Swift将泛型转换为可选参数,其值为nil会导致fatalError [英] Swift Casting Generic to Optional with a nil value causes fatalError
问题描述
使用Swift 2,在我设计的例子中,我将 String
转换为 Int
或者更具体地说是一个 Int
或一个 Int?
使用泛型。在 Int <
应该为零的情况下,强制转换将会失败并发生fatalError:致命错误:意外地发现nil,同时展开可选值 code>
这些看起来像是相似/重复的问题:
我的问题是:人们应该如何转换为可选项,即nil?
示例:
class Thing< T> {
var item:T
init(_ item:T){
self.item = item
struct Actions {
static func convertIntForThing< T>(string:String,thing:Thing< T>) - > T {
return convertStringToInt(string,to:T.self)
}
static func convertStringToInt< T>(string:String,to:T.Type) - > T {
debugPrint(Converting to ---> \(to))
if T.self == Int.self {
return Int(string) !如! T
}
//如果是Int?将是零,铸造
//在这里导致它炸毁:
//
//致命错误:意外地发现零,同时展开
//可选值甚至尽管在这种情况下T是可选的< Int>
返回Int(string)as! T
func testExample(){
//此工程:
let thing1 =物件< Int>(0)
thing1.item = Actions.convertIntForThing(1,thing:thing1)
//此失败:
//空字符串,其中value = Int ()
//将返回一个可选的< Int> (0)
thing2.item = Actions.convertIntForThing(,thing:thing2)
}
testExample()
。
我忘记了可选
是 NilLiteralConvertible
。所以我们可以在转换函数上提供2个变体,它不会失败。基本上,在 T
其中 T:NilLiteralConvertible
< pre $
class Thing< T> {
var item:T
init(_ item:T){
self.item = item
struct动作{
//提供2个变体之一,其中T:NilLiteralConvertible
//变体1 for non-optionals
static func convertIntForThing< T>(string:String,thing:Thing< T>) - > T {
return convertStringToInt(string,to:T.self)
}
//变化2 for optionals
static func convertIntForThing< T where T:NilLiteralConvertible> (string:String,thing:Thing< T>) - > T {
return convertStringToInt(string,to:T.self)
}
static func convertStringToInt< T>(string:String,to:T.Type) - > T {
debugPrint(Converting to ---> \(to))
return Int(string)!如! T
}
static func convertStringToInt< T where T:NilLiteralConvertible>(string:String,to:T.Type) - > T {
debugPrint(Converting to ---> \(to))
let value = Int(string)
if _ =价值{
返回值为! T
}
让其他:T =零
返回其他
}
}
func testExample(){
//此作品:
let thing1 = Thing< Int>(0)
thing1.item = Actions.convertIntForThing(1,thing:thing1)
let thing2 = Thing< Int?>(0)
thing2.item = Actions.convertIntForThing(,thing:thing2)
}
testExample()
Using Swift 2, in my contrived example I am converting a String
to an Int
or more specifically an Int
or an Int?
using a generic. In the case where the Int?
should be nil the cast will fail with a fatalError: fatal error: unexpectedly found nil while unwrapping an Optional value
These look like they may be similar/duplicate questions:
My question is: how is one supposed to cast to an optional that is nil?
Example:
class Thing<T>{
var item: T
init(_ item: T){
self.item = item
}
}
struct Actions{
static func convertIntForThing<T>(string: String, thing:Thing<T>) -> T{
return convertStringToInt(string, to: T.self)
}
static func convertStringToInt<T>(string: String, to: T.Type) -> T{
debugPrint("Converting to ---> \(to)")
if T.self == Int.self{
return Int(string)! as! T
}
// in the case of "" Int? will be nil, the cast
// here causes it to blow up with:
//
// fatal error: unexpectedly found nil while unwrapping an
// Optional value even though T in this case is an Optional<Int>
return Int(string) as! T
}
}
func testExample() {
// this WORKS:
let thing1 = Thing<Int>(0)
thing1.item = Actions.convertIntForThing("1", thing: thing1)
// This FAILS:
// empty string where value = Int("")
// will return an Optional<Int> that will be nil
let thing2 = Thing<Int?>(0)
thing2.item = Actions.convertIntForThing("", thing: thing2)
}
testExample()
I got something that does work.
I forgot that Optional
is a NilLiteralConvertible
. So we can provide 2 variations on the conversion function and it will not fail. Basically, provides a constraint on T
where T: NilLiteralConvertible
class Thing<T>{
var item: T
init(_ item: T){
self.item = item
}
}
struct Actions{
// Provide 2 variations one with T the other where T: NilLiteralConvertible
// variation 1 for non-optionals
static func convertIntForThing<T>(string: String, thing:Thing<T>) -> T{
return convertStringToInt(string, to: T.self)
}
// variation 2 for optionals
static func convertIntForThing<T where T: NilLiteralConvertible>(string: String, thing:Thing<T>) -> T{
return convertStringToInt(string, to: T.self)
}
static func convertStringToInt<T>(string: String, to: T.Type) -> T{
debugPrint("Converting to ---> \(to)")
return Int(string)! as! T
}
static func convertStringToInt<T where T: NilLiteralConvertible>(string: String, to: T.Type) -> T{
debugPrint("Converting to ---> \(to)")
let value = Int(string)
if let _ = value{
return value as! T
}
let other: T = nil
return other
}
}
func testExample() {
// this WORKS:
let thing1 = Thing<Int>(0)
thing1.item = Actions.convertIntForThing("1", thing: thing1)
let thing2 = Thing<Int?>(0)
thing2.item = Actions.convertIntForThing("", thing: thing2)
}
testExample()
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