> On Apr 4, 2016, at 4:57 PM, Jonathan Hull via swift-users
> <[email protected]> wrote:
>
> If this enum is going to be used by tens/hundreds of thousands of structs, am
> I actually saving any space by breaking out the rarer cases which store more
> data or is the footprint just equal to the largest case?
Enums are implemented a lot like C unions, so the size is equal to that of the
largest case, plus a byte for the tag.
It’s interesting that the size in Daniel’s example is 41 bytes. That implies
that the tag is placed at the end of the data, because putting a 1-byte field
at the beginning would require 7 extra bytes of padding before the first Int.
But the stride still needs to be 48 bytes to preserve the 8-byte-alignment of
subsequent values.
This implies that you might be able to make an enum use up to 7 fewer bytes in
practice, if you can make its largest case be one byte smaller than a multiple
of a power of 2. Obviously not always attainable, but if you’re really
groveling for space improvements you might be able to squeeze out that extra
byte.
—Jens
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