[swift-users] Mapping C semantics to Swift

[Matthew Johnson]

> On May 20, 2016, at 12:54 PM, Joe Groff via swift-users <swift-users at swift.org> wrote:
> 
> 
>> On May 20, 2016, at 7:33 AM, Ken Burgett via swift-users <swift-users at swift.org> wrote:
>> 
>> I have been told that the in-memory layout of a Swift struct is 'undefined', so the C style of aliasing an array over a struct and iterating across the struct is a definite no-no.  Not surprising, since aliasing violates all type-safe rules.
>> 
>> So, if you have this situation, you must address it on a case-by-case basis.  In my case, converting a hash algorithm from C to Swift, the required C struct morphs into a Swift class, with a member function 'as_C_byte_array' which answers an array of the required form.  Internally, this function will shift and mask class attributes in order to build the necessary result.
> 
> That's not necessary. You can leave the struct defined in C and import it into Swift. Swift will respect C's layout.

Any thought on introducing an attribute or something into the “generated header” for these C imports to make this more clear?  The same syntax could be used in the future if / when Swift supports more control over layout.

> 
> -Joe
> 
>> Ugly, but I must preserve the semantics exactly if I hope to generate the exact same hash as the C version.
>> 
>> -- 
>> Ken Burgett
>> Principal Software Engineer
>> Email: kenb at iotone.io
>> Office: 530.693.4449
>> Mobile: 831.332.6846
>> URL: www.iotone.co
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