在Swift中将String类型的字符串转换成floate或double的若干方法：
1._bridgeToObjectiveC()

let version: String = UIDevice.currentDevice().systemVersion
let versionFloat = version._bridgeToObjectiveC().floatValue
if versionFloat < 8.0 {
    cell.textLabel?.font = UIFont.preferredFontForTextStyle(UIFontTextStyleHeadline)
}

2.as NSString

let version: String = UIDevice.currentDevice().systemVersion
let versionFloat = (version as NSString).floatValue        // -> 8.1

3.CFStringGetDoubleValue()

let version: String = UIDevice.currentDevice().systemVersion
let versionFloat = CFStringGetDoubleValue(version)
println("\(versionFloat)")     // -> 8.1

4.atof()/strtod()
strtod()会扫描参数nptr字符串，跳过前面的空格字符，直到遇上数字或正负符号才开始做转换，到出现非数字或字符串结束时(‘\0’)才结束转换，并将结果返回。若endptr不为NULL，则会将遇到不合条件而终止的nptr中的字符指针由endptr传回。参数nptr字符串可包含正负号、小数点或E(e)来表示指数部分。如123.456或123e-2。

let str = "0xff"
atof(str)                      // -> 255.0
strtod(str,nil)               // -> 255.0
atof("123.4567fubar")          // -> 123.4567

注：atof()和atof()可以接受进制数字，如上所示

5.NSNumberFormatter

var formatter = NSNumberFormatter()

let valueStr = "1234.123"
let oldNumber = formatter.numberFromString(valueStr)

formatter.groupingSeparator = " "
formatter.decimalSeparator = "|"
formatter.numberStyle = .DecimalStyle
formatter.formatterBehavior = .BehaviorDefault

let numberStr = formatter.stringFromNumber(oldNumber!)    
println("\(numberStr!)")     //1 234|123