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How To Use TMap?

TMap is exactly what I need considering it works similar to the stl::map. The issue is that it seemes to work a bit differently. I get that it sorts based on hashes. What do I need to implement for my custom struct to be used as a key? The documentation is less than helpful. https://docs.unrealengine.com/latest/INT/API/Runtime/Core/Containers/TMap/index.html

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asked Sep 21 '14 at 01:31 AM in C++ Programming

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MJ_01
822 38 39 240

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1 answer: sort voted first

Here you go:

 TMap<struct FBoneIndexPair, TArray<uint32> > VertexInfluenceMapping;
 
 
 /** 
 * A pair of bone indices
 */
 struct FBoneIndexPair
 {
     int32 BoneIdx[2];
 
     bool operator==(const FBoneIndexPair& Src) const
     {
         return (BoneIdx[0] == Src.BoneIdx[0]) && (BoneIdx[1] == Src.BoneIdx[1]);
     }
 
     friend FORCEINLINE uint32 GetTypeHash( const FBoneIndexPair& BonePair )
     {
         return FCrc::MemCrc_DEPRECATED(&BonePair, sizeof(FBoneIndexPair));
     }
 
     /**
     * Serialize to Archive
     */
     friend FArchive& operator<<( FArchive& Ar, FBoneIndexPair& W )
     {
         return Ar << W.BoneIdx[0] << W.BoneIdx[1];
     }
 };


I think the most important part of this struct is == operator.

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answered Sep 21 '14 at 08:49 AM

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Pierdek
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avatar image MJ_01 Sep 21 '14 at 04:51 PM

You are correct! I did not have the == operator implemented correctly.

avatar image j_zeitler Oct 17 '16 at 03:43 PM

Exactly what I needed! Made a version based on Cantor pairs without the deprecated cyclic redundancy thing. Perhaps a tiny bit less performant, but should work well for bounded indices like the ones above:

     friend FORCEINLINE uint32 GetTypeHash(const IndexPair& indexPair)
     {
         return (indexPair.key + indexPair.value) * (indexPair.key + indexPair.value + 1) / 2 + indexPair.value;
     }
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