Go language variable

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1.  How variables are declared

1.1）var Statement

Its declaration form is as follows ：

var name type = expression

• among , type （type） And expressions （expression） One of them can be omitted , But we can't omit all of them

for example ：

var a int = 1
//  Omit  type
var b = "abc"

var c int
fmt.Println(a, b, c)

In omitting expression when , The variable will be assigned Zero value

• Zero value

stay Go In language , The corresponding zero values of each data type are as follows ：

bool The value is false;
int Integer type is 0;
float Floating point is 0.0;
string by "" Empty string ;
Interface and reference types （slice、 The pointer 、map、 passageway 、 function ） by nil;
The compound type （ Array 、 Structure ） by Zero value of all elements ;

• The declaration of multiple variables can be put into one () Inside

var (
	a int = 1
	b     = "abc"
	c int
)

• Multiple variables can also be placed on one line , Omit type

var a, b, c = 1, "abc", 0

1.2）:= Short variable declaration

Go Language also provides a way to declare short variables , The format is as follows ：

name := expression

• And var The difference is that , Short variable declarations can only be used for functions in

for instance ：

func main() {
    
	// :=  Declaration can only be used in functions 
	a, b, c  := 1, "abc", 0
	fmt.Println(a, b, c)
}

• Notes

stay Function body , As far as possible Declare with short variables

When short variables are declared , The variable on the left The quantity is greater than or equal to 2 when , For in The same grammar block in Existing variables , The behavior of short variable declarations Equal to assignment , Short variable declaration equals assignment （ Premise ： Short variable declarations declare at least one variable ）

for instance ：

func main() {
    
	var a int = 1
	var b string = "abc"
	//  At least one variable must be declared  ！！！
	a, b, c := 2, "Hello", 0
	fmt.Println(a, b, c)
}

1.3）new function

Go The language can also be used by using the built-in new Function to create a variable , The format is as follows ：

new(T)

expression new(T) Create an unnamed T Variable of type , Initialize to the corresponding zero value , Return its address , So the receiver on the left is a pointer variable .

for instance ：

a:= new(int)
fmt.Println(*a)

Its benefits ： There is no need to declare a virtual name

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2. Printf Common escape characters of output variables

Escape character	describe
%d	Decimal integer
%b, %o, %x	Binary system , octal , Hexadecimal integer
%f, %g, %e	Floating point numbers
%t	Boolean type ：（ Output true / false）
%c, %s	character , character string
%q	Quoted characters （“a”） Or a string （“abc”）
%v	Any value in built-in format
%T	The type of any value

• Notes

When using With ln At the end of the function （ Such as Println） Use %v The way To format parameters

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3.  Life cycle of variable

Life cycle ： The period of time during which variables exist during program execution

• Package level variables

Its life cycle is The execution time of the whole program

• local variable

Its life cycle is Dynamic , Variable always Survive until it is inaccessible （ Collect variables through garbage collection mechanism ）

Garbage collection mechanism ：
Path source of variables ：

1. Variables at each package level
2. Local variables of each currently executing function

Trace the path source of this variable , adopt Pointers and other references Variables can be found , If the variable path does not exist , Then the variable becomes inaccessible , At this time, it is recycled .