//test cpp:

    FBox SelectionBox(FVector(0.f, -4000.f, 0.f), FVector(3000, 5000, 600.f));
    FVector BoxExtent = SelectionBox.GetExtent();
    FVector BoxCenter = SelectionBox.GetCenter();
    TArray<int32> InstancesDetected = George_GetInstancesOverlappingBox(ISM1, SelectionBox, ISM2->GetComponentTransform());
    for (int32 i = 0; i < InstancesDetected.Num(); ++i) {
    FTransform T3;
    BenchEnemy->HISM2->GetInstanceTransform(InstancesDetected[i], T3, true);
    DrawDebugSphere(GetWorld(), T3.GetLocation(), 100.f, 8, FColor::Yellow, false, 6.f, 0, 6.f);

}


//.h

    TArray<int32> George_GetInstancesOverlappingBox(UInstancedStaticMeshComponent* MeshComponent, FBox& Box, FTransform BoxTransform);



//cpp

TArray<int32> ABenchmark::George_GetInstancesOverlappingBox(UInstancedStaticMeshComponent* MeshComponent, FBox& Box, FTransform BoxTransform)
{
    TArray<int32> Result;
    if (const UStaticMesh* Mesh = MeshComponent->GetStaticMesh())
    {
        const FVector StaticMeshBoundsExtent = Mesh->GetBounds().BoxExtent;
        for (int32 Index = 0; Index < MeshComponent->PerInstanceSMData.Num(); Index++)
        {
            const FMatrix& Matrix = MeshComponent->PerInstanceSMData[Index].Transform;
            const FVector InstanceOrigin = Matrix.GetOrigin();
            const FVector PositionInOrientedBoxSpace = BoxTransform.InverseTransformPosition(InstanceOrigin);
            if (Box.IsInside(PositionInOrientedBoxSpace))
            {
                Result.Add(Index);
            }
        }
    }
    return Result;
}