How do I pass a callback function to sqlite3_exec in swift?

Question

How do I pass a callback function to sqlite3_exec in Swift?

sqlite_str = sqlite_str + "\(sqlite_property_str))";

var str:NSString = sqlite_str;

var sqlite:COpaquePointer = share().sqlite3_db;
var errmsg:UnsafePointer<Int8> = nil

let rc = sqlite3_exec(sqlite, str.cStringUsingEncoding(NSUTF8StringEncoding), <#callback: CFunctionPointer<((UnsafePointer<()>, Int32, UnsafePointer<UnsafePointer<Int8>>, UnsafePointer<UnsafePointer<Int8>>) -> Int32)>#>, <#UnsafePointer<()>#>, <#errmsg: UnsafePointer<UnsafePointer<Int8>>#>)

Answer 1

Swift 2.2 provides two options for implementing the sqlite3_exec callback function: (1) a global, non-instance func procedure or (2) a non-capturing literal {} closure.

The sqlite.org's "SQLite in 5 minutes or less" example is implemented in a Swift Xcode7 project here.

Readable typealias

typealias sqlite3 = COpaquePointer
typealias CCharHandle = UnsafeMutablePointer<UnsafeMutablePointer<CChar>>
typealias CCharPointer = UnsafeMutablePointer<CChar>
typealias CVoidPointer = UnsafeMutablePointer<Void>

Callback Approach

func callback(
    resultVoidPointer: CVoidPointer, // void *NotUsed 
    columnCount: CInt,               // int argc
    values: CCharHandle,             // char **argv     
    columns: CCharHandle             // char **azColName
    ) -> CInt {
    for  i in 0 ..< Int(columnCount) {
        guard let value = String.fromCString(values[i]) 
        else { continue }
        guard let column = String.fromCString(columns[i]) 
        else { continue }
        print("\(column) = \(value)")
    }
    return 0 // status ok
}

func sqlQueryCallbackBasic(argc: Int, argv: [String]) -> Int {
    var db: sqlite3 = nil 
    var zErrMsg:CCharPointer = nil
    var rc: Int32 = 0 // result code

    if argc != 3 {
        print(String(format: "ERROR: Usage: %s DATABASE SQL-STATEMENT", argv[0]))
        return 1
    }

    rc = sqlite3_open(argv[1], &db)
    if  rc != 0 {
        print("ERROR: sqlite3_open " + String.fromCString(sqlite3_errmsg(db))! ?? "" )
        sqlite3_close(db)
        return 1
    }

    rc = sqlite3_exec(db, argv[2], callback, nil, &zErrMsg)
    if rc != SQLITE_OK {
        print("ERROR: sqlite3_exec " + String.fromCString(zErrMsg)! ?? "")
        sqlite3_free(zErrMsg)
    }

    sqlite3_close(db)
    return 0
}

Closure Approach

func sqlQueryClosureBasic(argc argc: Int, argv: [String]) -> Int {
    var db: sqlite3 = nil 
    var zErrMsg:CCharPointer = nil
    var rc: Int32 = 0

    if argc != 3 {
        print(String(format: "ERROR: Usage: %s DATABASE SQL-STATEMENT", argv[0]))
        return 1
    }

    rc = sqlite3_open(argv[1], &db)
    if  rc != 0 {
        print("ERROR: sqlite3_open " + String.fromCString(sqlite3_errmsg(db))! ?? "" )
        sqlite3_close(db)
        return 1
    }

    rc = sqlite3_exec(
        db,      // database 
        argv[2], // statement
        {        // callback: non-capturing closure
            resultVoidPointer, columnCount, values, columns in

            for i in 0 ..< Int(columnCount) {
                guard let value = String.fromCString(values[i]) 
                else { continue }
                guard let column = String.fromCString(columns[i]) 
                else { continue }
                print("\(column) = \(value)")
            }
            return 0
        }, 
        nil, 
        &zErrMsg
    )

    if rc != SQLITE_OK {
        let errorMsg = String.fromCString(zErrMsg)! ?? ""
        print("ERROR: sqlite3_exec \(errorMsg)")
        sqlite3_free(zErrMsg)
    }
    sqlite3_close(db)
    return 0
}