Swift Optional Type-standby

Let's start by looking at the following code:

 

1. var N1: Int = Ten
3. N1 = Nil//Compile error
7. Let str: String = Nil//Compile error

int and string types cannot accept nil, but sometimes it is unavoidable that the program is copied to nil during the run , andSwift provides an optional type (optional) for each data type. That is, after a data type with a question mark (?) or exclamation point (!), modify the preceding example code:

 

1. var n1:int? = 10
3. N1 = Nil
7. Let str:string! = Nil

int? and string! are both original type int and String optional types, they can accept nil.

Optional type value unpacking

What is the difference between a question mark (?) or an exclamation point (!) in an optional type? This is related to the "unpacking" (unwrapping) of the optional type, which is to turn the optionaltype into a normal type, and if we print a non-empty optional type value directly, the code is as follows:

 

1. var n1:int? = 10
3. Print (N1)

The result of the output is Optional (10), not 10. So trying to evaluate the expression n1+ 100 will compile error, the code is as follows:

 

1. var n1:int? = 10
3. Print (n1 + 100)//Compile error occurred

It is necessary to "disassemble" the optional type value.

"Unpacking" is divided into show unpacking and hidden unpacking.

an optional type declared with a question mark (?) that requires an exclamation point (!) when unpacking,which is called an explicit unpacking;

Anoptional type declared with an exclamation point (!) that can be split without using an exclamation point (!), which is called an implicit unpacking.

Take a look at the following code:

 

1. var n1:int? = 10
3. Print (n1! + 100)//Explicit unpacking
7. var n2:int! = 100
9. Print (n2 + 200)//implicit unpacking

Swift Optional Type-standby