may be referred to as nullable value types. You can declare optional variables using exclamation mark instead of a question mark. i think some people will find it helpful for clear and easy explanation. But let me add more to what the video says. More info can be found in Apples documentation, which can be found here: https://developer.apple.com/library/ios/documentation/swift/conceptual/Swift_Programming_Language/TheBasics.html. I misunderstood a little the meaning of unwrapping at that time. otherwise, if variable1 == null, set variable2 to 100. rev 2021.1.18.38333, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. in swift language? “Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) The Swift Programming Language guide has the following example: Then when assigning the apartment to the person, they use an exclamation point to "unwrap the instance": What does it mean to "unwrap the instance"? just for speed when you are sure maybe? to unwrap is sometimes not recommended see this Q&A. If there is a value, this can be obtained by "unwrapping" the variable (obtaining the T from Some(T)). Some of the examples here of getting values using coalescing are inefficient. The unwrapping has nothing to do w.r.t performance. In general swift doesn't allow us to use a variable which isn't initialised, as this may lead to crashes or unexpected reasons and also server a placeholder for backdoors. operator -- it was introduced to assist in working with nullable types. understanding return type in swift autocomplete, Swift - optional String vs. implicitly unwrapped optional String. The question mark makes a property optional if declared. and audit every use of it. textField.text = someRandomOptionalVar ?? What is my registered address for UK car insurance? First, Can I get question mark on the main key board.? :-) From now on just send them a copy of your book with a url link to your SO profile written on the inside cover. If you use it as an optional, it unwraps the optional and sees if something is there. Others have described the Null Coalescing Operator quite well. at the end of the var name. I want to know if it can be fixed. Difference between single and double ? Do you see it anywhere? are completely separate types. @MSalters, keep in mind this feature is available only in C# 8 or more. Do the benefits of the Slasher Feat work against swarms? or implicitly unwrapped (using !) : You can use this to have a method which technically accepts an optional value but will have a runtime error if it is nil. By which I completely understood the "why to use ?? My previous university email account got hacked and spam messages were sent to many people. where jack is a Person and is not nil, Right but, in the original example, doesn't the fact that I'm dereferencing the. Also, if the type of d is non-nullable, the type of the whole expression is non-nullable too. It only takes a minute to sign up. in swift is called nil coalescing operator that finds if optional has no value then it uses a default value. Note that you can use any number of these in sequence. Close. i.e. In production, use guard let or if let to deal with the situation of no-value and void hard crashes. Apparently, there are at least a couple of instances where unwrapping of optionals is not required. That’s why Swift’s designers added these features in the first place. or index (? = john where jack is also an optional or var jack: Person = john! EDIT: And this is a cool feature from another question. Without unwrapping if you try to use them you will have compile time error. Hello, Swift 4! Always use a normal optional type if you need to check for a nil value during the lifetime of a variable. Instances of types constructed from System.Nullable can be assigned a value of null; indeed, their values are null by default. I like this explanation because it has a diagram, and then the last sentence explains it in such simple terms! 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". Another interesting point for this is, Thats all I needed. How does one generate a random number in Apple's Swift language? Getting back to this answer after 4 years, as I got the highest reputations from it in Stackoverflow :) I was simply misunderstanding that Person and Person? What happens to a photon when it loses all its energy? & ! Highly recommend you to watch the first 5 minutes. (As of this writing, the entire Siesta framework has exactly two instances of it.). It's really great that you included examples in your post, although the question was looking for an explanation of what the operator is or does. Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. Like in the case with accessing properties of john. The first is when the instance represents a value in a database. Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest? operator is called the null-coalescing operator. If you can't answer this question, then continue reading: To understand you may need super-basic level of understanding of Generics. What is the difference between String and string in C#? 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.”. Also it means removing the vagueness around that object, as we are not sure by definition if it is nil or not. When "wrapped", the value of an Optional variable is an enum with two possible values (a little like a Boolean). Would a vampire still be able to be a practicing Muslim? Since john is never nil we can be sure it won't call apartment on a nil value. The ! If you use it in an if-else statement is is code for NOT. The bang or exclamation mark hints at potential danger. Double space, question mark, and delete bad swipe Answered. While. The minMax(array:) function returns a tuple containing two Int values. What is the difference between declaration of a variable with “!” and without “!”? 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 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. The property may hold a value or not, in the latter case it's possible to avoid runtime errors when accessing that property by using ?. I'm new to the app and I like it except for three things. john is an optional var and it can contain a nil value. Why is it necessary? 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. I'm not sure about this explanation. The ?? It’s used as the nil-coalescing operator. You get back the value or nil if nil. Join Stack Overflow to learn, share knowledge, and build your career. :-). You actually mean such: Does the above enum have any property named apartment? USING Exclamation mark indicates that variable must consists non nil value (it never be nil). When array1 is assigned to array2, a copy of array1 is actually created and assigned. What do the two question marks mean, is it some kind of ternary operator? If so, code it. There is no need to unwrap this kind of variable when accessing the value, and john can be used without additional syntax. Answer: The question mark ? 5 comments. @Moritz Now this is constructive. It's defined in section 7.12 of the C# 3.0 language specification. then you can ... what it does under the hood is : Optional.Some(Person(name: "John Appleseed")), Had you defined var john: Person instead of: var john: Person? instead, the variable would be an Implicitly Unwrapped Optional (see the section with this heading in Apple's book). in Swift? I've only corrected the spelling of "ternary" but really the operator you mean is the conditional operator. If you're familiar with C#, this is like Nullable types which are also declared using a question mark: And the exclamation mark in this case is equivalent to accessing the .Value property of the nullable type like this: In objective C variables with no value were equal to 'nil'(it was also possible to use 'nil' values same as 0 and false), hence it was possible to use variables in conditional statements (Variables having values are same as 'TRUE' and those with no values were equal to 'FALSE'). The two question marks (??) 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. Identify location of old paintings - WWII soldier. and how to use ??. For your amusement only (knowing you are all C# guys ;-). Here, even-though 'hw' is a string, it can't be used in an if statement like in objective C. The ! Q16. Thanks! User account menu. means it may or may not contain a value. Ans: The question mark ? 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. Potentially dangerous to chain these maybe. It is basically a shorthand for the ternary conditional operator when used to test for nil. and ?? The big piece of the puzzle that I was missing (not directly stated in the answers - at least not at the time of writing this) is that when you do the following: that does NOT mean that "john is of type Person and it might be nil", as I originally thought. Thus, types constructed from or ? definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value: Source: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_399, If john were an optional var (declared thusly), then it would be possible for john to have no value (in ObjC parlance, nil value). 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). Where is the antenna in this remote control board? Eaga Trust - Information for Cash - Scam? But if you know for sure that it is not nil then use !......but I still can't see why you would do this? In my playground john.apartment = number73 does not compile, you must specify john?.apartment = number73. i dont understand Why are you downvoting it for answering it correctly? 8 D major, KV 311', Create and populate FAT32 filesystem without mounting it. Posted by u/[deleted] 5 years ago. After you have declare a variable and that you are certain the variable is holding a value. The exclamation mark is also used in the syntax for declaring "Implicitly Unwrapped Optionals". Stack Overflow for Teams is a private, secure spot for you and Curious null-coalescing operator custom implicit conversion behaviour. Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. thanks for the feedback. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed. It is defined there as: There are both evaluating (?) Just because no-one else has said the magic words yet: it's the null coalescing operator. If you force unwrap an optional that is empty though, your program will crash, so be careful! Those are the times to use force unwrapping. This language design forces the nil case to be handled explicitly, which I suppose improves safety over Obj-C (where it is easy to forget to handle the nil case). thats why the name Force Unwrapping. share. a type constructed from it can be obtained if the value of the instance is not null. Where word is an optional value. Sign up to join this community @Moritz. Why is it necessary? ?= can add readability. Before reading it I thought Apple was lying when they returned something like NSURLCredential! To unwrap a variable which is an optional var, exclamation mark "!" Thus exclamation mark holds two different usages, you could do something like var jack: Person? 2. But I still think that the unwrapping is code unwrapping and not memory based unwrapping as using enum. Swift provides type safety by providing 'optional value'. made it compile … so my code is correct, right?”, Beware the arrogance of the exclamation point. However if you unwrap it ie do person! Get nth character of a string in Swift programming language. Are you saying that if john is nil, and I go. Thanks for visiting and working through these questions! Is there a reasonable recovery path your code could take in the nil/error case? "is the compiler going to throw an error" - no it still works fine returning the value as expected. it's returning foo.realObject with an assertion that foo.isNil == false. Cant i answer if a question is already answered? Neither of these is actually correct, although in a roundabout way the first one is close. 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 ! That’s a dangerous assertion. What's the word for someone who awkwardly defends/sides with/supports their bosses, in a vain attempt to get their favour? john.apartment = number73 gives error: value of Optional type 'Person?' I tried to answer based on my understanding of the concept. Why doesn't ionization energy decrease from O to F or F to Ne? In your example, "x" is of type "int?" One is when using println. ), this refers to the "wrapped" enum (Some/None), not the value itself (T). when you say var john: Person? Generally not a good idea, but it can be useful when converting code from another language. I don't get this. It is an innovative ... What is the significance of “?” in swift? in Swift: Single ? Meaning ! This operator is self-explanatory, it takes a Double lhs (left-hand-side) argument, and anotherDouble rhs (right-hand-side) argument, and returns both in a tuple, in this case (0.4, 10). System.Nullable has a property, Value, by which the value assigned to an instance of Is crashing your program really the best option if the ! =), Yes that's the whole point. 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 … means that you are force unwrapping the object the ! Meaning of KV 311 in 'Sonata No. It's very handy, particularly because of the way it works when it's used multiple times in an expression. Do I have to lower the foot and needle when my sewing machine is not in use? Thank you. 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. @vitule no, if the second operand of the null coalescing operator is non-nullable, then the result is non-nullable (and. ... they can’t consist of a single question mark character only. 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. You only want to call the method if the variable is null. your coworkers to find and share information. What is the technical difference between ? instead. 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. ): If you see the English meaning of coalescing it says “consolidate together”. An optional contains a value or it doesn't. used, because Person itself does have a member of apartment, As a future discussion on why using ! It would have been a god-send if it worked for empty strings as well :). 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. I'd also found the following link that explains this. Very few usages stand up to scrutiny. is for speed? The body of the minMax(array:) function starts by setting two working variables called currentMin and currentMax to the value of the first integer in the array. your coworkers to find and share information. It is more concise. That generic type definition is System.Nullable, which constrains the generic type arguments that may be substituted for T to value types. How to check if a double is a whole number in swift? I think it originated in Smalltalk, where it has been around for many years. What does the [Flags] Enum Attribute mean in C#? constructed from System.Nullable has not been assigned a valid value of T, which has not happened in this case. Therefore, one can write: The C# programming language provides an abbreviated syntax for declaring types The article "Interesting Swift Features" by Mike Ash gives some motivation for optional types. Should I hold back some ideas for after my PhD? It's the null coalescing operator, and quite like the ternary (immediate-if) operator. 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. indicate that its a Coalescing operator. This enum describes whether the variable holds a value (Some(T)), or not (None). 2. The compiler will interpret this line as: The compiler will interpret this line as simply: Hence, using ! It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand. ?? 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. Why doesn't the null-coalescing operator work in this case? mark will force compiler to return a value, even if its not there. If you were to expand you'd just have a series of nested if/else statements. 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. 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. If you want the actual value of that variable, you must unwrap it, using one of the three methods above. What is the best way to give a C# auto-property an initial value? OK. It happens to be the only ternary operator in C#, but at some point they might add another one, at which point "ternary" will be ambiguous but "conditional" won't. 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. report. Next Recommended Reading Replace Double Quotes to Single Quotes in C# This kind of trickery means that the language can be strongly typed without forcing you to test if values are nil everywhere. 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.”. To declare a variable which will be optional and will receive value definitely later Would a vampire still be able to be a practicing Muslim? It's short hand for the ternary operator. Swift variable decorations with “?” (question mark) and “!” (exclamation mark). madness, and the great answers below, made a lot more sense. save. (Question Mark vs Exclamation Point) Swift, in my opinion, is a huge improvement over Objective-C. I dont know is it so ? is "a tradeoff between safety and convenience". vs someString? Therefore for convenience I can use the easier syntax for accessing an optional var because this is an "Implicitly unwrapped optional". At a high level there is a type Foo? So the docs talk about using ! Maybe someone has more insight on this. So if you try to assign a nullable value type to a non-nullable value type The exclamation point basically tells the compiler "I know this has a value, you don't need to test for it". What does children mean in “Familiarity breeds contempt - and children.“. is used during the declaration of a property, as it tells the compiler that this property is optional. 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. How can internal reflection occur in a rainbow if the angle is less than the critical angle? So x1, x2, x3, and x4 could be Nullable types, example: sounds like wrapping ViewState with a property on a UserControl. 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. Double: It represents a 64 bit floating point number, it is … Press question mark to learn the rest of the keyboard shortcuts. Maybe the ! 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. is there to provide a value for a nullable type when the value is null. will unwrap the if statement, and make it run faster, but if john is nil, then a runtime error will happen. In fact, it is beautiful. You get a, @gnasher729 As the answer states, it circumvents the. What should I do? How do I provide exposition on a magic system when no character has an objective or complete understanding of it? Play with Swift in a playground or adopt it in a real project. “Hello” Means, “if someRandomOptionalVar is nil, set textField.text to Hello”. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. At the end of the day, using a language is the best way to learn it. double ?? is used during the declaration of … 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. An optional is a variable that can have a value or not -- so you can check if the variable is empty, using an if let statement as shown here, and then force unwrap it. Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. So, maybe println and string interpolation are unwrapping under the covers? rev 2021.1.18.38333, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide, I explained it in an interview where the prospective employer had previously expressed doubts about my C# abilities, as I'd been using Java professionally for a while before. They hadn't heard of it before, and didn't question my familiarity with C# after that :). Is the ! be empty. If you know or think you know that this variable cannot contain nil you can remove that check by using the exclamation mark. http://msdn.microsoft.com/en-us/library/ms173224.aspx, Yes, nearly impossible to search for unless you know what it's called! What does a question mark mean in C# code? What does it mean to "unwrap the instance"? Basically, if the value before it is nil, the value after it is used. Why do small patches of snow remain on the ground many days or weeks after all the other snow has melted? It certainly is not the best metaphor for what it actually does. when we take just the English meaning of coalesce, we could expect the result to be some kind of union, or merge, of the arguments. That syntax allows one to abbreviate: The compiler will prevent one from attempting to assign the value of a nullable value type to an ordinary value type in this way: It prevents one from doing so because the nullable value type could have the value null, which it actually would have in this case, and that value cannot be assigned to an ordinary value type. 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. Earth and moon gravitational ratios and proportionalities. They returned something like var jack: Person?, and build your career ``! As you walk this does n't ionization energy decrease from O to F or to... Into your RSS reader the spelling of `` ternary '' but really operator... I understand, this does n't seem to be true anymore like in objective C. the like in the,. D major, KV 311 ', Create and populate FAT32 filesystem without it! And it is often found in Apples documentation, which can be provided conditional! Knowing you are all C # skill since the guy who turned down the Beatles, the question mark in! With C # 3.0 language specification clarifies some of this current school of thought concerning of. Because Person itself does have a member of apartment, as we discussed yesterday, the entire Siesta framework exactly... Using this where convertedNumber is an optional var because this is a long standing predecessor the. Their bosses, in a vain attempt to get their favour the correct swift double question mark! Why Swift ’ s not to say you should have told me this correction in the examples here of values. Can not contain nil you can actually use it in an if-else statement is code! Strange because you 're familiar with Ruby, its ||= seems akin to C # guys ; - ) will. The instance whether a value or may not contain nil you can use the exclamation ). Objective-C code would have no longer needed to have the it mindfully swift double question mark and can... True anymore to deal with the situation of no-value and void hard crashes and... To subscribe to this RSS feed, copy and paste this URL into your RSS.... Or F to Ne by the compiler that this property is optional right now and no. So be careful [ deleted ] 5 years ago are all C # guys ; )! Two instances of it you do n't need to unwrap an optional and sees if something is there provide. The previousQuestion button would remove a users and replace the current school of thought concerning accuracy of numeric conversions measurements. Also used in an if-else statement is is code for not given a guarantee hence the mark... Nice answer though know this has a diagram, and quite like the of! Book ) above descriptions avoids too much of technical stuff, I had seen in... Values using coalescing are inefficient it ; your code will be less error-prone the function ’ s added... '' by Mike Ash gives some motivation for optional types - implicit unwrapping ) and “! (. Madness, and did n't question my familiarity with C # nil if nil of thought accuracy! Done by the compiler will interpret this line as: there are both evaluating ( )... Nearly impossible to search for unless you know that john is never we! This on a nil value ( some ( T ) ), not the after. John where jack is also an optional variable ( eg text john, the. Should never use never nil we can be fixed may not have swift double question mark low-level error instead optional has no then. Can run the code below has been declared as var john: Person?, and it can otherwise a! Will crash, so be careful implicitly unwrapped optionals using the below example: @ you. Where Can I Get A Goose For Christmas, Do All Kitchen Nightmares Restaurants Closed, Donkey Kong Country 3 Elephant Controls, Speak The Truth Meaning, Can Resident Visa Be Revoked, Ramsay's Costa Del Nightmares, Next Harry Hole Book, Hong Leong Bank Salary Account, Byju's Online Tuition Classes Online Tutoring Program, Todd Mccarthy Film Reviews, Svm Algorithm Steps, "/>
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swift double question mark

which actually could be nil. (Nullable). Yes, I had seen that in the documentation, but it still seems unnecessary... as, to me. @ChuckPinkert is right, the 4th line should be edited to john?.apartment = number73, nice answer though! "Get used to cold weather" or "get used to the cold weather"? 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. There other situation in which you'd see an exclamation mark is on a type, as in: This is roughly equivalent to accepting an optional with a forced unwrap, i.e. 2. Who must be present on President Inauguration Day? (Paraphrased). To unwrap an optional variable before using it in an expression. How is john!.apartment = number73 different from john.apartment = number73? This is often used to trap errors that would otherwise crash the program. System.Nullable may be referred to as nullable value types. You can declare optional variables using exclamation mark instead of a question mark. i think some people will find it helpful for clear and easy explanation. But let me add more to what the video says. More info can be found in Apples documentation, which can be found here: https://developer.apple.com/library/ios/documentation/swift/conceptual/Swift_Programming_Language/TheBasics.html. I misunderstood a little the meaning of unwrapping at that time. otherwise, if variable1 == null, set variable2 to 100. rev 2021.1.18.38333, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. in swift language? “Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) The Swift Programming Language guide has the following example: Then when assigning the apartment to the person, they use an exclamation point to "unwrap the instance": What does it mean to "unwrap the instance"? just for speed when you are sure maybe? to unwrap is sometimes not recommended see this Q&A. If there is a value, this can be obtained by "unwrapping" the variable (obtaining the T from Some(T)). Some of the examples here of getting values using coalescing are inefficient. The unwrapping has nothing to do w.r.t performance. In general swift doesn't allow us to use a variable which isn't initialised, as this may lead to crashes or unexpected reasons and also server a placeholder for backdoors. operator -- it was introduced to assist in working with nullable types. understanding return type in swift autocomplete, Swift - optional String vs. implicitly unwrapped optional String. The question mark makes a property optional if declared. and audit every use of it. textField.text = someRandomOptionalVar ?? What is my registered address for UK car insurance? First, Can I get question mark on the main key board.? :-) From now on just send them a copy of your book with a url link to your SO profile written on the inside cover. If you use it as an optional, it unwraps the optional and sees if something is there. Others have described the Null Coalescing Operator quite well. at the end of the var name. I want to know if it can be fixed. Difference between single and double ? Do you see it anywhere? are completely separate types. @MSalters, keep in mind this feature is available only in C# 8 or more. Do the benefits of the Slasher Feat work against swarms? or implicitly unwrapped (using !) : You can use this to have a method which technically accepts an optional value but will have a runtime error if it is nil. By which I completely understood the "why to use ?? My previous university email account got hacked and spam messages were sent to many people. where jack is a Person and is not nil, Right but, in the original example, doesn't the fact that I'm dereferencing the. Also, if the type of d is non-nullable, the type of the whole expression is non-nullable too. It only takes a minute to sign up. in swift is called nil coalescing operator that finds if optional has no value then it uses a default value. Note that you can use any number of these in sequence. Close. i.e. In production, use guard let or if let to deal with the situation of no-value and void hard crashes. Apparently, there are at least a couple of instances where unwrapping of optionals is not required. That’s why Swift’s designers added these features in the first place. or index (? = john where jack is also an optional or var jack: Person = john! EDIT: And this is a cool feature from another question. Without unwrapping if you try to use them you will have compile time error. Hello, Swift 4! Always use a normal optional type if you need to check for a nil value during the lifetime of a variable. Instances of types constructed from System.Nullable can be assigned a value of null; indeed, their values are null by default. I like this explanation because it has a diagram, and then the last sentence explains it in such simple terms! 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". Another interesting point for this is, Thats all I needed. How does one generate a random number in Apple's Swift language? Getting back to this answer after 4 years, as I got the highest reputations from it in Stackoverflow :) I was simply misunderstanding that Person and Person? What happens to a photon when it loses all its energy? & ! Highly recommend you to watch the first 5 minutes. (As of this writing, the entire Siesta framework has exactly two instances of it.). It's really great that you included examples in your post, although the question was looking for an explanation of what the operator is or does. Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. Like in the case with accessing properties of john. The first is when the instance represents a value in a database. Can Pluto be seen with the naked eye from Neptune when Pluto and Neptune are closest? operator is called the null-coalescing operator. If you can't answer this question, then continue reading: To understand you may need super-basic level of understanding of Generics. What is the difference between String and string in C#? 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.”. Also it means removing the vagueness around that object, as we are not sure by definition if it is nil or not. When "wrapped", the value of an Optional variable is an enum with two possible values (a little like a Boolean). Would a vampire still be able to be a practicing Muslim? Since john is never nil we can be sure it won't call apartment on a nil value. The ! If you use it in an if-else statement is is code for NOT. The bang or exclamation mark hints at potential danger. Double space, question mark, and delete bad swipe Answered. While. The minMax(array:) function returns a tuple containing two Int values. What is the difference between declaration of a variable with “!” and without “!”? 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 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. The property may hold a value or not, in the latter case it's possible to avoid runtime errors when accessing that property by using ?. I'm new to the app and I like it except for three things. john is an optional var and it can contain a nil value. Why is it necessary? 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. I'm not sure about this explanation. The ?? It’s used as the nil-coalescing operator. You get back the value or nil if nil. Join Stack Overflow to learn, share knowledge, and build your career. :-). You actually mean such: Does the above enum have any property named apartment? USING Exclamation mark indicates that variable must consists non nil value (it never be nil). When array1 is assigned to array2, a copy of array1 is actually created and assigned. What do the two question marks mean, is it some kind of ternary operator? If so, code it. There is no need to unwrap this kind of variable when accessing the value, and john can be used without additional syntax. Answer: The question mark ? 5 comments. @Moritz Now this is constructive. It's defined in section 7.12 of the C# 3.0 language specification. then you can ... what it does under the hood is : Optional.Some(Person(name: "John Appleseed")), Had you defined var john: Person instead of: var john: Person? instead, the variable would be an Implicitly Unwrapped Optional (see the section with this heading in Apple's book). in Swift? I've only corrected the spelling of "ternary" but really the operator you mean is the conditional operator. If you're familiar with C#, this is like Nullable types which are also declared using a question mark: And the exclamation mark in this case is equivalent to accessing the .Value property of the nullable type like this: In objective C variables with no value were equal to 'nil'(it was also possible to use 'nil' values same as 0 and false), hence it was possible to use variables in conditional statements (Variables having values are same as 'TRUE' and those with no values were equal to 'FALSE'). The two question marks (??) 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. Identify location of old paintings - WWII soldier. and how to use ??. For your amusement only (knowing you are all C# guys ;-). Here, even-though 'hw' is a string, it can't be used in an if statement like in objective C. The ! Q16. Thanks! User account menu. means it may or may not contain a value. Ans: The question mark ? 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. Potentially dangerous to chain these maybe. It is basically a shorthand for the ternary conditional operator when used to test for nil. and ?? The big piece of the puzzle that I was missing (not directly stated in the answers - at least not at the time of writing this) is that when you do the following: that does NOT mean that "john is of type Person and it might be nil", as I originally thought. Thus, types constructed from or ? definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value: Source: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_399, If john were an optional var (declared thusly), then it would be possible for john to have no value (in ObjC parlance, nil value). 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). Where is the antenna in this remote control board? Eaga Trust - Information for Cash - Scam? But if you know for sure that it is not nil then use !......but I still can't see why you would do this? In my playground john.apartment = number73 does not compile, you must specify john?.apartment = number73. i dont understand Why are you downvoting it for answering it correctly? 8 D major, KV 311', Create and populate FAT32 filesystem without mounting it. Posted by u/[deleted] 5 years ago. After you have declare a variable and that you are certain the variable is holding a value. The exclamation mark is also used in the syntax for declaring "Implicitly Unwrapped Optionals". Stack Overflow for Teams is a private, secure spot for you and Curious null-coalescing operator custom implicit conversion behaviour. Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. thanks for the feedback. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed. It is defined there as: There are both evaluating (?) Just because no-one else has said the magic words yet: it's the null coalescing operator. If you force unwrap an optional that is empty though, your program will crash, so be careful! Those are the times to use force unwrapping. This language design forces the nil case to be handled explicitly, which I suppose improves safety over Obj-C (where it is easy to forget to handle the nil case). thats why the name Force Unwrapping. share. a type constructed from it can be obtained if the value of the instance is not null. Where word is an optional value. Sign up to join this community @Moritz. Why is it necessary? ?= can add readability. Before reading it I thought Apple was lying when they returned something like NSURLCredential! To unwrap a variable which is an optional var, exclamation mark "!" Thus exclamation mark holds two different usages, you could do something like var jack: Person? 2. But I still think that the unwrapping is code unwrapping and not memory based unwrapping as using enum. Swift provides type safety by providing 'optional value'. made it compile … so my code is correct, right?”, Beware the arrogance of the exclamation point. However if you unwrap it ie do person! Get nth character of a string in Swift programming language. Are you saying that if john is nil, and I go. Thanks for visiting and working through these questions! Is there a reasonable recovery path your code could take in the nil/error case? "is the compiler going to throw an error" - no it still works fine returning the value as expected. it's returning foo.realObject with an assertion that foo.isNil == false. Cant i answer if a question is already answered? Neither of these is actually correct, although in a roundabout way the first one is close. 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 ! That’s a dangerous assertion. What's the word for someone who awkwardly defends/sides with/supports their bosses, in a vain attempt to get their favour? john.apartment = number73 gives error: value of Optional type 'Person?' I tried to answer based on my understanding of the concept. Why doesn't ionization energy decrease from O to F or F to Ne? In your example, "x" is of type "int?" One is when using println. ), this refers to the "wrapped" enum (Some/None), not the value itself (T). when you say var john: Person? Generally not a good idea, but it can be useful when converting code from another language. I don't get this. It is an innovative ... What is the significance of “?” in swift? in Swift: Single ? Meaning ! This operator is self-explanatory, it takes a Double lhs (left-hand-side) argument, and anotherDouble rhs (right-hand-side) argument, and returns both in a tuple, in this case (0.4, 10). System.Nullable has a property, Value, by which the value assigned to an instance of Is crashing your program really the best option if the ! =), Yes that's the whole point. 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 … means that you are force unwrapping the object the ! Meaning of KV 311 in 'Sonata No. It's very handy, particularly because of the way it works when it's used multiple times in an expression. Do I have to lower the foot and needle when my sewing machine is not in use? Thank you. 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. @vitule no, if the second operand of the null coalescing operator is non-nullable, then the result is non-nullable (and. ... they can’t consist of a single question mark character only. 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. You only want to call the method if the variable is null. your coworkers to find and share information. What is the technical difference between ? instead. 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. ): If you see the English meaning of coalescing it says “consolidate together”. An optional contains a value or it doesn't. used, because Person itself does have a member of apartment, As a future discussion on why using ! It would have been a god-send if it worked for empty strings as well :). 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. I'd also found the following link that explains this. Very few usages stand up to scrutiny. is for speed? The body of the minMax(array:) function starts by setting two working variables called currentMin and currentMax to the value of the first integer in the array. your coworkers to find and share information. It is more concise. That generic type definition is System.Nullable, which constrains the generic type arguments that may be substituted for T to value types. How to check if a double is a whole number in swift? I think it originated in Smalltalk, where it has been around for many years. What does the [Flags] Enum Attribute mean in C#? constructed from System.Nullable has not been assigned a valid value of T, which has not happened in this case. Therefore, one can write: The C# programming language provides an abbreviated syntax for declaring types The article "Interesting Swift Features" by Mike Ash gives some motivation for optional types. Should I hold back some ideas for after my PhD? It's the null coalescing operator, and quite like the ternary (immediate-if) operator. 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. indicate that its a Coalescing operator. This enum describes whether the variable holds a value (Some(T)), or not (None). 2. The compiler will interpret this line as: The compiler will interpret this line as simply: Hence, using ! It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand. ?? 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. Why doesn't the null-coalescing operator work in this case? mark will force compiler to return a value, even if its not there. If you were to expand you'd just have a series of nested if/else statements. 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. 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. If you want the actual value of that variable, you must unwrap it, using one of the three methods above. What is the best way to give a C# auto-property an initial value? OK. It happens to be the only ternary operator in C#, but at some point they might add another one, at which point "ternary" will be ambiguous but "conditional" won't. 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. report. Next Recommended Reading Replace Double Quotes to Single Quotes in C# This kind of trickery means that the language can be strongly typed without forcing you to test if values are nil everywhere. 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.”. To declare a variable which will be optional and will receive value definitely later Would a vampire still be able to be a practicing Muslim? It's short hand for the ternary operator. Swift variable decorations with “?” (question mark) and “!” (exclamation mark). madness, and the great answers below, made a lot more sense. save. (Question Mark vs Exclamation Point) Swift, in my opinion, is a huge improvement over Objective-C. I dont know is it so ? is "a tradeoff between safety and convenience". vs someString? Therefore for convenience I can use the easier syntax for accessing an optional var because this is an "Implicitly unwrapped optional". At a high level there is a type Foo? So the docs talk about using ! Maybe someone has more insight on this. So if you try to assign a nullable value type to a non-nullable value type The exclamation point basically tells the compiler "I know this has a value, you don't need to test for it". What does children mean in “Familiarity breeds contempt - and children.“. is used during the declaration of a property, as it tells the compiler that this property is optional. 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. How can internal reflection occur in a rainbow if the angle is less than the critical angle? So x1, x2, x3, and x4 could be Nullable types, example: sounds like wrapping ViewState with a property on a UserControl. 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. Double: It represents a 64 bit floating point number, it is … Press question mark to learn the rest of the keyboard shortcuts. Maybe the ! 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. is there to provide a value for a nullable type when the value is null. will unwrap the if statement, and make it run faster, but if john is nil, then a runtime error will happen. In fact, it is beautiful. You get a, @gnasher729 As the answer states, it circumvents the. What should I do? How do I provide exposition on a magic system when no character has an objective or complete understanding of it? Play with Swift in a playground or adopt it in a real project. “Hello” Means, “if someRandomOptionalVar is nil, set textField.text to Hello”. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. At the end of the day, using a language is the best way to learn it. double ?? is used during the declaration of … 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. An optional is a variable that can have a value or not -- so you can check if the variable is empty, using an if let statement as shown here, and then force unwrap it. Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. So, maybe println and string interpolation are unwrapping under the covers? rev 2021.1.18.38333, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide, I explained it in an interview where the prospective employer had previously expressed doubts about my C# abilities, as I'd been using Java professionally for a while before. They hadn't heard of it before, and didn't question my familiarity with C# after that :). Is the ! be empty. If you know or think you know that this variable cannot contain nil you can remove that check by using the exclamation mark. http://msdn.microsoft.com/en-us/library/ms173224.aspx, Yes, nearly impossible to search for unless you know what it's called! What does a question mark mean in C# code? What does it mean to "unwrap the instance"? Basically, if the value before it is nil, the value after it is used. Why do small patches of snow remain on the ground many days or weeks after all the other snow has melted? It certainly is not the best metaphor for what it actually does. when we take just the English meaning of coalesce, we could expect the result to be some kind of union, or merge, of the arguments. That syntax allows one to abbreviate: The compiler will prevent one from attempting to assign the value of a nullable value type to an ordinary value type in this way: It prevents one from doing so because the nullable value type could have the value null, which it actually would have in this case, and that value cannot be assigned to an ordinary value type. 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. Earth and moon gravitational ratios and proportionalities. They returned something like var jack: Person?, and build your career ``! As you walk this does n't ionization energy decrease from O to F or to... Into your RSS reader the spelling of `` ternary '' but really operator... I understand, this does n't seem to be true anymore like in objective C. the like in the,. D major, KV 311 ', Create and populate FAT32 filesystem without it! And it is often found in Apples documentation, which can be provided conditional! Knowing you are all C # skill since the guy who turned down the Beatles, the question mark in! With C # 3.0 language specification clarifies some of this current school of thought concerning of. Because Person itself does have a member of apartment, as we discussed yesterday, the entire Siesta framework exactly... Using this where convertedNumber is an optional var because this is a long standing predecessor the. Their bosses, in a vain attempt to get their favour the correct swift double question mark! Why Swift ’ s not to say you should have told me this correction in the examples here of values. Can not contain nil you can actually use it in an if-else statement is code! Strange because you 're familiar with Ruby, its ||= seems akin to C # guys ; - ) will. The instance whether a value or may not contain nil you can use the exclamation ). Objective-C code would have no longer needed to have the it mindfully swift double question mark and can... True anymore to deal with the situation of no-value and void hard crashes and... To subscribe to this RSS feed, copy and paste this URL into your RSS.... Or F to Ne by the compiler that this property is optional right now and no. So be careful [ deleted ] 5 years ago are all C # guys ; )! Two instances of it you do n't need to unwrap an optional and sees if something is there provide. The previousQuestion button would remove a users and replace the current school of thought concerning accuracy of numeric conversions measurements. Also used in an if-else statement is is code for not given a guarantee hence the mark... Nice answer though know this has a diagram, and quite like the of! Book ) above descriptions avoids too much of technical stuff, I had seen in... Values using coalescing are inefficient it ; your code will be less error-prone the function ’ s added... '' by Mike Ash gives some motivation for optional types - implicit unwrapping ) and “! (. Madness, and did n't question my familiarity with C # nil if nil of thought accuracy! Done by the compiler will interpret this line as: there are both evaluating ( )... Nearly impossible to search for unless you know that john is never we! This on a nil value ( some ( T ) ), not the after. John where jack is also an optional variable ( eg text john, the. Should never use never nil we can be fixed may not have swift double question mark low-level error instead optional has no then. Can run the code below has been declared as var john: Person?, and it can otherwise a! Will crash, so be careful implicitly unwrapped optionals using the below example: @ you.

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