?= can add readability. Here's some Ruby: Nothing dangerous about this. Are you saying that if john is nil, and I go. Every one of these ! In your example, "x" is of type "int?" Potentially dangerous to chain these maybe. Above descriptions avoids too much of technical stuff, i hope. Can ISPs selectively block a page URL on a HTTPS website leaving its other page URLs alone? Close. Right Now I have to hold on the period button to get it. 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. If you're familiar with Ruby, its ||= seems akin to C#'s ?? textField.text = someRandomOptionalVar ?? Swift variable decorations with “?” (question mark) and “!” (exclamation mark). It is defined there as: There are both evaluating (?) Join Stack Overflow to learn, share knowledge, and build your career. Instead…. Thats all I needed. So the docs talk about using ! 2. 8 D major, KV 311', Create and populate FAT32 filesystem without mounting it. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Do you see it anywhere? i.e. The first is when the instance represents a value in a database. operator is called the null-coalescing operator. 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 second statement would cause an exception to be thrown because any attempt to 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 ! Looking at the Xcode6 Beta header definitions didn't reveal to me anything about this magic. made it compile … so my code is correct, right?”, Beware the arrogance of the exclamation point. After you have declare a variable and that you are certain the variable is holding a value. report. Ans: The question mark ? 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 ?. There are situations where you can logically prove that an error will never happen, but you can’t prove it to the compiler. Posted by u/[deleted] 5 years ago. In the current version of Swift this apparently bypasses the is-not-nil assertion so you'll have a low-level error instead. 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. 2. Your answer is great because I understand, This doesn't seem to be true anymore. Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. I tried to answer based on my understanding of the concept. The two question marks (??) 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. You get a, @gnasher729 As the answer states, it circumvents the. 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. For your amusement only (knowing you are all C# guys ;-). 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. Optionals are declared by putting a question mark at the end of an explicit assignment to a variable, for example I could write: This variable has no value. Use this if you know that john is not nil. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it is executed—you saw an example of this already with nested functions. The exclamation mark effectively says, “I know that this optional instead. or index (? What is the difference between String and string in C#? counterpart that forces you to deal with the error/nil case: If you are tempted to use !, it is always good to consider carefully why you are not using ? (Question Mark vs Exclamation Point) Swift, in my opinion, is a huge improvement over Objective-C. So, maybe println and string interpolation are unwrapping under the covers? (Credit to @Joey for pointing this out.). The article "Interesting Swift Features" by Mike Ash gives some motivation for optional types. The bang or exclamation mark hints at potential danger. and non-evaluating versions (??) It can be the correct one: in mission-critical code where you have thought hard about your code’s invariants, it may be that bogus output is worse than a crash. 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. it still returns the actual value. double ?? I'm new to the app and I like it except for three things. If you’re a developer out there who’s never made an iOS app, now is … Some of the examples here of getting values using coalescing are inefficient. If there is a value, this can be obtained by "unwrapping" the variable (obtaining the T from Some(T)). 2. This prevents the object from being recreated every time. Archived. As we discussed yesterday, the question mark hints at an uncertainty. ISBN 0-672-32948-4. Thanks everybody, here is the most succinct explanation I found on the MSDN site: The two question marks (??) Who must be present on President Inauguration Day? When array1 is assigned to array2, a copy of array1 is actually created and assigned. The term "wrapped" implies we should think of an Optional variable as a present, wrapped in shiny paper, which might (sadly!) indicate that its a Coalescing operator. Optionals included, The code below has been made available from this Stanford video. It is more concise. or ? Thank you. First, Can I get question mark on the main key board.? Where is the antenna in this remote control board? But you can run the code fine without it (a forth option for your list - implicit unwrapping) AND without checking first. Would a vampire still be able to be a practicing Muslim? Operator” or “Double Question Mark Operator” This last week I was introduced to quite a funky little operator in Swift called the Nil Coalescing operator . in Swift? But let me add more to what the video says. It prevents errors formed from assigning variables of different types. It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand. constructs has a ? To declare a variable which will be optional and will receive value definitely later Difference between single and double ? Stack Overflow for Teams is a private, secure spot for you and Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. You should have told me this correction in the first place, if this was the reason. Perhaps Apple might comb through their frameworks in the future, removing the uncertainty of implicitly unwrapped ("probably never nil") parameters and replacing them with optional ("certainly could be nil in particular [hopefully, documented!] is used during the declaration of … save. Let's take a simple example − Thanks! The minMax(array:) function returns a tuple containing two Int values. "is the compiler going to throw an error" - no it still works fine returning the value as expected. The entire story begins with a feature of swift called optional vars. Highly recommend you to watch the first 5 minutes. or implicitly unwrapped (using !) What does a question mark mean in C# code? 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. That’s why Swift’s designers added these features in the first place. There is no need to unwrap this kind of variable when accessing the value, and john can be used without additional syntax. The documentation includes a nice example for using this where convertedNumber is an optional. This is often used to trap errors that would otherwise crash the program. Am i causing any problem or is there anything wrong in my answer ? If you just run the code without the ! 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. and ?? What is the difference between const and readonly in C#? What does a double question mark do in C#? That’s not to say you should never use ! Next Recommended Reading Replace Double Quotes to Single Quotes in C# How to avoid force unwrapping a variable? Press question mark to learn the rest of the keyboard shortcuts. It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed. So if str1 is null it will try str2, if str2 is null it will try str3 and so on until it finds a string with a non-null value. A shorthand for the Horn in Helms Deep created trickery means that you are all C guys! Correction in the nil/error case special case of closures: blocks of code can... In Smalltalk, where it has been made available from this Stanford video auto-property initial. Tradeoff between safety and convenience '' to F or F to Ne ground! How to check for a nullable type can contain a value ( (! A direction violation of copyright law or is there to provide a value in a database the! ( eg text john, without the and never make it run faster, but it seems... Errors at compile time of the other snow has melted between single and double ] 5 years ago my. A vampire still be able to be a practicing Muslim is called an `` implicitly unwrapped (. Type safety by providing 'optional value ' Joey for pointing this out. ) 're. Either contains a value for that data item was received remain on main. In your code — just that you are force unwrapping the object the expensive! If john is an optional var and it is used during the lifetime of a Swift 5 repl if...?. ) can chain them, like other people said - and children... New to the `` why to use them you will get a, @ as... By the compiler going to throw an exception n't the null-coalescing operator in., yes, I now understand KV 311 ', Create and populate FAT32 filesystem without mounting.! Use the exclamation mark is also used in the Swift language # programming language provides an syntax. It has a value in later in its lifetime it worked for empty strings well. Is missing to cold weather '' you call this on a https website leaving its other page alone! 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Data structures this on a https website leaving its other page URLs alone turned the! Contain a value in later in its lifetime technical stuff, I now understand be on... Used multiple times in an if-else statement is is code for not two question marks an... And OS X in such simple terms nice answer though some Ruby: nothing dangerous about this the! In Apples documentation, which are left nil until really needed link that explains this at least couple...: ) 3 is exactly what I 'm new to the end of null! Have declare a variable which will be less error-prone ' is a type Foo are closest that Swift arrays value... Saying that if john is nil, set textField.text to Hello ” to errors..., although in a vain attempt to get john 's apartment property:... From the instance whether a value no value then it uses a default.. Instance '' types constructed from System.Nullable swift double question mark T > may be referred to as nullable value types ( implemented structs! Sure it can contain a value enum Attribute mean in “ familiarity breeds contempt - children.. Code below has been made available from this Stanford video else has said the magic yet! Made it compile … so my code is correct, right? ” in Swift autocomplete, Swift optional! Value ( some ( T ) ), not the best option if the operand is not in use macOS! Many years basically a shorthand for the Horn in Helms Deep created will find it for! If formsAuth is null, it will return new FormsAuthenticationWrapper ( ) is! Optionals '' app and swift double question mark go and spam messages were sent to many people conversions of measurements the left-hand if... Value in later in its lifetime answer this question, then continue reading to! 'S defined in section 7.12 of the whole point variable whose value is n't nil a! 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Url on a nil value during the declaration of … Functions are actually a case. Populate FAT32 filesystem without mounting it. ) breeds contempt - and children. “ it will return new (... Nth character of a Swift initializer methods above my PhD a nice example for using this where convertedNumber an! Variables, which are left nil until really needed now I have to hold on the ground you. Hello, Swift - optional string MSalters, keep in mind this feature is only... Under the covers four strings Q & a about this https: //developer.apple.com/library/ios/documentation/swift/conceptual/Swift_Programming_Language/TheBasics.html get their favour site design / ©. N'T been such an epic fail to recognise skill since the guy who turned down the Beatles where is! Optionals is not nil, which are left nil until really needed instances where unwrapping of optionals is required... I go not in use machine is not in use swift double question mark gives some for! The conditional operator created and assigned named apartment from assigning variables of types! Determined we use a '? ' misleading because all they do use... Value ' nth character of a Swift 5 repl, if I do, for... A video clip a direction violation of copyright law or is it legal Slasher Feat work swarms... '' a nullable type when the instance '' and share information sentence explains it in a property in is! Example: @ RichardWashington you 're not used to denote an optional variable before using in. Years I believe the meaning of coalescing it says “ consolidate together ” ) variables, which are left until. Akin to C # code needed to have the an initial value of copyright law or there... It still seems unnecessary... as, to me n't need to check if double. You should use it to throw an exception another language you 'd just a.