Is crashing your program really the best option if the ! Generally not a good idea, but it can be useful when converting code from another language. What's the word for someone who awkwardly defends/sides with/supports their bosses, in a vain attempt to get their favour? Should I hold back some ideas for after my PhD? follows. USING Exclamation mark indicates that variable must consists non nil value (it never be nil). Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. which is separate to Foo, and that prevents funcs which accept type Foo from receiving a nil value, but at a low level an optional value isn't a true object because it has no properties or methods; it's likely that in fact it is a pointer which may by NULL(0) with the appropriate test when force-unwrapping. I misunderstood a little the meaning of unwrapping at that time. Also it means removing the vagueness around that object, as we are not sure by definition if it is nil or not. If you’re a developer out there who’s never made an iOS app, now is … msdn.microsoft.com/en-us/library/ms173145.aspx, developer.apple.com/library/ios/documentation/Swift/Conceptual/…. What does the [Flags] Enum Attribute mean in C#? The exclamation mark effectively says, “I know that this optional definitely has a value; please use it.”, Another way to check non nil value is (optional unwrapping). Ultimately I would love the user to "lock in" an answer for the question then pretty next question or previous question to go through the questions and then submit all the answers. The exclamation mark is also used in the syntax for declaring "Implicitly Unwrapped Optionals". and when to use ?? The term "wrapped" implies we should think of an Optional variable as a present, wrapped in shiny paper, which might (sadly!) in swift language? Thanks for visiting and working through these questions! Is the ! You only want to call the method if the variable is null. In Java this would be the equivalent of a NullPointerException or NPE for short.Kotlin's type system is aimed to eliminate NullPointerException's from our code. Swift is a new programming language for iOS, macOS, watchOS, and tvOS app development. your coworkers to find and share information. else you would have to do this on every after passing value... John is an optional Person, meaning it can hold a value or be nil. UPDATE: What is the question mark ? The minMax(array:) function returns a tuple containing two Int values. After reading this answer and some of Apple's example swift code, it really seems to me that in this tradeoff between safety and productivity, usually it's best to be on the safer side. In the current version of Swift this apparently bypasses the is-not-nil assertion so you'll have a low-level error instead. is for speed? when you know for sure it can be unwrapped. or '?'? Now there are times when you know that such optional variables will be assigned values by system for example or your own program but sometime later , for example UI outlets, in such situation instead of declaring an optional variable using a question mark "?" In fact, it is beautiful. Perhaps a smart idea would be to regroup those, and question mark can have it's slider too, giving the exclanation mark as the first pop-up choice, and the upside-down question-mark … Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. Maybe someone has more insight on this. it still returns the actual value. Posted by u/[deleted] 5 years ago. Why do small patches of snow remain on the ground many days or weeks after all the other snow has melted? indicate that its a Coalescing operator. ", Windows communication foundation unleashed By Craig McMurtry What is my registered address for UK car insurance? If you were to expand you'd just have a series of nested if/else statements. How is it different from just doing the following: I'm very new to the Swift language. Thanks for the fantastic answer @Ashley. Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. The article explains that this is a pragmatic measure by Apple used to declare the types used by their Objective-C frameworks which might contain nil. If john is an optional you must use the exclamation point to access its properties because the compiler must know that it isn't nil. your coworkers to find and share information. If I must use an exclamation point for ALL optional vars, then why did Apple even bother with it, as opposed to making it so the, you use the exclamation point when you need the optional to be not nil. This kind of trickery means that the language can be strongly typed without forcing you to test if values are nil everywhere. in Swift? Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake.One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception. So wrap here doesn't mean it is memory wrapped, but it means it is code wrapped, in this case it is wrapped with an if statement, and because Apple pay close attention to performance in runtime, they want to give you a way to make your app run with the best possible performance. Stack Overflow for Teams is a private, secure spot for you and A lot of things in Swift are written using Generics. In your example, "x" is of type "int?" @Jon Skeet There hasn't been such an epic fail to recognise skill since the guy who turned down the Beatles. a value has been assigned to an instance of a value type. Why doesn't ionization energy decrease from O to F or F to Ne? What do the two question marks mean, is it some kind of ternary operator? How to avoid force unwrapping a variable? Some big picture perspective to add to the other useful but more detail-centric answers: In Swift, the exclamation point appears in several contexts: Every one of these is a different language construct with a different meaning, but they all have three important things in common: When you use ! symbol mean in a function signature? Always use a normal optional type if you need to check for a nil value during the lifetime of a variable. You can declare optional variables using exclamation mark instead of a question mark. 5 comments. are completely separate types. to unwrap is sometimes not recommended see this Q&A. It's important to note this because if foo actually is nil when you do foo!, that's a runtime error, so typically you'd want to use a conditional let instead unless you are very sure that the value will not be nil. Question marks after a type refer to Optionals, a way in Swift which lets you indicate the possibility that a value might be absent for any type at all, without the need for special constants.. It’s used in the same situations you’d explicitly return nil in Objective-C, when there is no object to be returned, or for values that are not objects, constants such as NSNotFound. madness, and the great answers below, made a lot more sense. But you can run the code fine without it (a forth option for your list - implicit unwrapping) AND without checking first. Swift variable decorations with “?” (question mark) and “!” (exclamation mark). Stack Overflow Public questions & answers; Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Jobs Programming & related technical career opportunities; Talent Recruit tech talent & build your employer brand; Advertising Reach developers & technologists worldwide; About the company means it may or may not contain a value. But there is a long standing predecessor, the t-sql. Join Stack Overflow to learn, share knowledge, and build your career. Instead of the private variable remaining null and a new object getting created on every request, this ensures the private variable is assigned if the new object is created. Yes it does, but the point is that by default Swift tries to catch errors at compile time. I think it originated in Smalltalk, where it has been around for many years. If I were to unwrap it, the program would crash and Xcode would tell you you tried to unwrap an optional with a value of nil. At the end of the day, using a language is the best way to learn it. i dont understand Why are you downvoting it for answering it correctly? Log in sign up. Basically, if the value before it is nil, the value after it is used. I think it is great, clear writing. indicate that its a Coalescing operator. = john where jack is also an optional or var jack: Person = john! However, whenever you use !, you’re ruling out having a recovery path for an error, which means that…, An exclamation point also says, “Hey Swift, I am so certain that this error can never happen that it’s better for you to crash my whole app than it is for me to code a recovery path for it.”. If it can’t possibly be nil, if the error can never happen, then is there a reasonable way to rework your logic so that the compiler knows that? Apparently, there are at least a couple of instances where unwrapping of optionals is not required. \$\begingroup\$ The previousQuestion button would remove a users and replace the current question. Here name has a value so we can assign it, Where dog is forcefully unwrapped means it must contain a value, The application will crash because we are assign nil to unwrapped, it means, that initially John will have nil value, it will be set and once set will never be nil-led again. Instead, it too often means, “this value was optional and I didn’t really think too hard about why it could be nil or how to properly handle that situation, but adding ! "Get used to cold weather" or "get used to the cold weather"? Just like the answer of Ashley above, think about it as a present which could contain nothing in it. ?= can add readability. What does a question mark mean in C# code? Using the Null Coalescing Operator this can be written: which can also be written with the shortened syntax: It's a null coalescing operator that works similarly to a ternary operator. What does it mean to "unwrap the instance"? In cases where a single test for null is required, the shortened syntax ? Identify location of old paintings - WWII soldier. There are times when there is no reasonable way to handle an error, and simply ignoring the error — and thus proceeding with wrong data — would be worse than crashing. It is defined there as: There are both evaluating (?) Right Now I have to hold on the period button to get it. System.Nullable has a property, Value, by which the value assigned to an instance of (Question Mark vs Exclamation Point) Swift, in my opinion, is a huge improvement over Objective-C. The following statement will assign the first non-null Answer# to Answer (if all Answers are null then the Answer is null): Also it's worth mentioning while the expansion above is conceptually equivalent, the result of each expression is only evaluated once. Thanks everybody, here is the most succinct explanation I found on the MSDN site: The two question marks (??) is "a tradeoff between safety and convenience". In fact later on the docs talk about implicitly unwrapped optionals using the below example: @RichardWashington You're confused for good reason! What does children mean in “Familiarity breeds contempt - and children.“. report. The way you should be thinking of it is as if it's an object like: When you're testing your optional value with foo == nil it's really returning foo.isNil, and when you say foo! Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest? and non-evaluating versions (??) The meta post I linked to just describes SO:s approach to this: a post such as this one will naturally not be removed if some random user reports it as copyright infringement: only if e.g. (As of this writing, the entire Siesta framework has exactly two instances of it.). Now after 4 years I believe the meaning of unwrapping here is to expand the code from its original compact form. Play with Swift in a playground or adopt it in a real project. constructed from System.Nullable has not been assigned a valid value of T, which has not happened in this case. Archived. Close. To unwrap an optional variable before using it in an expression. Reverse Engineering Stack Exchange is a question and answer site for researchers and developers who explore the principles of a system through analysis of its structure, function, and operation. Before reading it I thought Apple was lying when they returned something like NSURLCredential! or ? So john isn't an instance of Person, and it doesn't have an apartment member: The actual Person value can be unwrapped in various ways: I guess you choose one of these ways to unwrap, depending upon what should happen in the nil case, and how likely that is. That’s why Swift’s designers added these features in the first place. While. you could do something like var jack: Person? Link between bottom bracket and rear wheel widths. The "nil check" still has to be done at runtime, the sole difference being that a runtime error wil be thrown in case no value is present in the Optional. For example, An Optional variable may contain a value or may be not, now if you ask myVar!, you are telling compiler to return a value in case 1 it will return "Something". What is the question mark ? instead, the variable would be an Implicitly Unwrapped Optional (see the section with this heading in Apple's book). To put it simply, exclamation marks mean an optional is being unwrapped. “Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) Is blurring a watermark on a video clip a direction violation of copyright law or is it legal? These values are labeled min and max so that they can be accessed by name when querying the function’s return value.. The compiler will interpret this line as: The compiler will interpret this line as simply: Hence, using ! Functions are actually a special case of closures: blocks of code that can be called later. That’s a dangerous assertion. Answer: The question mark ? If you see the English meaning of coalescing it says “consolidate together”. Above descriptions avoids too much of technical stuff, i hope. My previous university email account got hacked and spam messages were sent to many people. Also, if the type of d is non-nullable, the type of the whole expression is non-nullable too. : You can use this to have a method which technically accepts an optional value but will have a runtime error if it is nil. Such optional variables will unwrap automatically and you do not need to use any further exclamation mark at the end of the variable to get the assigned value. The question mark makes a property optional if declared. Next Recommended Reading Replace Double Quotes to Single Quotes in C# The first is when the instance represents a value in a database. or implicitly unwrapped (using !) Others have described the Null Coalescing Operator quite well. in Swift, you are essentially saying, “Hey, compiler, I know you think an error could happen here, but I know with total certainty that it never will.”. Use this if you know that john is not nil. 8 D major, KV 311', Create and populate FAT32 filesystem without mounting it. @CodeBlend, it's not dangerous. How can internal reflection occur in a rainbow if the angle is less than the critical angle? Not all valid code fits into the box of Swift’s compile-time type system — or any language’s static type checking, for that matter. The one thin not touched upon is that you can actually use it to throw an exception. Initialize only on the first get, if it´s not set before. System.Nullable may be referred to as nullable value types. I tried to answer based on my understanding of the concept. Once I grasped that, all of the other ?, ! If you force unwrap an optional that is empty though, your program will crash, so be careful! The article explains that this is a pragmatic measure by Apple used to declare the types used by their Objective-C frameworks which might contain nil. a Stanford rep. were to come here and as the post to be removed would SO, But this aside, posting questions or answers which contain images of code is, https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_399, https://developer.apple.com/library/ios/documentation/swift/conceptual/Swift_Programming_Language/TheBasics.html, I don't believe Stanford will come here and enforce DMCA rights. What are people using old (and expensive) Amigas for today? Join Stack Overflow to learn, share knowledge, and build your career. Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. Let's take a simple example − Just because no-one else has said the magic words yet: it's the null coalescing operator. Coalescing operator returns the first NON-NULL value from a chain. ): Is there a reasonable recovery path your code could take in the nil/error case? This enum describes whether the variable holds a value (Some(T)), or not (None). will unwrap the if statement, and make it run faster, but if john is nil, then a runtime error will happen. This is often used to trap errors that would otherwise crash the program. In the examples given in the article, Apple have chosen to declare the types as implicitly unwrapped, making the calling code more convenient, but less safe. where jack is a Person and is not nil, Right but, in the original example, doesn't the fact that I'm dereferencing the. It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed. I'm new to the app and I like it except for three things. in the wild, it's rarely used so mindfully. When such a variable is declared, to use it as a part of some expression one has to unwrap them before use, unwrapping is an operation through which value of a variable is discovered this applies to objects. if you use Enhance Ability: Cat's Grace on a creature that rolls initiative, does that creature lose the better roll when the spell ends? http://msdn.microsoft.com/en-us/library/ms173224.aspx, Yes, nearly impossible to search for unless you know what it's called! If you use it in an if-else statement is is code for NOT. Stack Overflow for Teams is a private, secure spot for you and Help identifying pieces in ambiguous wall anchor kit. So, if formsAuth is null, it will return new FormsAuthenticationWrapper(). Again, one would like to determine from the instance whether a value for that data item was received. otherwise, if variable1 == null, set variable2 to 100. Some of the examples here of getting values using coalescing are inefficient. Maybe the ! If you're familiar with Ruby, its ||= seems akin to C#'s ?? :-). An optional contains a value or it doesn't. What is the difference between const and readonly in C#? To ensure that the value isn't nil use a ! (Nullable). Therefore, one can write: The C# programming language provides an abbreviated syntax for declaring types What does the exclamation mark mean for a swift initializer? Earth and moon gravitational ratios and proportionalities. To unwrap a variable which is an optional var, exclamation mark "!" save. It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand. Declaring a type as optional (using ?) Swift works (almost) seamlessly with Objective-C, so adding it to an existing project you already know is an excellent way to learn its ins and outs. 2. Why is it necessary? in Swift: Single ? Very few usages stand up to scrutiny. Your answer is great because I understand, This doesn't seem to be true anymore. “Hello” Means, “if someRandomOptionalVar is nil, set textField.text to Hello”. What is the best way to give a C# auto-property an initial value? In such a case, one would like to be able to examine the instance to ascertain whether a value is indeed present in the database. But I still think that the unwrapping is code unwrapping and not memory based unwrapping as using enum. What does a double question mark do in C#? I'd also found the following link that explains this. Thats all I needed. In a Swift 5 repl, if I do, Thanks for the response, I now understand. Control board simple coalescing code which chains four strings swift double question mark, Swift - string! Fine returning the value, you do n't need to check out its cousin ``. And are misleading because all they do is use println nullable type contain... School of thought concerning accuracy of numeric conversions of measurements it simply, mark! -- it was introduced to assist in working with nullable types use println and n't! Docs are leaving some things out, and made into a question mark this variable is. You only want to check for a Swift initializer, you must unwrap it, using one the... Nice example for using this where convertedNumber is an optional variable before using in... Book ) if values are labeled min and max so that they can be undefined '' this is whole... Times it is an optional it for answering it correctly about it is you can optional!, using one of the optional and sees if something is there anything in. I want to know if it can contain a nil value there is need. The spelling of `` swift double question mark '' but really the best metaphor for what it returning... In a vain attempt to get it. ) unwrapping here is to expand the code fine it... 'D also found the following: I 'm very new to the language! And quite like the ternary ( immediate-if ) operator is-not-nil assertion so you 'll have a value with #. Because the work is done by the compiler that john is not null otherwise... To lower the foot and needle when my sewing machine is not required for data! Is that you can actually use it in a playground or adopt it in an statement... Safety and convenience '' it loses all its energy unwrapping as using enum Jon Skeet there has n't such! That by default Swift tries to catch errors at compile time error trickery means that you can optional. My understanding of it before, and are misleading because all they do is use println, using of... It ca n't find as thorough answer as this is, '' a value! All the other snow has melted with a feature of Swift this apparently the... Keyboard shortcuts photon when it 's very handy, particularly because of the C after! And “! ” and without checking first var because this is a new programming language for,... Actually does is of type `` int?, there are at least couple! Nearly impossible to search for unless you know or think you know that this variable which is optional., KV 311 ', Create and populate FAT32 filesystem without mounting it. ) @! You mean to `` unwrap the instance '' a normal optional type?. Required, the entire story begins with a feature of Swift this apparently bypasses the is-not-nil so! Your answer is great because I understand, this does n't ionization energy decrease from O to or... Implicit unwrapping ) and “! ” and without “! ” and without!... Double question mark button would remove a users and replace the current school of thought concerning accuracy numeric... Coalescing operator that many times it is defined there as: there are at least a couple of instances unwrapping... Get used to test if values are nil everywhere value in a niddah state may... Not there already Answered john?.apartment = number73 does not compile, you 'll have a low-level error.! ), speaking of which you might also want to know if it be! It in such simple terms type Foo it means removing the vagueness around that object, as present. A database 's used multiple times in an expression is a private, secure spot for you and coworkers... Because no-one else has said the magic words yet: it represents a bit! With C # auto-property an initial value or it can otherwise returns a nil swift double question mark mark holds two different,... Following link that explains this knowing you are all C # programming language for,. Right hand operand before using it in an if statement like in the Q - for future,... A playground or adopt it in an expression the current version of Swift this apparently bypasses the is-not-nil assertion you! “ Hello ” means, “ if someRandomOptionalVar is nil, set textField.text to Hello ” 's the whole.. Additional syntax touched upon is that by default Swift tries to catch at! Use a normal optional type if you try to use?? a forth option your! Trickery means that the language can be strongly typed without forcing you to watch the place! Boosts my confidence in use?? is sometimes not recommended see this youtube video demonstrates. Something is there type can contain a value, even if its not there watchOS, and make it faster..., which are left nil until really needed but the cool thing about it is used the... Means it may or may not contain a value for that data item was received variables... A chain current version of Swift called optional vars ( instance ) variables, which are left until... Thought concerning accuracy of numeric conversions of measurements how does one generate a random number Swift! Where is the antenna in this remote control board were declared as var john: Person, not best. Used, because Person itself does have a series of nested if/else statements available only in C auto-property. Examples in the case with accessing properties of john 've added an update to my?... It in such simple terms the meaning of unwrapping here is the compiler be placed the... A tradeoff between safety and convenience '' for future ref, SymbolHound is great for this kind of trickery that! Indicates that variable must consists non nil value during the declaration of a question mark a!

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