operator== and GetTypeHash() proper implementation

Development Programming & Scripting C++
1

I have a Struct FCombinableKey which contains two Items.
this struct should be used as a Key for Map.

The struct should be equal when the two items are equal, regardless of their order.
This is my struct:



USTRUCT(BlueprintType)
struct FCombinableKey {
GENERATED_BODY()

public:
 UPROPERTY(EditAnywhere, BlueprintReadWrite)
 UItem* ItemA;
 UPROPERTY(EditAnywhere, BlueprintReadWrite)
 UItem* ItemB;

 bool operator== (const FCombinableKey& Other)
 {
  return ((this->ItemA == Other.ItemA) && (this->ItemB == Other.ItemB)) || ((this->ItemA == Other.ItemB) && (this->ItemB == Other.ItemA));
 }
 friend uint32 GetTypeHash(const FCombinableKey& Other)
 {
  return GetTypeHash(Other.ItemA) + GetTypeHash(Other.ItemB);
 }
};

However, A TMap only returns the object if the Combinable Key is completely equal:

unknown.png
unknown.png1109×509 109 KB

unknown.png

Can someone tell me how to fix this?

2

After some debugging I found out that the Has Value is correct, but the == operator is not being called!

For both Array.Find() and Map.Find() the operator isnt being called

3

Can’t remember the details of how it works offhand, but I think for blueprints, those C++ methods are unknown and the comparisons will be done via the reflection system by default. Which would be per-property, hence the behaviour.

I think this can be overridden. Have a search in the engine codebase for CppStructOps. It’s a way of specifying custom C++ behaviour that the reflection system can access. I suspect there is one for equality testing.

4

I tried using C++ code (hiding Map.find behind a C++ function) but that doesnt seem to work either.

5

Try adding this right below the declaration.



template<>
struct TStructOpsTypeTraits<FCombinableKey> : public TStructOpsTypeTraitsBase2<FCombinableKey>
{
    enum
    {
        WithIdenticalViaEquality = true,
    };
};

I’m not 100% certain but I think without this, it will just do memcmp to see if they are identical.

6

Your == operator is just outsourcing the check to UItem’s == operator (that you might not have implemented).
It’s just better to give your UItem class a FGuid Id which you can compare within operators or simply compare UItems by FName there.

7

UItem is just a DataAsset, which means same pointer means same item.
Also thats not the problem. the == operator is never called as stated in my second post.

the solution from TheJamsh seems to be solving my issue though.

8

lol that is very weird behavior, operator not called.

9

Doesnt seem too weird. Multiple people seem to have the same problem.