[swift-users] Passing Data to a f(void *) function

[Martin R]

> On 30. Jun 2017, at 16:57, Daniel Dunbar <daniel_dunbar at apple.com> wrote:
> 
>> 
>> On Jun 30, 2017, at 7:40 AM, Martin R via swift-users <swift-users at swift.org> wrote:
>> 
>> I have a C function 
>> 
>>   void myfunc(const void *ptr);
>> 
>> which is imported to Swift as
>> 
>>   func myfunc(_ ptr: UnsafeRawPointer!)
>> 
>> This compiles and runs without problems:
>> 
>>   let data = Data(bytes: [1, 2, 3, 4])
>>   data.withUnsafeBytes { (ptr) in myfunc(ptr) } // (A)
>> 
>> and the type of `ptr` is inferred as `UnsafePointer<Void>`. But adding an explicit type
>> annotation produces a compiler warning:
> 
> How do you know the inferred type is `UnsafePointer<Void>`? I think it is more likely it is `UnsafePointer<UInt8>`, and the following compiles:
> ```
> let data = Data(bytes: [1, 2, 3, 4])
> data.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in
>    myfunc(ptr)
> }
> ```
> 
> - Daniel

Command-click on "ptr" in "myfunc(ptr)" (or the Quick Help inspector) shows

    let ptr: (UnsafePointer<()>)

> 
>> 
>>   data.withUnsafeBytes { (ptr: UnsafePointer<Void>) in myfunc(ptr) } // (B)
>>   // warning: UnsafePointer<Void> has been replaced by UnsafeRawPointer
>> 
>> which is understandable in the view of "SE-0107 UnsafeRawPointer API".
>> 
>> The "Fix-it" replaces `UnsafePointer<Void>` by `UnsafeRawPointer`, and that does not
>> compile anymore:
>> 
>>   data.withUnsafeBytes { (ptr: UnsafeRawPointer) in myfunc(ptr) } // (C)
>>   // error: cannot convert value of type 'Void' to closure result type '_'
>> 
>> because there is no `withUnsafeBytes()` method taking a `(UnsafeRawPointer)->ResultType`
>> closure.
>> 
>> 
>> My questions are:
>> 
>> 1. Why are (A) and (B) treated differently?
>> 
>> 2. Is (A) "legal", or should one use some non-void pointer
>> 
>>   data.withUnsafeBytes { (ptr: UnsafePointer<Int8>) in myfunc(ptr) } // (D)
>> 
>>  (which feels wrong to me because it is converted back to a void pointer when
>>  calling the function).
>> 
>> 3. Or should there be a `withUnsafeRawPointer()` method which makes (C) compile as
>> 
>>     data.withUnsafeRawBytes { (ptr: UnsafeRawPointer) in myfunc(ptr) }
>> 
>>  This would also allow to access the data at byte offsets more easily, e.g.
>> 
>>      data.withUnsafeRawBytes { ptr in
>>          let u16 = ptr.load(fromByteOffset: 4, as: UInt16.self)
>>      }
>> 
>>  Does that makes sense?
>> 
>> Regards, Martin
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
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