[GitHub] [arrow] sanjibansg commented on pull request #14758: ARROW-18119: [C++] Utility method to ensure an array object meetings an alignment requirement

["sanjibansg (via GitHub)" <gi...@apache.org>]

sanjibansg commented on PR #14758:
URL: https://github.com/apache/arrow/pull/14758#issuecomment-1473639765

   > In most cases the alignment will already be correct. We want this method to be really fast in that scenario. So this means that methods should look like...
   > 
   > ```
   > std::shared_ptr<Array> Foo(std::shared_ptr<Array> thing) {
   >   if (needs_alignment(...)) {
   >     // a copy needs to be made
   >   } else {
   >     return std::move(thing);
   >   }
   > }
   > ```
   > 
   > This way we can call it like so...
   > 
   > ```
   > std::shared_ptr<Array> aligned = EnsureAlignment(std::move(unaligned), ...);
   > ```
   > 
   > Then, if `unaligned` is already aligned it will be a straight move into `aligned` without ever making any copies (not even copies of `shared_ptr`)
   
   The current implementation might be a bit complicated, so comments and suggestions for optimized approaches will be very helpful. Thanks in advance! 
   
   Currently, we have `Check` functions for various Arrow objects, which check their constituents for alignments. For `ChunkedArray`, `RecordBatch`, and `Table`, we use a boolean vector to track the constituents which require alignment thus avoiding unnecessary function calls.


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