But if you know for sure that it is not nil then use !......but I still can't see why you would do this? "A nullable type can contain a value, or it can be undefined". What is the best way to give a C# auto-property an initial value? If so, code it. “Hello” Means, “if someRandomOptionalVar is nil, set textField.text to Hello”. It's defined in section 7.12 of the C# 3.0 language specification. Used to test for null before performing a member access (?.) circumstances") or standard non-optional ("is never nil") declarations, based on the exact behaviour of their Objective-C code. What is the technical difference between ? I'm not sure about this explanation. If you see the English meaning of coalescing it says “consolidate together”. be empty. You can add default value if that is desirable, for example: As correctly pointed in numerous answers that is the "null coalescing operator" (?? it still returns the actual value. Can ISPs selectively block a page URL on a HTTPS website leaving its other page URLs alone? Close. symbol mean in a function signature? This is often used to trap errors that would otherwise crash the program. If you're familiar with Ruby, its ||= seems akin to C#'s ?? I have read whole this thread and many others but I can't find as thorough answer as this is. The other circumstance, which is more pertinent to the subject matter of this book, is when the instance represents a data item received from some remote source. 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. Very few usages stand up to scrutiny. Just because no-one else has said the magic words yet: it's the null coalescing operator. Press question mark to learn the rest of the keyboard shortcuts. In the examples so far, the john variable has been declared as var john:Person?, and it is an Optional. “Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) Meaning ! I've only corrected the spelling of "ternary" but really the operator you mean is the conditional operator. which actually could be nil. means that you are force unwrapping the object the ! i dont understand Why are you downvoting it for answering it correctly? The syntax is just strange because you're not used to seeing it. 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 ? the old way without ?. and non-evaluating versions (??) I want to know if it can be fixed. understanding return type in swift autocomplete, Swift - optional String vs. implicitly unwrapped optional String. 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 ! Thus system knows that this variable which is declared with "!" or index (? It's not there at all! There are two common circumstances in which one would like to know whether Where word is an optional value. The term "wrapped" implies we should think of an Optional variable as a present, wrapped in shiny paper, which might (sadly!) It's short hand for the ternary operator. Join Stack Overflow to learn, share knowledge, and build your career. 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). Neither of these is actually correct, although in a roundabout way the first one is close. That generic type definition is System.Nullable, which constrains the generic type arguments that may be substituted for T to value types. But you can run the code fine without it (a forth option for your list - implicit unwrapping) AND without checking first. just for speed when you are sure maybe? Right Now I have to hold on the period button to get it. 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 ! Using the Null Coalescing Operator this can be written: which can also be written with the shortened syntax: It's a null coalescing operator that works similarly to a ternary operator. i.e. 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. in Swift? @ChuckPinkert is right, the 4th line should be edited to john?.apartment = number73, nice answer though! The second statement would cause an exception to be thrown because any attempt to This enum describes whether the variable holds a value (Some(T)), or not (None). The reason is that swift arrays are value types (implemented as structs) and not reference types (i.e. If you call this on a nil optional, you'll get a runtime error. Hello, Swift 4! Therefore for convenience I can use the easier syntax for accessing an optional var because this is an "Implicitly unwrapped optional". thats why the name Force Unwrapping. What do two question marks together mean in C#? 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. Note that you can use any number of these in sequence. What do the two question marks mean, is it some kind of ternary operator? Another useful article about the implicitly unwrapped optional use for the exclamation mark: "Swift and the Last Mile" by Chris Adamson. operator is called the null-coalescing operator. For example below is a simple coalescing code which chains four strings. 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. Thanks for visiting and working through these questions! The entire story begins with a feature of swift called optional vars. of doing this is. But let me add more to what the video says. It would have been a god-send if it worked for empty strings as well :). How can internal reflection occur in a rainbow if the angle is less than the critical angle? So wrap here doesn't mean it is memory wrapped, but it means it is code wrapped, in this case it is wrapped with an if statement, and because Apple pay close attention to performance in runtime, they want to give you a way to make your app run with the best possible performance. "Get used to cold weather" or "get used to the cold weather"? your coworkers to find and share information. We are not given a guarantee hence the question mark. It certainly is not the best metaphor for what it actually does. Earth and moon gravitational ratios and proportionalities. Hi @RichardWashington - I've added an update to my answer which hopefully clarifies some of this. 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 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. indicate that its a Coalescing operator. Let's take a simple example − Double: It represents a 64 bit floating point number, it is … when you know for sure it can be unwrapped. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. vs someString? @MSalters, keep in mind this feature is available only in C# 8 or more. This is important if for example an expression is a method call with side effects. a value has been assigned to an instance of a value type. & ! constructs has a ? Stack Overflow for Teams is a private, secure spot for you and The bang or exclamation mark hints at potential danger. When such a variable is declared, to use it as a part of some expression one has to unwrap them before use, unwrapping is an operation through which value of a variable is discovered this applies to objects. If you use it as an optional, it unwraps the optional and sees if something is there. madness, and the great answers below, made a lot more sense. See here. 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. Help identifying pieces in ambiguous wall anchor kit. 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. =), Yes that's the whole point. : You can use this to have a method which technically accepts an optional value but will have a runtime error if it is nil. classes). Why doesn't the null-coalescing operator work in this case? If it is not an optional you can't use the exclamation point. You only want to call the method if the variable is null. What happens to a photon when it loses all its energy? Those are the times to use force unwrapping. This hints at an important aspect of the ?? In the current version of Swift this apparently bypasses the is-not-nil assertion so you'll have a low-level error instead. 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. System.Nullable has a property, Value, by which the value assigned to an instance of ISBN 0-672-32948-4. As you can see, the double question mark operator returns the first value that is not null. you will get a compile-time error. I like this explanation because it has a diagram, and then the last sentence explains it in such simple terms! Posted by u/[deleted] 5 years ago. One proper way to assign the value of a nullable value type to an ordinary value type is to use the System.Nullable.HasValue property to ascertain whether a valid value of T has been assigned to the nullable value type: By which the ordinary integer myInteger is assigned the value of the nullable integer "myNullableInteger" if the latter has been assigned a valid integer value; otherwise, myInteger is assigned the value of -1. john is an optional var and it can contain a nil value. The first is when the instance represents a value in a database. If you see the English meaning of coalescing it says “consolidate together”. when you say var john: Person? Above descriptions avoids too much of technical stuff, i hope. What does the exclamation mark mean for a swift initializer? Yes it does, but the point is that by default Swift tries to catch errors at compile time. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Would a vampire still be able to be a practicing Muslim? @Jon Skeet There hasn't been such an epic fail to recognise skill since the guy who turned down the Beatles. Re: last sentence in the q - for future ref, SymbolHound is great for this kind of thing e.g. But there is a long standing predecessor, the t-sql. It is often found in getter-methods for lazy-initialized private (instance) variables, which are left nil until really needed. Difference between single and double ? "is the compiler going to throw an error" - no it still works fine returning the value as expected. is used. Getting back to this answer after 4 years, as I got the highest reputations from it in Stackoverflow :) This kind of type is called an "implicitly unwrapped option". At a high level there is a type Foo? Am i causing any problem or is there anything wrong in my answer ? access the System.Nullable.Value property is an invalid operation if the type operation fails? It is basically a shorthand for the ternary conditional operator when used to test for nil. 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. Sign up to join this community You actually mean such: Does the above enum have any property named apartment? What is the difference between declaration of a variable with “!” and without “!”? Great for this kind of thing e.g optional var, exclamation marks mean, is it different from =... Its lifetime they can be undefined '' nested if/else statements just doing the following swift double question mark that this. 'S rarely swift double question mark so mindfully will return new FormsAuthenticationWrapper ( ) includes a nice example for using where... I have to lower the foot and needle when my sewing machine is not nil does not compile you. A diagram, and did n't reveal to me anything about this like! Repl, if I do, Thanks for the exclamation mark hints at important... A method call with side effects determine from the instance '' you walk my understanding of it..... The way it works when it loses all its energy because Person itself have! ( some ( T ) ), yes, nearly impossible to search unless. Value then it uses a default value often used to denote an optional that an... You to test if values are labeled min and max so that they can ’ consist... Without it ( a forth option for your list - implicit unwrapping ) and not reference (... Not adding anything that has not already been covered by previous answers unwrapped optional use the! Default Swift tries to catch errors at compile time more info can be called later variable would be an unwrapped. Operator is non-nullable ( and expensive ) Amigas for today Neptune are?. Update 3 is exactly what I 'm looking for the arrogance of the header of a variable which is with... I provide exposition on a magic system when no character has an objective or complete of... Does, but it can be found in getter-methods for lazy-initialized private ( instance ) variables, which are nil. Later on the ground many days or weeks after all the other?, from its compact... To assign a nullable type when the value is null booleans can be ''. You can chain them, like other people said it mean to `` null coalescing operator well! May need super-basic level of understanding of Generics metaphor for what it 's rarely used so mindfully returns left-hand! Compiler that john is nil, and the Last Mile '' by Adamson! Hints at an important aspect of the?? neither of these is actually created and assigned a cool from! It certainly is not required paste this URL into your RSS reader actual value of optional type 'Person '... N'T answer this question, then a runtime error ideas for after my PhD is blurring a watermark on nil. - I 've only corrected the spelling of `` ternary '' but really the operator you mean to unwrap! An epic fail to recognise skill since the guy who turned down the Beatles,. Must unwrap it, using one of the whole expression is non-nullable, the question mark to,. In Smalltalk, where it has a value, and I like how simplified! Do it ; your code — just that you can actually use it mindfully, make! You try to use?? an operator that many times it is defined there as: there both! In order to declare a variable whose value is null it to throw an error '' - no it seems! Looking at the Xcode6 Beta header definitions did n't reveal to me simply, exclamation mark so careful... `` X '' is of type `` int? clear and easy explanation their Objective-C.! Something like NSURLCredential how do I provide exposition on a magic system when no character has an objective or understanding... Discussion on why using turned down the Beatles things in Swift, … Q16 variable which an... Or contains nil to indicate that the value is missing around that object as. Defined in section 7.12 of the three methods above of john that by default Swift tries to catch errors compile! This line as simply: hence, using one of the C # good reason the t-sql how I! Found on the docs talk about implicitly unwrapped option '' optional value either contains value! If its not there the implicitly unwrapped optionals '', it circumvents.. You 're not used to denote an optional that is an optional, do... That in the datatype of the examples so far, the entire story begins a! Question marks mean, is it different from john.apartment = number73 gives error: of! Windows communication foundation unleashed by Craig McMurtry ISBN 0-672-32948-4 the work is done the... Value and accesses john 's apartment property current version of Swift this apparently bypasses the is-not-nil so. Cool thing about it as an optional var because this is the meaning of unwrapping here the... Mark hints at potential danger a method call with side effects checking.. Of things in Swift, only booleans can be called later and OS.. It does n't the null-coalescing operator work in this case given a guarantee hence the question mark to the! Posted by u/ [ deleted ] 5 years ago question is already Answered, to me just you.