9.6. Swift optional type

发布时间 ： 2025-10-25 13:33:50 UTC      

Page Views: 9 views

The optional (Optional) type of Swift, which is used to handle cases where values are missing. Optional means “there is a value, and it is equal to x” or “there is no value”.

Swfit language definition suffix? As an abbreviation for named type Optional, in other words, the following two declarations are equal:

var optionalInteger: Int? var optionalInteger: Optional    
 In both cases, the variable optionalInteger is an optional integer type. Note that in the type and   ?   with no space between. 
 
 Optional is an enumeration with two cases, None and   Some(T)   used to indicate that there may or may not be a value Any type can be explicitly declared (or implicitly converted) as an optional type. When declaring an optional type, be sure to give it in parentheses. An appropriate range of operators. For example, declare an array of optional integers, which should be written as (Int [])? write Int[] ? Will make a mistake. 
 
 When you declare an optional variable or optional property without providingan initial value, its value defaults to nil. 
 
 Optional compliance with the LogicValue protocol, so it can appear in a Boolean environment. In this case, if the optional type T? Contains any value of type T (that is, its value is   Optional.Some(T)   this optional type is equal to   true   instead of being regarded as   false   . 
 
 If an instance of an optional type contains a value, you can use the suffix operator! To access this value, as follows: 
 
 
 
optionalInteger = 42 optionalInteger! // 42 
 
 
 
 Use operators! To get an optional variable with a value of nil will have a run-time error. 
 
 You can optionally perform operations on optional expressions with optional links and optional bindings. If the value is   nil   no action will be performed, and no run error will be reported 
 
 Let’s take a closer look at the following examples to understand the application of optional types in Swift: 
 
 
 
import Cocoa var myString:String? = nil if myString != nil { print(myString) }else{ print("The string is nil") } 
 
 
 
 The execution results of the above procedures are as follows: 
 
 
 
The string is nil 
 
 
 
 The optional type is similar to the pointer in Objective-C   nil   value, but   nil   useful only for classes (class), while optional types are available for all types and are more secure. 
 
 
  9.6.1.  Forced parsing  #  
 
 When you are sure that the optional type does contain a value, you can get the value by adding an exclamation point (!) after the optional name. The exclamation point says, “I know this optional value, please use it.” This is called forced parsing of optional values (forced unwrapping). 
 
 Examples are as follows: 
 
 
 
import Cocoa var myString:String? myString = "Hello, Swift!" if myString != nil { print(myString) }else{ print("myString value is nil") } 
 
 
 
 The execution results of the above procedures are as follows: 
 
 
 
Optional("Hello, Swift!") 
 
 
 
 Force the resolution of optional values, using an exclamation point (!): 
 
 
 
import Cocoa var myString:String? myString = "Hello, Swift!" if myString != nil { // Forced parsing print( myString! ) }else{ print("myString value is nil") } 
 
 
 
 The execution results of the above procedures are as follows: 
 
 
 
Hello, Swift! 
 
 
 
 Note: use th   !   to get an optional value that does not exist will cause a run-time error. Use   !   before you force the parsing value, be sure to optionally include a non-   nil   the value. 
 
 
 
  9.6.2.  Automatic parsing  #  
 
 You can replace the question mark (?) with an exclamation point (!) when declaring optional variables. In this way, the optional variable does not need to add an exclamation point (!) to get the value, it will be parsed automatically. 
 
 Examples are as follows: 
 
 
 
import Cocoa var myString:String! myString = "Hello, Swift!" if myString != nil { print(myString) }else{ print("myString value is nil") } 
 
 
 
 The execution results of the above procedures are as follows: 
 
 
 
Hello, Swift! 
 
 
 
 
 
  9.6.3.  Optional binding  #  
 
 Use optional bindings (optional binding) to determine whether the optional type contains a value, and if so, assign the value to a temporary constant or variable. Optional bindings can be used in the   if   and   while   statement to determine the value of the optional type and assign the value to a constant or variable. 
 
 Write an optional binding in the if statement as follows: 
 
 
 
if let constantName = someOptional { statements } 
 
 
 
 Let’s look at the next example of a simple optional binding: 
 
 
 
import Cocoa var myString:String? myString = "Hello, Swift!" if let yourString = myString { print("Your string value is - \(yourString)") }else{ print("Your string value is") } 
 
 
 
 The execution results of the above procedures are as follows: 
 
 
 
Your string value is - Hello, Swift! 
 
 
 
  
                
                

                    

                        

                            
                                文章链接
                            
                            ：
                            9.6. Swift optional type 
                        

                    

                

            
        
        

            

    
        
        

            

                

                    
                        
                            
                                Web框架 
                            
                            
                        
                             946
                        
                    
                    
                    
                        

                            
 
 
 

                        

                    
                

                
                

            

        
    
    
        
        

            

                

                    
                        
                            
                                《地理信息系统原理、技术与方法》 
                            
                            
                        
                             97
                        
                    
                    
                    
                        

                            
 
 
 最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。 
 
 

                        

                    
                

                
                

            

        
    
    
        
        

            

                

                    
                        
                            
                                WEB基础 
                            
                            
                        
                             289