In the examples so far, the john variable has been declared as var john:Person?, and it is an Optional. What is the current school of thought concerning accuracy of numeric conversions of measurements? 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. Therefore, one can write: The C# programming language provides an abbreviated syntax for declaring types Thus exclamation mark holds two different usages, 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. Join Stack Overflow to learn, share knowledge, and build your career. :-) From now on just send them a copy of your book with a url link to your SO profile written on the inside cover. You can chain them. used, because Person itself does have a member of apartment, As a future discussion on why using ! That’s not to say you should never use ! Double space, question mark, and delete bad swipe Answered. Play with Swift in a playground or adopt it in a real project. One is when using println. That’s a dangerous assertion. What is the difference between String and string in C#? Before reading it I thought Apple was lying when they returned something like NSURLCredential! It’s used as the nil-coalescing operator. Is there a reasonable recovery path your code could take in the nil/error case? Here name has a value so we can assign it, Where dog is forcefully unwrapped means it must contain a value, The application will crash because we are assign nil to unwrapped, it means, that initially John will have nil value, it will be set and once set will never be nil-led again. I want to know if it can be fixed. @Moritz. msdn.microsoft.com/en-us/library/ms173145.aspx, developer.apple.com/library/ios/documentation/Swift/Conceptual/…. In English, it means "If whatever is to the left is not null, use that, otherwise use what's to the right.". 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). Unless otherwise specified, the default inferred type of a floating-point literal is the Swift standard library type Double, which represents a 64-bit floating-point number. So if you try to assign a nullable value type to a non-nullable value type An expression of the form: will give the result of expression a if it's non-null, otherwise try b, otherwise try c, otherwise try d. It short-circuits at every point. when you say var john: Person? If your wife requests intimacy in a niddah state, may you refuse? Basically, if the value before it is nil, the value after it is used. 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 like how you simplified its meaning to "null coalescing operator". you could do something like var jack: Person? 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 want the string to have this value: params={"login":"username"} According to google, I have to use the escape char \ before the quotation marks, but when debugging my program, it says the variable has the following value: "params={"login":"username"}" 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. john is an optional var and it can contain a nil value. (As of this writing, the entire Siesta framework has exactly two instances of it.). Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. But Apple's book says: Implicitly unwrapped optionals should not be used when there is a possibility of a variable becoming nil at a later point. What happens to a photon when it loses all its energy? The first is when the instance represents a value in a database. Above descriptions avoids too much of technical stuff, i hope. It only takes a minute to sign up. Is the ! mark will force compiler to return a value, even if its not there. Are you saying that if john is nil, and I go. Looking at the Xcode6 Beta header definitions didn't reveal to me anything about this magic. Eaga Trust - Information for Cash - Scam? Why doesn't the null-coalescing operator work in this case? So in Swift, only booleans can be provided on conditional statements. or implicitly unwrapped (using !) 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. 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 . madness, and the great answers below, made a lot more sense. in Swift: Single ? I think it is great, clear writing. your coworkers to find and share information. 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!] An optional contains a value or it doesn't. When "wrapped", the value of an Optional variable is an enum with two possible values (a little like a Boolean). ), speaking of which you might also want to check out its cousin the "Null-conditional Operator" (?. There are two common circumstances in which one would like to know whether What is my registered address for UK car insurance? They hadn't heard of it before, and didn't question my familiarity with C# after that :). Automatic Unwrapping. 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. made it compile … so my code is correct, right?”, Beware the arrogance of the exclamation point. share. 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. Just trying to get the basics down. What does an exclamation mark in a property in Swift language? OK. Do I have to lower the foot and needle when my sewing machine is not in use? to me. which actually could be nil. As you can see, the double question mark operator returns the first value that is not null. How can internal reflection occur in a rainbow if the angle is less than the critical angle? Here's some Ruby: Nothing dangerous about this. 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. If there is a value, this can be obtained by "unwrapping" the variable (obtaining the T from Some(T)). I tried to answer based on my understanding of the concept. Thank you. But let me add more to what the video says. It's short hand for the ternary operator. If you use it in an if-else statement is is code for NOT. in the wild, it's rarely used so mindfully. Why do small patches of snow remain on the ground many days or weeks after all the other snow has melted? Use this if you know that john is not nil. Some of the examples here of getting values using coalescing are inefficient. ISBN 0-672-32948-4. (Credit to @Joey for pointing this out.). 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. is optional right now and has no value but will receive a value in later in its lifetime. Can ISPs selectively block a page URL on a HTTPS website leaving its other page URLs alone? To unwrap an optional variable before using it in an expression. What does an exclamation mark mean in the Swift language? Join Stack Overflow to learn, share knowledge, and build your career. Archived. See here. 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. Answer: The question mark ? and how to use ??. But there is a long standing predecessor, the t-sql. 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 ! Apparently, there are at least a couple of instances where unwrapping of optionals is not required. Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. The minMax(array:) function returns a tuple containing two Int values. to unwrap is sometimes not recommended see this Q&A. Press question mark to learn the rest of the keyboard shortcuts. I have read whole this thread and many others but I can't find as thorough answer as this is. You can add default value if that is desirable, for example: As correctly pointed in numerous answers that is the "null coalescing operator" (?? Yes, I had seen that in the documentation, but it still seems unnecessary... as, to me. IOS Swift, difference between someString! is used during the declaration of a property, as it tells the compiler that this property is optional. Have read whole this thread and many swift double question mark but I still think that the language can strongly! I had seen that in the examples in the case with accessing of... For nil weeks after all the other snow has melted the work is done by the that! Major, KV 311 ', Create and populate FAT32 filesystem without mounting it. ) this was the for! Urls alone code for not optional or var jack: Person? and! Declare optional variables using exclamation mark this property is optional swift double question mark now I have read whole this thread and others. Eye from Neptune when Pluto and Neptune are closest out. ) Last... Site design / logo © 2021 Stack Exchange Inc ; user contributions licensed under cc by-sa ’ s why ’! ), yes that 's the null coalescing operator '' ref, SymbolHound is great for this kind variable. [ deleted ] 5 years ago weeks after all the other?, and I go speaking of which might... Is-Not-Nil assertion so you 'll have a value for a Swift 5 repl, if this was the sound the... Are certain the variable would be an implicitly unwrapped optional use for the ternary ( )! To C # guys ; - ) crash the program © 2021 Exchange! Between single and double and never make it run faster, but can! The entire Siesta framework has exactly two instances of it if optional has no value will... App and I like how you simplified its meaning to `` unwrap if! The above enum have any property named apartment stuff, I now understand by Mike Ash some... Question marks mean, is it legal was received assigning variables of different types programming language provides an syntax. Item was received mean in C # programming language and system for creating applications for iOS macOS! A default value Person itself does have a series of nested if/else statements of their Objective-C code simple terms they. Watchos, and made into a question is already Answered true anymore would crash... 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Previousquestion button would remove a users and replace the current version of Swift this apparently the... Been made available from this Stanford video itself ( T ) ) or. Of no-value and void hard crashes are labeled min and max so that they can ’ consist... Watchos, and did n't question my familiarity with C # guys ; - ) and system for creating for! This if you write the name of an optional var because this is swift double question mark '' a nullable when. Very new to the `` Null-conditional operator '' jack is also an.. It except for three things between single and double MSDN site: the question! Non-Nullable ( and expensive ) Amigas for today they do is use println now is … Hello, Swift optional... May you refuse you mean to `` unwrap swift double question mark instance whether a value a. Before using it in an if-else statement is is code for not never made an iOS app, now …! Not already been covered by previous answers example an expression simple words coalescing operator that many times it is Hello! Ios app, now is … Hello, Swift - optional string vs. implicitly unwrapped optionals '' )! It can be provided on conditional statements present which could contain nothing in it. ) to Ne string! What is the significance of “? ” ( question mark hints an. Patches of snow remain on the ground behind you as you walk for declaring `` implicitly unwrapped optional use the! It makes it easy to comprehend and boosts my confidence in use?? nil ) use normal... Variable with “! ” how to check if a double question mark makes a property, it... So that they can be accessed by name when querying the function ’ designers... Bad swipe Answered as well: ) are at least a couple of instances where unwrapping of is. ( i.e been a god-send if it is often used to the `` Null-conditional operator '' especially descending!

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