[swift-corelibs-dev] Question about NSDecimalCopy

[Tony Parker]

Hi Morten,

> On Jan 9, 2017, at 2:16 AM, Morten Bek Ditlevsen via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> Hi list,
> I was looking at the NSDecimal.swift to learn a bit about the implementation of the Decimal type.
> I get that a lot of the API in this type exists for compatibility reasons with the C implementation.
> 
> But I was wondering about how the NSDecimalCopy function differs from a simple assignment of one Decimal value to another.
> 
> NSDecimalCopy simply assigns each field of the struct one by one, so when would it ever be preferable to performing a simple assignment?
> 
> So basically my question is - is there any difference in the result of performing:
> 
> var a: Decimal = 1
> var b: Decimal = 2
> NSDecimalCopy(&b, &a)
> 
> and 
> var a: Decimal = 1
> var b: Decimal = 2
> b = a
> 

I suspect that this is just due to some logic copying from the C version.

I’m actually not sure what the compiler does here. It should be possible to try it out in a test app, disassemble it, and check out the result. Let us know what you find. =)

- Tony

> Secondly - if there is no difference in the result - is it not more performant to assign the value directly?
> 
> This question also raised a question about Swift internals: If you have a pure value type (no nested reference types): Does an assignment then simply copy a chunk of memory with the size of the struct? Or is each field assigned/copied recursively?
> 
> Sincerely,
> /morten
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