[GitHub] [arrow] dchigarev commented on pull request #8417: WIP: [C++] Get rid of code duplication in Decimal##bit_width

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dchigarev commented on pull request #8417:
URL: https://github.com/apache/arrow/pull/8417#issuecomment-707133198


   @emkornfield thanks for your comment! 
   
   1. I've changed this PR a bit and get rid of declaring API via macroses. I've left it only at declaring decimals at [`type_fwd.h`](https://github.com/dchigarev/arrow/blob/4346e4e7553af0ef053668c24ca32df61a8deeb1/cpp/src/arrow/type_fwd.h#L147-L157) and [`type_traits.h`](https://github.com/dchigarev/arrow/blob/4346e4e7553af0ef053668c24ca32df61a8deeb1/cpp/src/arrow/type_traits.h#L285-L297) because these files [already using](https://github.com/dchigarev/arrow/blob/4346e4e7553af0ef053668c24ca32df61a8deeb1/cpp/src/arrow/type_fwd.h#L183-L202) this pattern to declare alike classes, if this is acceptable I can leave macroses instead of code duplicating at these two files.
   
   2. These changes should not break `BasicDecimal` at Gandiva, because everything that was added to base decimals is [`decimal_meta.h`](https://github.com/dchigarev/arrow/blob/d256/cpp/src/arrow/util/decimal_meta.h) which I guess do not have references to anything that could break Gandiva
   
   Moving forward, I think that it could be useful if we'll declare some interface for `BasicDecimal` and `Decimal` classes, which will be implemented by `BasicDecimal/Decimal128, 256, 64 ...`. For now, amount of methods in BasicDecimal128 and 256 are noticeably different, is it a kind of TODO to implement in 256 all methods that 128 has, or 256-bit decimal will have less methods on purpose? If it is, is there a list of basic methods that any decimal must have and whose declaration could be moved to a decimal interface?
   
   


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