[swift-users] Memory Address of value types and reference types

[somu subscribe]

Thanks a lot Andrew and Quinn,

After your explanation, I understand the following better now: 

- It makes sense to get the memory address of a reference type as it represents identity. Just realised there is also === identity operator to check if it is the same reference.

- The need to get the memory address of a value type might not be of the same significance (as it is a value) and the emphasis would be more on the equality of the value.

- And model based on whether the entity represents identity / value

Thanks and regards,
Muthu


> On 13 Sep 2017, at 1:19 AM, Andrew Trick <atrick at apple.com> wrote:
> 
>> 
>> On Sep 12, 2017, at 9:55 AM, somu subscribe via swift-users <swift-users at swift.org> wrote:
>> 
>> Hi Quinn,
>> 
>> Thanks for the reply,
>> 
>> It is an iOS Swift project (uses Foundation, UIKit, CloudKit and other native frameworks) in which I would like to check the memory address for debugging (and out of enthusiasm). There is no C code I am using.
>> 
>> I have some asynchronous call back functions from CloudKit frameworks which return CKUserIdentity objects.
>> 
>> So thought it would be nice if I could print the memory address of CKUserIdentity objects and to check if it was unique.
>> 
>> And there are some other custom Swift Structs which I would like to know the memory address of.
>> 
>> Thanks and regards,
>> Muthu
> 
> For classes, use the Unmanaged API as Quinn’s suggested.
> 
> Your structs, tuples, and enums only have an address during mutation. So, for example, if you wrap all of your code in a function that takes the variable `inout`, you’ll see a consistent address within a single call to that function. There’s an implicit cast from `inout` to Unsafe[Mutable]Pointer arguments, so you can inspect the pointer value...
> 
> func foo(p: Unsafe[Mutable]Pointer) { print(p) }
> 
> foo(&s2)
> 
> As you noticed, between calls to `foo` you could see a different address.
> 
> If you really want to give your structs an “identity” you would need to wrap them in a class.
> 
> -Andy
> 
>>> On 12 Sep 2017, at 10:35 PM, Quinn The Eskimo! via swift-users <swift-users at swift.org> wrote:
>>> 
>>> 
>>> On 12 Sep 2017, at 13:44, somu subscribe via swift-users <swift-users at swift.org> wrote:
>>> 
>>>> 1. Is the above shown the correct way to get reference type memory address ?
>>>> 2. What Is it the correct way to get value type memory address ?
>>> 
>>> It’s hard to answer that without knowing how you’re intended to use these techniques.  If you can explain more about where you’re going with this, I should be able to make some specific recommendations.
>>> 
>>> For example, if you’re goal were to pass a Swift object to a C API that takes a callback function pointer and a ‘cookie’ value, and hen calls that function with that cookie (like the `qsort_r` function), the to/from opaque mechanism provider by `Unmanaged` is the recommended way to pass a Swift object through such a cookie value.
>>> 
>>>> 3. Is it possible to get the memory address of immutable value type (declared as let)
>>> 
>>> No, because such values don’t necessarily exist in memory.
>>> 
>>> Share and Enjoy
>>> --
>>> Quinn "The Eskimo!"                    <http://www.apple.com/developer/>
>>> Apple Developer Relations, Developer Technical Support, Core OS/Hardware
>>> 
>>> 
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