UnsafePointer<Uint8> 的问题在 Swift 的 SQLite 项目中 [英] Issue with UnsafePointer<Uint8> in SQLite project in Swift

问题描述

我们正在 iOS 和 Swift 中实现 SQLite，不使用包装器或 Objective-C 桥接.一切正常，除了在执行查询和提取结果时.问题在于从 Swift 中的 SQLite 返回的 UnsafePointer 如下:

We are implementing SQLite in iOS, in Swift, without using wrappers or Objective-C bridging. Everything works fine, except when doing a query and extracting the result. The issue is with the UnsafePointer<UInt8> that is returned from SQLite in Swift as follows:

var querySQL = "SELECT address, phone FROM CONTACTS WHERE NAME = 'myName'"
var cQuery = querySQL.cStringUsingEncoding(NSUTF8StringEncoding)
var statement: COpaquePointer = nil
if sqlite3_prepare_v2(contactsDB, cQuery!, -1, &statement, nil) == SQLITE_OK {
   if sqlite3_step(statement) == SQLITE_ROW {
   var address : UnsafePointer<UInt8> = sqlite3_column_text(statement, 0)
   var data = NSData(bytes: address, length: 10)
   var string = NSString(data: data, encoding: NSUTF8StringEncoding)
   println(string)

如您所见，如果我们知道对象的长度(在本例中为 10)，我们可以将指针转换为 String

As you can see, we can convert the pointer to String if we know the length of the object (in this case 10)

为了深入研究这个问题，我有以下例子

To dig into this issue, I have the following example

let pointerFromString: UnsafePointer<Int8> = "xyz".cStringUsingEncoding(NSUTF8StringEncoding)
let stringFromPointer = String.fromCString(anotherPointerFromString_Int8)                    println(stringFromPointer!)

鉴于 CChar 是 Int8 的别名，我可以将 String 转换为 UnsafePointer使用 .cStringUsingEncoding()，然后使用 .fromCString()

Given that CChar is an alias of Int8, I can convert a String to UnsafePointer<Int8> using .cStringUsingEncoding(), and then back to String using .fromCString(<UnsafePointer_CChar>)

问题是我的 SQLite 结果是一个 UnsafePointer_UInt8，不能与 .fromCString()

The problem is that my SQLite result is a UnsafePointer_UInt8, that can´t be used with .fromCString()

底线问题是:是否可以将 UnsafePointer_UInt8 转换或转换为 UnsafePointer_Int8

The bottom line question is: Is it possible to convert or cast a UnsafePointer_UInt8 to UnsafePointer_Int8

推荐答案

这应该有效:

let address = sqlite3_column_text(statement, 0)
let string = String.fromCString(UnsafePointer<CChar>(address))

Swift 3 (Xcode 8) 的更新， 比较 Swift 3:转换以空字符结尾的 UnsafePointer到一个字符串:

let string = String(cString: sqlite3_column_text(statement, 0))

更新:

sqlite3_column_text() 返回一个隐式解包的可选项，这就是您可以将它直接传递给 String(cString:) 的原因.

sqlite3_column_text() returns an implicitly unwrapped optional, and that is why you can pass it to String(cString:) directly.

但根据列方法、错误和NULL处理#41，返回value 可以 为 null(在内存不足的情况下，或者如果使用 NULL 列类型调用).因此这样做更安全

But according to Column methods, error and NULL handling #41, the return value can be null (in an out-of-memory situation, or if called with a NULL column type). Therefore it is safer to do

if let text = sqlite3_column_text(statement, 0) {
    let string = String(cString: text)
    // ...
} else {
    // sqlite3_column_text() returned NULL
}

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