在iOS Swift中声明变量之间的区别？

Question

I am learning Swift recently and I found that there are two types of declaration for a variable.

var indexArray = NSMutableArray()     //and
var indexArray : NSMutableArray = NSMutableArray()

Am just wondering what is the difference between them? Will it replicate in any kind of assigning values to the variable?

Answer 1

Here is a simple explanation

var indexArray = NSMutableArray()

As the above, indexArray variable can be any one , String , Int , ....... You didn't specifically give any type for that variable.

var indexArray : NSMutableArray = NSMutableArray()

In here you specifically give that indexArray is a NSMutableArray

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

 var welcomeMessage: String

The colon in the declaration means “…of type…,” so the code above can be read as:

Declare a variable called welcomeMessage that is of type String.

The phrase “of type String” means “can store any String value.” Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

 welcomeMessage = "Hello" 

You can define multiple related variables of the same type on a single line, separated by commas, with a single type annotation after the final variable name:

var red, green, blue: Double”

* Note *

It is rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it is defined, Swift can almost always infer the type to be used for that constant or variable, as described in Type Safety and Type Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 2 Prerelease).” iBooks. https://itun.es/us/k5SW7.l

Answer 2

One of the important language feature of Swift is Type Inference . What it means is that a variable can identify what is its type based on the value its assigned with.

var indexArray = NSMutableArray()

By the above statement, it is implicitly known that the variable indexArray is of type NSMutableArray without being specified.

Whereas in the other type of declaration,

var indexArray : NSMutableArray = NSMutableArray()

you are explicitly specifying that the variable indexArray is of type NSMutableArray before assigning a value to it. If you assign a different type to it, the compiler is throw an error.

var indexArray : NSMutableArray = NSString() // This is wrong 

A very good starting point is to go over the Swift Language document by Apple.

Answer 3

No difference, both are same..

there is difference between the follwing declarations,

var welcomeMessage: String? - can be nil or assigned nil at any point of time

var welcomeMessage: String! - can never be nil at any point of time when using that variable.

var welcomeMessage: String - will throw error saying, class has no initializers

But there is no difference between the following two,

var welcomeMessage: String = String()

var welcomeMessage = String()

Answer 4

In the first case, indexArray takes the type of the assignment source automatically, so in your example case the statements are exactly the same.

The difference besides the second version being more explicit is that you could tweak the type of indexArray - for example to a base class - in the second case;

var indexArray = NSMutableArray()            // indexArray is type NSMutableArray

var indexArray : NSArray = NSMutableArray()  // indexArray is type NSArray
                                             // but contains an NSMutableArray

Of course, if you declare a variable without assigning it you can't take the type automatically from the assignment source, so then you need the explicit type declaration.