Hello,
I was upgrading my MegaBoids plugin ( MegaBoids - A general purpose group AI movement plugin for MassEntity | Fab ) for 5.7 and a significant change to MassEntity memory safety checks has me scratching my head a little bit.
The change has to do with regular fragments and the fact they are being copied around with Memcpy if they change archetype at runtime or if some other entities get moved within the same chunk. To ensure the fragment data remains valid, a check on trivially copyable types was added in 5.7 and a trait (struct TMassFragmentTraits<>) was also introduced to signify it was developer validated as being safe, when needed. This is all good for memory safety and data coherence when copying the fragments around. Fine.
These checks however raise issues for using TArray within a fragment. We also encounter this problem with using FInstancedStruct. Both are not trivially copyable because they create copies of their respective allocated memory when copied. However, since MassEntity doesn’t destroy the fragment when Memcpying them, they are also safe to Memcpy as far as I can tell since they will simply copy the allocated memory pointer until they are effectively destroyed, at which point the allocation will get Freed.
So my question is twofold.
First, is my above understanding right? Should we be able to use TArray or FInstancedStruct within a fragment safely by specifying ‘AuthorAcceptsItsNotTriviallyCopyable = true’? I see this is done in ‘FMassReplicationViewerInfoFragment‘ for TArray.
Second, how can one tell MassEntity to use the copy operator instead of Memcpy? I don’t think this is possible at the moment (and I don’t need it either) but would it be possible to reuse TMassFragmentTraits to declare a fragment type as requiring use of the copy operator? Something like ‘TMassFragmentTraits<>::UseCopyOperator’ could open the possibility to voluntarily use a less performant but necessary part of C++.
Thanks