optional

问题 I understand what optional are in Swift but I just encountered a ”Double Wrapped Optional’ , where if I don’t use two '!' Xcode gives an complier error Value of optional type 'String?' not unwrapped; did you mean to use '!' or ‘?'? I have the following code, where app is of type NSRunningApplication . let name: String = app.localizedName! Why should I have to use two ! ? Isn’t one enough to unwrap the variable because it is of type var localizedName: String? . Context: Xcode want me to use

问题 I'm wondering whether or not it is possible to have an optional array. Let's assume a schema like this: { "type": "record", "name": "test_avro", "fields" : [ {"name": "test_field_1", "type": "long"}, {"name": "subrecord", "type": [{ "type": "record", "name": "subrecord_type", "fields":[{"name":"field_1", "type":"long"}] },"null"] }, {"name": "simple_array", "type":{ "type": "array", "items": "string" } } ] } Trying to write an avro record without "simple_array" would result in a NPE in the

问题 Obviously, std::optional is the best choice to return an optional value from a function if one uses C++17 or boost (see also GOTW #90) std::optional<double> possiblyFailingCalculation() But what and why would be the best alternative if one is stuck with an older version (and can't use boost)? I see a few options: STL smart pointers (C++11 only) std::unique_ptr<double> possiblyFailingCalculation(); (+) virtually the same usage as optional (−) confusing to have smart pointers to non-polymorphic

问题 @RequestMapping(value = "/contact.html", method = RequestMethod.POST) public final ModelAndView contact( @RequestParam(value = "name", required = false) Optional<String> name) { How does Spring's @RequestMapping handle an Optional from Guava library if the parameter value is not required and nothing is sent? Will it be: Set to null Set to Optional.absent() Can Optional.fromNullable(T) be used to accept the request? 回答1: EDIT (October 2015): Spring 4 handles java.util.Optional (from Java 8)

问题 @RequestMapping(value = "/contact.html", method = RequestMethod.POST) public final ModelAndView contact( @RequestParam(value = "name", required = false) Optional<String> name) { How does Spring's @RequestMapping handle an Optional from Guava library if the parameter value is not required and nothing is sent? Will it be: Set to null Set to Optional.absent() Can Optional.fromNullable(T) be used to accept the request? 回答1: EDIT (October 2015): Spring 4 handles java.util.Optional (from Java 8)

问题 I was very confused about forced unwrapping and implicit unwrapping at first. Now, the following understanding comes from my self-study: There is no action available for implicit unwrapping, but there is something called implicitly unwrapped Optionals. Implicitly unwrapped Optionals and normal Optionals are both Optionals, the difference being when accessing an implicitly unwrapped Optional, you confidently know that there is a valid value under the hood and ready for use. Normal Optionals

问题 When I want to check if an Optional Bool is true, doing this doesn't work: var boolean : Bool? = false if boolean{ } It results in this error: Optional type '@IvalueBool?' cannot be used as a boolean; test for '!= nil' instead I don't want to check for nil; I want to check if the value returned is true. Do I always have to do if boolean == true if I'm working with an Optional Bool? Since Optionals don't conform to BooleanType anymore, shouldn't the compiler know that I want to check the value

问题 Below is my snippet // MARK: - Location Functions func getCurrentLocation() -> (String!, String!) { let location = LocationManager.sharedInstance.currentLocation?.coordinate return (String(location?.latitude), String(location?.longitude)) } func setCurrentLocation() { let (latitude, longitude) = getCurrentLocation() let location = "\(latitude!),\(longitude!)" print(location) } Though I unwrap optional using latitude! and longitude! , it prints me Optional(37.33233141),Optional(-122.0312186) I

问题 This question already has answers here : can I split a numeric string using multiple separators in a Swift closure? (3 answers) Closed 2 years ago . I want a way to detect input errors in a string and notify the user. Take the following example: let fraction = "15/8" let fractionArray = fraction.components(separatedBy: "/") let numerator = Double(fractionArray[0]) let denominator = Double(fractionArray[1]) var linearFactor = numerator! / denominator! print(numerator!, "/", denominator!, " = "

问题 I have some types which need to be handled with their own special deserializers and serializers, but how do I instruct Jackson to use them when the type is nested inside an Optional? I am using the JDK8Module which works great for any type not needing any special handling. The @JsonDeserialize and @JsonSerialize annotations don't seem to have any way to apply to the value inside an Optional when they are used on an Optional field: @JsonDeserialize(?????) Optional<SpecialType> myField 回答1: The