在Swift中使用C字符串，或者：如何转换UnsafePointer< CChar>到CString [英] Working with C strings in Swift, or: How to convert UnsafePointer<CChar> to CString

问题描述

在Swift中使用标准C库函数时，在传递C字符串时遇到
的问题。作为一个简单示例（仅用于演示问题），标准C库函数

While playing with Standard C Library functions in Swift, I came across problems when passing C strings around. As a simple example (just to demonstrate the problem), the Standard C Library function

char * strdup(const char *s1);

暴露给Swift的原因是

is exposed to Swift as

func strdup(_: CString) -> UnsafePointer<CChar>

这意味着 strdup（）无法传递给另一个 strdup（）调用：

which means that the return value of strdup() cannot be passed to another strdup() call:

let s1 : CString = "abc"
let s2 = strdup(s1) // OK, s2 is a UnsafePointer<CChar>
let s3 = strdup(s2) // error: could not find an overload for '__conversion' that accepts the supplied arguments

我的问题是：如何从 UnsafePointer< CChar>创建Swift CString ，
，以便可以将一个标准库函数返回的C字符串传递给另一个函数？

My question is: How to create a Swift CString from a UnsafePointer<CChar>, so that the C string returned by one standard library function can be passed to another function?

可以找到的是（使用如何使用Swift语言将字符串转换为CString？）：

The only way that I could find is (using code from How do you convert a String to a CString in the Swift Language?):

let s2a = String.fromCString(s2).bridgeToObjectiveC().UTF8String
let s3 = strdup(s2a)

但是我发现这并不令人满意，有两个原因：

But I do not find this satisfying for two reasons:

• 对于一个简单的任务来说太复杂了。


• （主要原因：）上述转换仅在C字符串为有效的UTF-8
  字符串，否则它将因运行时异常而失败。但是C字符串是一个任意
  字符序列，由NUL字符分隔。

• It is too complicated for a simple task.
• (Main reason:) The above conversions works only if the C string is a valid UTF-8 string, otherwise it fails with a runtime exception. But a C string is an arbitrary sequence of characters, delimited by a NUL character.

备注/背景：当然，使用Swift等高级数据结构的高级函数 String 或Objective-C NSString 为佳。但是
标准C库中有BSD函数，在Foundation框架中没有完全对应的函数。

Remarks/Background: Of course, high-level functions using high-level data structures like Swift String or Objective-C NSString are preferable. But there are BSD functions in the Standard C Library which do not have an exact counterpart in the Foundation frameworks.

我在尝试回答时遇到了这个问题在Swift中访问临时目录。
在这里， mkdtemp（）是一个BSD函数，没有确切的 NSFileManager 替代品存在
（我所知道的）。
mkdtemp（）返回 UnsafePointer< CChar> ，该值必须传递给
NSFileManager 函数 stringWithFileSystemRepresentation 需要一个 CString

I came across this problem while trying to answer Accessing temp directory in Swift. Here, mkdtemp() is a BSD function for which no exact NSFileManager replacement exists (as far as I know). mkdtemp() returns a UnsafePointer<CChar> which has to be passed to the NSFileManager function stringWithFileSystemRepresentation which takes a CString argument.

更新：截至Xcode 6 beta 6，此问题不再存在因为简化了C字符串到Swift的映射。您可以只写

Update: As of Xcode 6 beta 6, this problem does not exist anymore because the mapping of C-Strings into Swift has been simplified. You can just write

let s1 = "abc"      // String
let s2 = strdup(s1) // UnsafeMutablePointer<Int8>
let s3 = strdup(s2) // UnsafeMutablePointer<Int8>
let s4 = String.fromCString(s3) // String

推荐答案

Swift 1.1（或更早的版本）甚至具有更好的C字符串桥接：

Swift 1.1 (or perhaps earlier) has even better C string bridging:

let haystack = "This is a simple string"
let needle = "simple"
let result = String.fromCString(strstr(haystack, needle))

CString 类型已完全消失。

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