Swift Error fatal error: unexpectedly found nil while unwrapping an Optional value

致命的错误: 对一个为空的optional值进行解包.

强制解包(forced unwrapping): 在optional后面加叹号. 比如 "value!"

错误代码:

let testString ="lalala"

let a = testString.toInt();

println("\(a!)")

不能对为空的optional进行解包,否则会报运行时错误.所以在对optional进行解包之前进行判断是否为空.

正确写法:

let testString ="lalala"

let a = testString.toInt();

if (a != nil) {

println("\(a!)")

}