConverter, e.g.

#include "Runtime/JsonUtilities/Public/JsonObjectConverter.h"

Example

The examples below will use this following struct as a guide:

USTRUCT()
struct FPlayer
{
  GENERATED_BODY()

  UPROPERTY()
  FString Name;

  UPROPERTY()
  int32 Level;

  UPROPERTY()
  TArray<FString> Friends;

  UPROPERTY(Transient)
  FString PropertyToIgnore;
};

Serialization

To serialize a struct to a JSON payload use FJsonObjectConverter::UStructToJsonObjectString. This method will operate recursively, so if you have anything liked nested structs, arrays inside of arrays inside of other structs it will operate as expected. Note that the PropertyToIgnore property above is marked as Transient, this will prevent the property from being serialized. For example:

FPlayer Player;
Player.Name = "Frank";
Player.Level = 3;
Player.Friends.Add("Jeff");
Player.PropertyToIgnore = "Gotta Go Fast";

FString JSONPayload;
FJsonObjectConverter::UStructToJsonObjectString(Player, JSONPayload, 0, 0);

Will result in the following JSON payload being written to the JSONPayload FString:

{
  "name": "Frank",
  "level": 3,
  "friends": ["Jeff"]
}

Deserialization

To deserialize a JSON payload back to a struct use FJsonObjectConverter::JsonObjectStringToUStruct. After this above serialization example this method works exactly how you would expect it to. For example:

FString JSONPayload = "...";  // The actual payload
FPlayer Player;

FJsonObjectConverter::JsonObjectStringToUStruct(JSONPayload, &Player, 0, 0);

// Player.Name == "Frank"
// Player.Level == 3
// etc..

Final Notes

You will have noticed above that we are passing two 0’s as the final arguments to both methods above. These 0’s are the CheckFlags and SkipFlags respectively. These can be used to provide more fine-graned control over what properties are either included (whitelisted) or skipped (blacklisted).

Above we used Transient to skip a property above without having to pass this value into SkipFlags. This is because FJsonObjectConverter uses Transient by default when no SkipFlags are specified (a very sane default).