如何正确处理UnsafeMutablePointer

Question

I am a little confused. When do I have to call free and when destroy/dealloc? I am working on a short code snippet learning core audio. I thought if I call UnsafeMutablePointer<Type>.alloc(size) then I should call destroy & dealloc . But if I use malloc() or calloc() I am supposed to call free() .

In this example from Learning Core Audio the following code snippet makes me wonder:

var asbds = UnsafeMutablePointer<AudioStreamBasicDescription>.alloc(Int(infoSize))
audioErr = AudioFileGetGlobalInfo(kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat,
                              UInt32(sizeof(fileTypeAndFormat.dynamicType)), &fileTypeAndFormat,
                              &infoSize, asbds)

Here I use alloc . To free up memory free is called.

free(asbds)

But why not

asbds.destroy(Int(infoSize))
asbds.dealloc(Int(infoSize))

which I would expect following the rule.

I would appreciate any help, because this makes my head spin. The documentation says I am responsible for destroying and dealloc so that part is clear, but in which way?

Answer 1

See the UnsafeMutablePointer Structure Reference .

The pointer can be in one of the following states:

• Memory is not allocated (for example, pointer is null, or memory has been deallocated previously).

• Memory is allocated, but value has not been initialized.

• Memory is allocated and value is initialized.

You can use the pointed region safely when "allocated and initialized". So, if you want to use Swift's UnsafeMutablePointer correctly, you need 2 steps before using and 2 steps after using.

(1) Allocate: alloc(_:) .

(2) Initilize: initialize...()

You can safely use the allocated and initialized region here.

(3) Deinitialize: destroy(_:)

(4) Deallocate: dealloc(_:)

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And why you can use free() for alloc(_:) ed memory, that's because Swift uses malloc(_:) in the current implementation of alloc(_:) . So, using free means that your app depends on the current implementation of the Swift runtime.

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So, using UnsafeMutablePointer is sort of difficult and annoying. You should consider passing an array as a pointer. In your case, you can write something like this:

    let elementCount = Int(infoSize) / strideof(AudioStreamBasicDescription)
    var asbds: [AudioStreamBasicDescription] = Array(count: elementCount, repeatedValue: AudioStreamBasicDescription())
    audioErr = AudioFileGetGlobalInfo(kAudioFileGlobalInfo_AvailableStreamDescriptionsForFormat,
                                  UInt32(sizeof(fileTypeAndFormat.dynamicType)), &fileTypeAndFormat,
                                  &infoSize, &asbds)

(I think you should use this elementCount even when using UnsafeMutablePointer . alloc(_:) or dealloc(_:) uses "number of elements", not "byte size".)