Things to know about TMap

TMap supports multiple values per hash key

You may be concerned about collisions due to using 32-bit hash values for your TMap keys. Don’t be.

Consider: TMap<KeyType, ValueType>

UE5’s underlying TMap implementation considers not only the underlying 32-bit hash key, but also the result of the KeyType’s operator==.

This means if you have multiple ValueType values that hash to the same 32-bit value, you won’t see value collisions as long as the KeyType contains an operator== that takes the full value into consideration.

While it’s not ideal to have many values use the same hash key, it is also not catastrophic for infrequent key collisions to occur.

Sample code

Prove it to yourself using this code:

struct FMyKey
{
    uint32 ID;
    
    bool operator==(const FMyKey& Other) const
    {
        return ID == Other.ID;
    }
};

FORCEINLINE uint32 GetTypeHash(const FMyKey& Key)
{
    // Every entry in the map will use the same 32-bit hash value (zero)
    return 0;
}

TMap<FMyKey, int32> Map;

Map.Add(FMyKey(1), 1);  // set FMyKey 1 = value 1
Map.Add(FMyKey(2), 2);  // set FMyKey 2 = value 2
Map.Add(FMyKey(2), 3);  // set FMyKey 2 = value 3  <-- overwrite

for (const auto& Pair : Map)
{
    UE_LOG(LogTemp, Log, TEXT("%d, %d"), Pair.Key.ID, Pair.Value);
}

Result

1, 1
2, 3

As you can see, there are 2 entries in the map, even though the result of the 32-bit hash value is the same for both entries. Furthermore, the FMyKey(2) value was overwritten with value 3 as you would expect.

What constitutes a unique entry in the TMap is actually the combination of the hash value AND the operator== result for the KeyType.

Thank you!

Thanks to Brian | PDS on the Unreal Source Discord for the sample code that illustrates how TMap actually works when there are hash collisions.