An article analyzing the init () function in Golang

Detailed description of Go init function

After initializing each package, the init() function will be automatically executed, and the execution priority is higher than The execution priority of the main function. [Related recommendations: Go video tutorial]

The init function is usually used for:

• Variable initialization
• Checking/repairing status
• Registrar
• Run Calculation

Package Initialization

In order to use an imported package, it must first be initialized. Initialization is always performed in a single thread and in the order of package dependencies. This is controlled by Golang's runtime system, as shown below:

• Initialize imported packages (recursive import)
• Calculate and assign initial values ​​to variables declared in blocks
• Execute the initialization function in the package

##initial.go

package main
import "fmt"
var _ int64=s()
func init(){
  fmt.Println("init function --->")
}
func s() int64{
  fmt.Println("function s() --->")
  return 1
}
func main(){
  fmt.Println("main --->")
}

Execution result

function s() —>

init function —>
main —>

Even if the package is imported multiple times, Initialization only needs to be done once.

Features

The init function does not need to pass in parameters, nor does it need to return any value. In contrast to main, init is not declared and therefore cannot be referenced.

package main
import "fmt"
func init(){
  fmt.Println("init")
}
func main(){
  init()
}

An error occurred while compiling the above function "

undefined:init".

Each source file can contain more than one init function, please remember that the "way of march" written in each source file can only contain one init function, which is a bit different, so proceed to the next verification .

package main
import "fmt"
func init(){
  fmt.Println("init 1")
}
func init(){
  fmt.Println("init2")
}
func main(){
  fmt.Println("main")
}
/* 实施结果:
init1
init2
main */

From the above example, you can see that each source file can contain multiple init functions.

A common example of an initialization function is to set the value of an initial expression.

var precomputed=[20]float64{}
func init(){
  var current float64=1
  precomputed[0]=current
  for i:=1;i<len(precomputed);i++{
    precomputed[i]=precomputed[i-1]*1.2
  }
}

Because it is not possible to use a for loop as a precomputed value in the above code (which is a statement), you can use the init function to solve this problem.

Side effects of Go package import rules

Go is very strict and does not allow references to unused packages. But sometimes you reference the package just to call the init function to do some initialization. The purpose of the empty identifier (i.e. the underscore) is to solve this problem.

import _ "image/png"

Abstract

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Original address: https://developpaper.com/detailed-explanation-of-init-function-in-go-language/

Translation address: https://learnku .com/go/t/47178

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