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Using FOnlineJsonSerializable on a struct


I'm trying to serialize a very simple struct, but I'm guessing UE4's preproccessor scripts are borking this.

My struct:

 struct FInventoryItemEntry : public FOnlineJsonSerializable
     UPROPERTY(EditDefaultsOnly, BlueprintReadWrite, Category = "Inventory")
     FString ItemId;
     UPROPERTY(EditDefaultsOnly, BlueprintReadWrite, Category = "Inventory")
     int32 Quantity;
     FInventoryItemEntry() { }
     FInventoryItemEntry(FString id)
         ItemId = id;
         Quantity = 1;
     FInventoryItemEntry(FString id, int32 count)
         ItemId = id;
         Quantity = count;
         ONLINE_JSON_SERIALIZE("ItemId", ItemId);
         ONLINE_JSON_SERIALIZE("Quantity", Quantity);

My compilation error:

 Inventory.h(59): error : In Inventory: 'struct': Can't find struct 'FOnlineJsonSerializable'

Just below this struct I have a class that extends FOnlineJsonSerializable, and that compiles just fine, so it definitely has a reference to FOnlineJsonSerializable, I'm guessing this is a bug with the preprocessors.

Has anyone seen this or found a workaround? This is currently blocking me, and I'd rather not have to write a custom serializer method.

Thanks for any help!

Product Version: Not Selected
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asked Apr 11 '15 at 12:50 AM in C++ Programming

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avatar image phil.harlow Apr 13 '15 at 06:01 PM


avatar image phil.harlow Apr 13 '15 at 11:29 PM

Hello? Epic?

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2 answers: sort voted first

Yay for Epic's wonderful support. Just switched to using a UObject instead. Yucky, wasteful hack for what we need, but quicker than waiting for a reply.

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answered Apr 22 '15 at 11:50 PM

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This is an old question now, but I ran into this issue and found the problem and an alternate solution. The issue is that the type is not a USTRUCT, and so attempting to make a new USTRUCT that inherits from it will confuse the preprocessor.

TL;DR: It can't find the struct because it hasn't preprocessed it.

If you absolutely need the type to take advantage of the preprocessor (eg. make it exposed to Blueprint), you can make it a UObject inheriting class. Eg.

 class UMyJsonSerializable : public FOnlineJsonSerializable, public UObject

But if you don't need your type to take advantage of the preprocessor, just making it a regular old struct will do the job. Eg.

 struct FMyJsonSerializable : public FOnlineJsonSerializable

You can still use it as you would any other C++ struct, because that's exactly what it is. You don't even have to use the prefix if you don't want to, the preprocessor won't enforce it.

Hope this helps someone, happy coding.

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answered May 18 '16 at 06:15 PM

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