I can understand the statement, but as is good enough to cast any type, why followed by a question mark like:
if let foo = object as? String
It's really bugging me. Does it mean object is optional or the return value of the variable foo is an optional string type? Or it really means nothing just a language syntax? It must means something otherwise why is it there in the first-place? What does it mean? The question mark after word as in the if let statement.
I did search and can't find any.
i was confused course i know object is already an optional and be assigned to variable foo then if let unwrap it if it can unwrap,or jumped out the following statement.
so the if let unwrapping procedure is happening after as? type casting right? after as?casting a optional string from the example, then if let is unwrapping that not the optional object class assigned to the variable.
Your code:
if let foo = object as? String
breaks into two significant parts: object as? String and if let.
The first is a downcast which allows for failure by producing an optional type (String? here). From The Swift Programming Language (Swift 3.1): Typecasting:
Use the conditional form of the type cast operator (as?) when you are not sure if the downcast will succeed. This form of the operator will always return an optional value, and the value will be nil if the downcast was not possible. This enables you to check for a successful downcast.
The second, if let, is an optional binding which unwraps an optional, the body of the if let is only executed if the optional value being unwrapped is not nil and with the bound variable being bound to the unwrapped value. See Optional Binding in The Swift Programming Language (Swift 3.1): Basics.
So your sense that something is happening twice here is correct. The as? cast may fail and produce a nil optional value, the if let will test that optional value for nil. We can only hope that the compiler handles the combination of if let and as? together and no unneccesary optionals are actually created and destroyed by the implementation.
HTH
Related
This question already has answers here:
Swift variable decorations with "?" (question mark) and "!" (exclamation mark)
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What is the difference between String? and String! (two ways of creating an optional variable)?
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I noticed that some coding examples for Swift declare optional properties in different ways. However, I seem to have trouble trying to tell them apart (or, rather, when to use each.)
class MyClass {
// I believe this can start as nil but should always be set
// once it is.
var optionalProperty1: Type!
// I believe this can be set or nil anytime.
var optionalProperty2: Type?
// I think this is the same as
// var optionalProperty3: Type!
lazy var optionalProperty3: Type
}
How does each one differ from the others and when should each one be used?
var optionalProperty1: Type!
When you're sure you will have value for this property such as timestamp it will be something for sure. And Yes it can be nil too.
var optionalProperty2: Type?
When you're not sure about the value (Or this field is not mandatory) take it as optional for example:- If I make a Person class address can be optional and name will not.
lazy var optionalProperty3: Type
This syntax is wrong you can not declare lazy property in this way. You must assign something to it initially. See below example:
/// First way
lazy var optionalProperty3: String = {
return "Hello"
}()
/// Second way
lazy var optionalProperty4 = "Hello"
A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration. lazy variables are great for things that need to be setup once, then never re-set.
One more thing you really don't need to specify type in modern Swift. Means if you will assign 0 it will be an integer itself, if you will assign 0.0 it will take it as double and same for String, Array etc.
The third declaration is not an optional.
The third one declares a lazy property. Lazy properties will only be initialised when they are first used. Example:
class A {
lazy var a: Int = {
print("hello")
return 1 + 1
}()
}
let a = A()
print(a.a)
If you remove the last line, hello will not be printed.
The first declaration is a implicitly unwrapped optional while the second declaration is a normal optional.
When you access members of a normal optional, you need to unwrap it with ? or !:
var myInt: Int? = ...
print(myInt.description) // doesn't compile, must unwrap
print(myInt!.description) // will crash if myInt is nil
print(myInt?.description) // will print "nil" if myInt is nil
On the other hand, implicitly unwrapped optionals do this implicitly - whenever you try to access a member, it will implicitly unwrap it with ! for you:
var myInt: Int! = ...
print(myInt.description) // equivalent to "myInt!.description"
From Apple's Swift documentation:
Optionals (?): "You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all."
Implicitly Unwrapped Optionals (!): "As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.
Sometimes it’s clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it’s useful to remove the need to check and unwrap the optional’s value every time it’s accessed, because it can be safely assumed to have a value all of the time.
These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.
Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties."
Read more at https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html
Explicitly Unwrapped Variable
var optionalProperty1: Type!
This means that the value should never be nil. It will always be initialized but it is better to use var optionalProperty1: Type
Optional
var optionalProperty2: Type?
This is an optional value, meaning that it can either have a value or be nil. normally to use the value you would need to unwrap it. using either conditional unwrapping or a guard statement...
if let value = optionalProperty2 {
print("optionalProperty2 had a value, value is set")
}
guard let value = optionalProperty2 else {
print("optionalProperty2 was nil value is nil")
}
or you can just check if it has a value and explicitly unwrap it (which is not considered best practice)
if optionalProperty2 != nil {
print("optionalProperty2 is set to \(optionalProperty2!)")
}
Lazy Variable
lazy var optionalProperty3: Type a lazy variable is different, it is not an optional unless you define it to be optional. Lazy variables do not initialise to a value until the value is first used. So if you do not call/use the variable it will never be set.
I found an interesting function in the Swift Tests source code:
func test(_ v: A????, _ cast: (A????) -> B?)
Since Type? is just syntax sugar for Optional<Type>, this means that the type of the argument v can be rewritten as Optional<Optional<Optional<Optional<A>>>>.
I know that this function is used to test optional types, so it is definitely going overboard with the arguments v and cast, but what would be the actual use of having "an optional of an an optional, etc." type in Swift (if any)?
These occur sometimes in Swift when you are accessing data.
One example is if you have a dictionary with an optional type for the value, and then you look up a value in that dictionary:
let rockStars: [String: String?] = ["Sting": nil, "Elvis": "Presley", "Bono": nil, "Madonna": nil]
let lastName = rockStars["Elvis"]
print(lastName as Any)
Optional(Optional("Presley"))
This happens because a dictionary look up can fail when the key is not present, and it returns nil in that case. So the return type of a dictionary lookup has to be the value type wrapped in an optional. A look up from the rockStars dictionary returns a String??.
In Xcode 8 release version, i found a strange scene.
Here is the code,
let implicitlyUnwrappedOptionalString: String! = "implicitlyUnwrappedOptionalString"
let foo = implicitlyUnwrappedOptionalString
print(implicitlyUnwrappedOptionalString)
print(foo)
and here is the result:
implicitlyUnwrappedOptionalString
Optional("implicitlyUnwrappedOptionalString")
These above shows that when i assign a implicitly unwrapped optional to a variable without a explicit type, the type will be inferred to an optional type, not the type which it originally is, aka implicitly unwrapped optional.
My Xcode has been updated to 8. Anyone can verify the behavior in Xcode 7.x?
The change is due to the Swift version changing or the Xcode?
This is a consequence of SE-0054 Abolish ImplicitlyUnwrappedOptional type which has been implemented in Swift 3. Extract from that proposal (emphasis added):
However, the appearance of ! at the end of a property or variable declaration's type no longer indicates that the declaration has IUO type; rather, it indicates that (1) the declaration has optional type, and (2) the declaration has an attribute indicating that its value may be implicitly forced. ...
If the expression can be explicitly type checked with a strong optional type, it will be. However, the type checker will fall back to forcing the optional if necessary. The effect of this behavior is that the result of any expression that refers to a value declared as T! will either have type T or type T?. For example, in the following code:
let x: Int! = 5
let y = x
let z = x + 0
… x is declared as an IUO, but because the initializer for y type checks correctly as an optional, y will be bound as type Int?. However, the initializer for z does not type check with x declared as an optional (there's no overload of + that takes an optional), so the compiler forces the optional and type checks the initializer as Int.
In your case, the assignment
let foo = implicitlyUnwrappedOptionalString
makes foo a strong optional, as in the example let y = x
from the proposal.
You could make foo an IUO by adding an explicit type annotation
let foo: String! = implicitlyUnwrappedOptionalString
but generally you should try to get rid from IUOs in your code,
as stated in the same proposal:
Except for a few specific scenarios, optionals are always the safer bet, and we’d like to encourage people to use them instead of IUOs.
I understand the difference between String! type and String? type. But what about String type? How does it differs from String! and String? in swift? Does String! type identical to String type?
Say, I have a class like this:
class Person {
private var _name: String!
var name: String {
return _name
}
init(name: String) {
_name = name
}
}
There is no compiler error, looks like String type is identical to String! type. But I am not sure...
String and String! are not identical. There is just happens to be enough sugar in the language to convert between them. Similarly there is sugar in the language to convert between String and String? (but not in reverse).
Start with the basics. There is String. Unless there is some strong reason, you should use String when you mean a string of characters. Everything else is "more stuff" and you shouldn't add it unless you need it.
There is Optional<String>. This is just an enum with two cases, one with a value, and one without a value:
public enum Optional<Wrapped> : _Reflectable, NilLiteralConvertible {
case None
case Some(Wrapped)
// ...
}
There is a postfix operator ! for Optional which will return Wrapped if it's available, and crash if it is not. So far, no magic. This is stuff you could build yourself.
There are are few pieces of magic around Optional. First, the type Wrapped? is magically converted to Optional<Wrapped> by the compiler (for any Wrapped). This is just syntactic sugar. The two notations are identical. Second, there is optional-chaining with the ?. "operator" (it's not really an operator; it's part of the language and you couldn't build it yourself). And then there's optional promotion. Any type Wrapped can be automatically and implicitly converted to Wrapped? if needed. There are are few other pieces of magic around Optional like if-let syntax, and there's nil which is a synonym for Optional.None (I believe they're actually identical). But Optional really is just a generic type, implemented as an enum. It's just a type the compiler knows special things about.
Then there is ImplicitlyUnwrappedOptional<Wrapped>. This is also just an enum (in Swift 2.2; this will change in Swift 3).
public enum ImplicitlyUnwrappedOptional<Wrapped> : _Reflectable, NilLiteralConvertible {
case None
case Some(Wrapped)
// ...
}
This is not the same as Optional and it is not the same as Wrapped. It's a completely different type. But it also has some magic associated with it. First, the type Wrapped! is syntactic sugar for ImplicitlyUnwrappedOptional<Wrapped>. Again, it's just sugar. The two are the same (in Swift 2.2, not in Swift 3). Next, if IUO<Wrapped> is found in a place that Wrapped is expected, it will automatically be converted to Wrapped or crash if there is no value. If it is found in a place that Wrapped? is expected, it will automatically be converted to Wrapped?. Those are magical, and it's why sometimes String and String! seem to be the same type. That's just the compiler magically "making it work" for you by adding an invisible conversion step. It doesn't mean they're really the same type.
IUO is mostly useful in bridging to certain Objective-C patterns, especially involving Storyboards, and should be avoided outside of those situations. Even in those situations, IUO is just there for convenience. You could do all the same things using regular Optionals, you'd just have to check them with if-let more often. Using Optionals is much safer than using IUO. It's easy to think "I know for certain this value will always be set before it is used." And just this week I chased down a crasher due to being wrong about that. There's a difference between "it should be" and "it must be." But, being totally safe with Optionals in Storyboards could be very inconvenient and might mask some bugs (by doing nothing rather than crashing), so that's the most common place for IUO.
IUO properties used to be valuable for dealing with failable init methods. That's no longer a problem in Swift 2.2, so that use has gone away. The last "pure Swift" use that I run into is when you must pass self to the initializer of something you store as a property (you can't pass self at that point because all your properties have been initialized). That's an unfortunate use-case and very messy, and I hope we come up with a fix for it. Outside of these kinds of cases, you should avoid Implicitly Unwrapped Optionals.
String and String! are same and different at the same time. If you declare let or var as String, you will have to set some value at init method. If you declare it as String! you can set value when you want but you must do it before reading this value.
So, if your _name will be nil, your app will crash.
If you declare it as String, compiler guarantees that _name != nil before the first reading. If you declare it as String!, you should guarantee it.
This things have same type because of this:
var test1: String = "test"
var test2: String! = "test"
if test1 is String { // True
print("test1 is String")
}
if test2 is String { // True
print("test2 is String")
}
This types are equal but this things are different
Note the difference between these two functions:
func f() {
let x: String? = nil
guard let y: String = x else {
print("failed")
return
}
}
func g() {
let x: String? = nil
guard let y: String! = x else {
print("failed")
return
}
print(y.dynamicType, y)
}
f() # failed
g() # Optional<String> nil
You are better to read swift documentation than any answer here. Properly undestandig swift's type system is very important, so do it first.
both String! and String? are optional types.
Using String? requires you to test whether the value is nil (.None) or a value (.Some). For example using guard statements or if-let clauses.
String! is an optional that is assumed to be non-nil and is unwrapped automatically. This allows you to write cleaner-looking code and avoid unwrapping wherever the value is used. BUT: if your value is nil a fatal error will occur.
The name of the ! always confuses me: it's called an 'implicitly unwrapped optional'. However, what is implicit about it? Implicit means "implied though not plainly expressed." However, does not adding a ! plainly express its purpose? Does not adding a ! make it explicit what we are trying to accomplish?
In Swift, trailing exclamation marks (!) are used in two different ways. One is called Forced Unwrapping. This is when you have a variable defined as an Optional and you want to basically assert that the value is not nil so that you can use it as if it were not an optional:
var optionalName: String? = "World"
if optionalName != nil {
sayHelloTo(optionalString!)
}
An alternate way you could describe "Forced Unwrapping" is "Explicit Unwrapping", but forced adds the extra hint that the whole program will crash if you try to "Force Unwrap" an optional that is nil.
The second is when you are actually declaring the type of a variable and it is called an Implicitly Unwrapped Optional. This is for when you want to declare a variable that is technically optional, but that you will treat as if it is not optional. Every time you use this Implicitly Unwrapped Optional, it is in reality, doing the "Force Unwrapping" I described above:
var name: String! = "World"
if name != nil {
sayHelloTo(name)
}
You can still test an Implicitly Unwrapped Optional for nil, but you can also use it directly as if it were not optional. That is why it is considered implicit. Every time you use it, it is automatically, implicitly unwrapped for you.
Like "Forced Unwrapping" an "Implicitly Unwrapped Optional" will still crash the entire program if you try to use it when it is nil
Normal Optional
var foo : Foo? = Foo()
foo!.doSomething() // explicitly unwrap it and call the method
Implicitly Unwrapped Optional
var foo : Foo! = Foo()
foo.doSomething() // implicitly unwrap it and call the method just like it is not optional
In both case, you need something to declare an optional (? or !). Implicitly Unwrapped Optional does not add more. But you can omit unwrap operator when use it.