Array of Swift-Strings in const char * const *

Question

Ich habe Probleme eine Reihe von Swift-Strings in einem Array für AC-Funktion mit der Signatur umzuwandeln:

PGconn *PQconnectStartParams(const char * const *keywords, const char * const *values, int expand_dbname) 

In Swift, die const char * const * zeigt sich als:

<UnsafePointer<UnsafePointer<Int8>> 

Also versuche ich, den Inhalt eines Dictionary zu konvertieren [String: String] genannt ‚Optionen‘ und Futtermittel, die an die Funktion wie folgt:

var keys = [[Int8]]() 
var values = [[Int8]]() 
for (key, value) in options { 
    var int8Array = key.cStringUsingEncoding(NSUTF8StringEncoding)! 
    keys.append(int8Array) 
    int8Array = value.cStringUsingEncoding(NSUTF8StringEncoding)! 
      values.append(int8Array) 
} 
pgConnection = PQconnectStartParams(UnsafePointer(keys), UnsafePointer(values), 0) 

Es kompiliert und läuft, aber die Funktion funktioniert nicht.

Jeder Einblick würde sehr geschätzt werden.

Answer 1

Das ist nicht perfekt, aber zumindest funktioniert es.

let options = ["key1": "value1", "key2": "value2", "key3": "value3", "key4": "value4"] 

var keys = [String]() 

for (key, value) in options { 

    keys.append(key) 
} 

//you need to identify how many paramenters should be provided and set them following "static way" 
//I did not find how to prepare this dynamically 
let cKey1 = keys[0].cStringUsingEncoding(String.defaultCStringEncoding())! 
let key1Pointer = UnsafePointer<CChar>(cKey1) 

let cKey2 = keys[1].cStringUsingEncoding(String.defaultCStringEncoding())! 
let key2Pointer = UnsafePointer<CChar>(cKey2) 

let cKey3 = keys[2].cStringUsingEncoding(String.defaultCStringEncoding())! 
let key3Pointer = UnsafePointer<CChar>(cKey3) 

let cKey4 = keys[3].cStringUsingEncoding(String.defaultCStringEncoding())! 
let key4Pointer = UnsafePointer<CChar>(cKey4) 


let keysCArray = [key1Pointer, key2Pointer, key3Pointer, key4Pointer] 

f(keysCArray) 

/* 
C - code 

void f(const char * const *keywords) { 

printf("%s\n", keywords[0]); 
printf("%s\n", keywords[1]); 
printf("%s\n", keywords[2]); 
printf("%s\n", keywords[3]); 

} 
*/ 

Ich hoffe, es hilft. Ich kann meine Beispiel-App teilen, wenn Sie möchten.

Answer 2

Hier ist eine Erweiterung Array in swift3, dass es allgemein tun:

public extension Array { 

    // Translate [String] to (const char * const *), which translates to Swift as 
    public func cStringArray() throws -> ArrayBridge<Element,CChar> { 
     return try ArrayBridge<Element,CChar>(array:self) { 
      guard let item = $0 as? String, 
        let translated = item.cString(using: .utf8) else { 
       fatalError() 
      } 
      return translated 
     } 
    } 
} 

/* 
We need to have this intermediate object around to hold on to the translated objects, otherwise they will go away. 
The UnsafePointer won't hold on to the objects that it's pointing to. 
*/ 
public struct ArrayBridge<SwiftType,CType> { 

    let originals :[SwiftType] 
    let translated :[[CType]] 
    let pointers :[UnsafePointer<CType>?] 
    public let pointer :UnsafePointer<UnsafePointer<CType>?> 

    init(array :[SwiftType], transform: @noescape (SwiftType) throws -> [CType]) throws { 
     self.originals = array 
     self.translated = try array.map(transform) 

     var pointers = [UnsafePointer<CType>?]() 
     for item in translated { 
      pointers.append(UnsafePointer<CType>(item)) 
     } 
     pointers.append(nil) 
     self.pointers = pointers 
     self.pointer = UnsafePointer(self.pointers) 
    } 
} 

Dann können Sie den richtigen C-kompatiblen Zeiger erhalten, indem

tun

try stringArray.cStringArray().pointer