NSData（bytes：length :)如何将[Byte]转换为UnsafePointer< Void>在Swift？ [英] How does NSData(bytes:length:) convert [Byte] to UnsafePointer<Void> in Swift?

问题描述

在Swift中使用原始数据我遇到了一些我不明白的问题。

NSData 有一个构造函数：

  init（bytes：UnsafePointer< Void> ;, length：Int）
  

$ b b

其中第一个字节参数显然是 UnsafePointer 类型。

但是，如果我通过 [Byte] >对象到这个构造函数，不仅编译器不会抱怨，而且它工作正常。

但是如果我尝试转换 [Byte] 到 UnsafePointer
这是如何工作的？

例如（你可以在Playgrounds试试）：

  let buffer：[Byte] = [0x00,0xff] 
 let data = NSData（bytes：buffer，length：buffer.count）没有错误
 data.description 
 
 var指针：UnsafePointer< Void> 
 //注释此行以避免编译器错误
指针=缓冲区//错误
  

我知道我可以做

  UnsafePointer< Void>（缓冲区）
  / pre> 
 
 
但我的问题是，NSData构造函数隐式地做什么，我不必这样做。
解决方案
当你调用需要它们的C函数时，Swift将自己的类型映射到C指针。 Apple有一些关于此互动的文档：与C API交互。
 
 
 
总之，当你调用一个函数， c $ c> UnsafePointer< Type> ，它可以接受 nil ， inout 那种类型的数组（即 [Type] ，你正在使用什么），或者其中一个通用的Swift指针类型。一个函数取 UnsafePointer< Void> 可以取任何类型，对类型没有约束，所以你需要阅读函数的文档，看看它期望/将返回。
 
Playing with raw data in Swift I came across something I don't understand.

NSData has a constructor:
init(bytes: UnsafePointer<Void>, length: Int)
where the first bytes parameter is clearly of UnsafePointer type.

However, if I pass [Byte] object to this constructor, not only does the compiler not complain, but it works ok.

But if I try to cast [Byte] to UnsafePointer, I fail.

How does this work?

For example (you can try it in Playgrounds):
let buffer: [Byte] = [0x00, 0xff]
let data = NSData(bytes: buffer, length: buffer.count) // no error
data.description

var pointer: UnsafePointer<Void>
// comment this line to avoid compiler error
pointer = buffer // error
I know I can do
UnsafePointer<Void>(buffer)
but my question is, what does NSData constructor do implicitly, that I don't have to do that.
解决方案
Swift maps its own types to C pointers when you're calling a C function that requires them. Apple has a bit of documentation on this interaction: Interacting with C APIs.

In short, when you're calling a function that takes an UnsafePointer<Type>, it can accept nil, an inout variable of that type, an array of that type (i.e., [Type], what you're using), or one of the generic Swift pointer types. A function taking UnsafePointer<Void> can take any of those, with no constraint on the type at all, so you need to read the function's documentation to see what it expects/will return.

                        
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