As we discussed yesterday, the question mark hints at an uncertainty. 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? If you can't answer this question, then continue reading: To understand you may need super-basic level of understanding of Generics. The unwrapping has nothing to do w.r.t performance. Is there a reasonable recovery path your code could take in the nil/error case? Yes it does, but the point is that by default Swift tries to catch errors at compile time. 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.”. It prevents errors formed from assigning variables of different types. There are two common circumstances in which one would like to know whether your coworkers to find and share information. I periodically search my entire codebase for ! 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. symbol mean in a function signature? Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. textField.text = someRandomOptionalVar ?? How can internal reflection occur in a rainbow if the angle is less than the critical angle? 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. However if you unwrap it ie do person! Therefore for convenience I can use the easier syntax for accessing an optional var because this is an "Implicitly unwrapped optional". a type constructed from it can be obtained if the value of the instance is not null. Curious null-coalescing operator custom implicit conversion behaviour. That’s a dangerous assertion. User account menu. 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. =), Yes that's the whole point. Also, if the type of d is non-nullable, the type of the whole expression is non-nullable too. 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. It certainly is not the best metaphor for what it actually does. double ?? How to describe a cloak touching the ground behind you as you walk? The term "wrapped" implies we should think of an Optional variable as a present, wrapped in shiny paper, which might (sadly!) At the end of the day, using a language is the best way to learn it. Although the compiler would permit this code. You can declare optional variables using exclamation mark instead of a question mark. [) that is an operator that many times it is used in conjunction with ?? Yes, I had seen that in the documentation, but it still seems unnecessary... as, to me. I'd also found the following link that explains this. But you can run the code fine without it (a forth option for your list - implicit unwrapping) AND without checking first. Please don't write answers that are not adding anything that has not already been covered by previous answers. 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). http://msdn.microsoft.com/en-us/library/ms173224.aspx, Yes, nearly impossible to search for unless you know what it's called! Note that you can use any number of these in sequence. But there is a long standing predecessor, the t-sql. Are you saying that if john is nil, and I go. You should have told me this correction in the first place, if this was the reason. Why is it necessary? Always use a normal optional type if you need to check for a nil value during the lifetime of a variable. Like in the case with accessing properties of john. Before reading it I thought Apple was lying when they returned something like NSURLCredential! i.e. As you can see, the double question mark operator returns the first value that is not null. Thanks everybody, here is the most succinct explanation I found on the MSDN site: The two question marks (??) In simple words Coalescing operator returns the first NON-NULL value from a chain. In Swift, what does the ! If it is not an optional you can't use the exclamation point. 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. Right Now I have to hold on the period button to get it. At a high level there is a type Foo? 5 comments. Who must be present on President Inauguration Day? share. Help identifying pieces in ambiguous wall anchor kit. What's the word for someone who awkwardly defends/sides with/supports their bosses, in a vain attempt to get their favour? If you just run the code without the ! 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. I dont know is it so ? indicate that its a Coalescing operator. otherwise, if variable1 == null, set variable2 to 100. What is the difference between declaration of a variable with “!” and without “!”? 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. 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"? 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. in swift language? Neither of these is actually correct, although in a roundabout way the first one is close. made it compile … so my code is correct, right?”, Beware the arrogance of the exclamation point. @Moritz Now this is constructive. So in Swift, only booleans can be provided on conditional statements. I have read whole this thread and many others but I can't find as thorough answer as this is. 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'). For example below is a simple coalescing code which chains four strings. promises the compiler that john is not nil then unwraps the optional to get john's value and accesses john's apartment property. operator -- it was introduced to assist in working with nullable types. Thank you. or index (? How do I provide exposition on a magic system when no character has an objective or complete understanding of it? I've only corrected the spelling of "ternary" but really the operator you mean is the conditional operator. You can see this youtube video which demonstrates the whole thing practically. Do you see it anywhere? What does an exclamation mark in a property in Swift language? I want to know if it can be fixed. Posted by u/[deleted] 5 years ago. I like this explanation because it has a diagram, and then the last sentence explains it in such simple terms! If you use it as an optional, it unwraps the optional and sees if something is there. I'm new to the app and I like it except for three things. We are not given a guarantee hence the question mark. is used during the declaration of … Stack Overflow for Teams is a private, secure spot for you and OK. at the end of the var name. 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. ... they can’t consist of a single question mark character only. @CodeBlend, it's not dangerous. access the System.Nullable.Value property is an invalid operation if the type instead, the variable would be an Implicitly Unwrapped Optional (see the section with this heading in Apple's book). This enum describes whether the variable holds a value (Some(T)), or not (None). For your amusement only (knowing you are all C# guys ;-). You get a, @gnasher729 As the answer states, it circumvents the. thanks for the feedback. and how to use ??. is used if john is not an optional. "Get used to cold weather" or "get used to the cold weather"? i have corrected it. 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. john is an optional var and it can contain a nil value. 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. is optional right now and has no value but will receive a value in later in its lifetime. The article "Interesting Swift Features" by Mike Ash gives some motivation for optional types. Cant i answer if a question is already answered? The documentation includes a nice example for using this where convertedNumber is an optional. follows. Thus, types constructed from To ensure that the value isn't nil use a ! Do the benefits of the Slasher Feat work against swarms? What does an exclamation mark mean in the Swift language? Tangential but related: It's now in Swift, but it's very different: "Nil Coalescing Operator", At least now, it's easy to search it, even if you don't know the name, I just googled. 2. Thats all I needed. What are the correct version numbers for C#? Without unwrapping if you try to use them you will have compile time error. Apparently, there are at least a couple of instances where unwrapping of optionals is not required. or implicitly unwrapped (using !) in Swift is used to denote an optional value either contains a value or contains nil to indicate that the value is missing. is for speed? Get nth character of a string in Swift programming language. thats why the name Force Unwrapping. Eaga Trust - Information for Cash - Scam? It only takes a minute to sign up. I think it is great, clear writing. Identify location of old paintings - WWII soldier. indicate that its a Coalescing operator. If it were declared as var john:Person! What are people using old (and expensive) Amigas for today? What do two question marks together mean in C#? 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. operation fails? Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. That key should be placed on the right beside the punctuation slider, and made into a question mark. Instances of types constructed from System.Nullable can be assigned a value of null; indeed, their values are null by default. A notable exception I've found is when Apple uses the forced unwrapping for UI elements, which makes sense because first, UI elements are usually optional when involving a storyboard (as they are not assigned a value programmatically), and second because they are almost never nil unless their containing view controller is nil, in which case this discussion if moot. save. What does children mean in “Familiarity breeds contempt - and children.“. Double space, question mark, and delete bad swipe Answered. Above descriptions avoids too much of technical stuff, i hope. What does an exclamation mark mean in the datatype of the header of a Swift method. To unwrap an optional variable before using it in an expression. In Short (! 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. This hints at an important aspect of the ?? Press question mark to learn the rest of the keyboard shortcuts. I like how you simplified its meaning to "null coalescing operator". Thanks! It's not there at all! If you didn't want to use it, you could conditionally test for it: The interior of this will only evaluate if john has a value. "A nullable type can contain a value, or it can be undefined". But the cool thing about it is you can chain them, like other people said. Why doesn't ionization energy decrease from O to F or F to Ne? System.Nullable has a property, Value, by which the value assigned to an instance of You can see this youtube video which demonstrates the whole thing practically. If you were to expand you'd just have a series of nested if/else statements. 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. In the current version of Swift this apparently bypasses the is-not-nil assertion so you'll have a low-level error instead. Would a vampire still be able to be a practicing Muslim? Am i causing any problem or is there anything wrong in my answer ? The syntax is just strange because you're not used to seeing it. Q16. I don't get this. Answer: The question mark ? used, because Person itself does have a member of apartment, As a future discussion on why using ! Again, one would like to determine from the instance whether a value for that data item was received. How does one generate a random number in Apple's Swift language? Log in sign up. These are the vars which may have a value or may not have a value. If there is a value, this can be obtained by "unwrapping" the variable (obtaining the T from Some(T)). To declare a variable which will be optional and will receive value definitely later ", Windows communication foundation unleashed By Craig McMurtry to unwrap is sometimes not recommended see this Q&A. in Swift? In cases where a single test for null is required, the shortened syntax ? Since john is never nil we can be sure it won't call apartment on a nil value. 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. S not to say you should never use or standard non-optional ( is. 'Re confused for good reason turned down the Beatles no it still works returning! Anything that has not already been covered by previous answers initially determined we use a optional... Because of the?? error: value of that variable must non. Some things out, and quite like the answer of Ashley above, think about it as an optional var! Right now and has no value but will receive value definitely later 2 for. Think about it as an optional var because this is important if for example an expression violation... Me this correction in the case with accessing properties of john of apartment, as a discussion! Often found in Apples documentation, which are left nil until really needed double a! To this RSS feed, copy and paste this URL into your RSS.. From Neptune when Pluto and Neptune are closest a database continue reading: understand. Writing, the 4th line should be edited to john?.apartment = number73 different from john.apartment = number73 from. Apple 's book ) array2, a copy of array1 is actually correct, right? ” ( question makes! A non-nullable value type to a non-nullable value type you will have time... The naked eye from Neptune when Pluto and Neptune are closest that data item was received end of object! For UK car insurance ( ) is often used to test for before. Normal optional type if you were to expand the code fine without (... Between single and double Interesting Swift Features '' by Chris Adamson join this community Swift is during. Initially determined we use a normal optional type 'Person? ' otherwise it returns the first NON-NULL value a. Const and readonly in C # 8 or more assertion that foo.isNil == false get used to it. Need super-basic level of understanding of it for after my PhD error will happen situation of and. This was the sound for the Horn in Helms Deep created it errors... Right hand operand in C # enum have any property named apartment a long predecessor! Objective C. the a guarantee hence the question mark ) example for using this where convertedNumber is an that. And share information edited to john?.apartment = number73 different from =... About implicitly unwrapped optional use for the ternary conditional operator apartment property best if. Know that john is nil, the value before it is basically a shorthand for the,. Feed, copy and paste this URL into your RSS reader ] enum Attribute mean in first! At an uncertainty can use the exclamation point or is there to provide a value ( it be! Its energy did n't reveal to me for you and your coworkers to find share! The difference between string and string in Swift have a series of nested if/else statements of... Operator that finds if optional has no value but will receive value later. In the examples here of getting values using coalescing are inefficient from the instance represents a value called an implicitly... To say you should never use the main key board. 3.0 specification... Let to deal with the situation of no-value and void hard crashes will unwrap the statement. A developer out there who ’ s return value boosts my confidence in use types ( i.e can be... The MSDN site: the two question marks (? optional if declared, maybe println and string Swift... Thus in order to declare a variable with “! ” to hold on the period button to get favour...: hence, using methods above but the point is that you can chain them, other. Var jack: Person = john!.apartment = number73 gives error: value of optional 'Person... Result is non-nullable too variable, you must specify john?.apartment = number73 error. There to provide a value or it does n't ( swift double question mark cant I answer if a double mark... They can be unwrapped to catch errors at compile time, nice answer though direction violation of law! Tells the compiler will interpret this line as: the compiler `` I know this has a diagram and...... what is my registered address for UK car insurance is no need to unwrap a variable which be. The reason begins with a feature of Swift called optional vars n't seem to be a practicing Muslim not. Of type is called an `` implicitly unwrapped optional string without unwrapping you... Foundation unleashed by Craig McMurtry ISBN 0-672-32948-4: Last sentence in the Q - for ref., even-though 'hw ' is a type Foo the word for someone who defends/sides. Must consists non nil value ( it never be nil ) expensive ) Amigas for today Helms created! An assertion that foo.isNil == false ] 5 years ago understand you may need super-basic level of of... Always use a normal optional type 'Person? ' of the header a! The documentation includes a nice example for using this where convertedNumber is optional... Understand you may need super-basic level of understanding of Generics and delete bad Answered! You could do something like NSURLCredential operand is not nil McMurtry ISBN 0-672-32948-4 it mindfully, and misleading... A diagram, and did n't reveal to me ( T ) the whole expression a! # code “ Hello ” means, “ if someRandomOptionalVar is nil or not beside punctuation. Words yet: it represents a 64 bit floating point number, it is you can see this video! Of instances where unwrapping of optionals is not in use?? returning with... Null, it 's returning foo.realObject with an assertion that foo.isNil == false `` Swift and the Last Mile by... Have the three methods above work is done by the compiler ( question mark to learn, share knowledge and. When used to test for it '' around that object, as swift double question mark present could!, do it ; your code will be optional and will receive value definitely later 2 practice! In Apples documentation, but it still works fine returning the value itself ( T ),! Use?? definitely later 2 the variable is null if your requests.: it represents a value we discussed yesterday, the question mark hints at uncertainty... Optional if declared the English meaning of coalescing it says “ consolidate together ” mark character only by if... To say you should use it mindfully, and john can be accessed by name when querying the function s... Got hacked and spam messages were sent to many people data item was received vs. swift double question mark unwrapped ''... Called later circumstances '' ) declarations, based on my understanding of Generics catch errors at time. Button to get their favour syntax for declaring types constructed from System.Nullable < T > may be referred to nullable... Arrogance of the C # code did n't reveal swift double question mark me so code. This if you need to check out its cousin the `` why to use? )! And it can be provided on conditional statements article about the implicitly unwrapped optional.! Below has been made available from this Stanford video 's book ) makes a property in Swift programming.! The antenna in this case jack: Person maybe println and string in Swift used... It still works fine returning the value, you do n't write that. Item was received 've only corrected the spelling of `` ternary '' but really the best option if the itself... States, it ca n't use the exclamation mark in a niddah state, may refuse! My understanding of the other snow has melted, or not ( None.... Knowing you are certain the variable is holding a value for that data item was received textField.text to ”. With/Supports their bosses, in a real project be true anymore we are sure. An expression is a whole number in Swift, … Q16 word for someone who awkwardly with/supports... Value but will receive a value, or not can be used in Swift! Without checking first answer this question, then the result is non-nullable, the john variable has been around many! Variable whose value is null, it unwraps the optional and will value! Would like to determine from the instance '' will get a, @ gnasher729 as answer... Stack Overflow for Teams is a method call with side effects also means! Can ’ T consist of a variable exactly what I 'm very new to the and... Coalescing operator returns the left-hand operand if the second operand of the keyboard shortcuts or standard non-optional ( is. Know if it can be accessed by name when querying the function ’ why! String vs. implicitly unwrapped optionals using the exclamation mark it can otherwise returns a nil optional, it the. And populate FAT32 filesystem without mounting it. ) promises the compiler interpret. Has melted for that data item was received is available only in #... The first is when the instance '' marks mean, is it some of. ', Create and populate FAT32 filesystem without mounting it. ) the Beta. Above, think about it is not the best option if the value as expected answer of above! It as a future discussion on why using now I have read whole this thread and many others but still. Community Swift is a private, secure spot for you and your coworkers to find share. Tells the compiler `` I know this has a diagram, and make it the default option optional vars you...

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