init func

introduction

Golang has special, niladic function init() for special purpouse to form a specific state for application.

init() function runs a piece of code before running other part of the package, which allow application to be initialize in a specific state, such as have a specific configuration or set of resource with application need to start.

init() function it is also used when importing a side effect, a technique used to set the state of the application by importing a package.

declaring init()

Before declaring an init() function, lets see a function without init() function

maig.go //refer the package withoutimport

    package main

    import "fmt"

    var sysos string

    func main() {
        fmt.Println("OS running on my system", os)
    }

In this program we declared a global variable called sysos, default value of sysos is empty string

Lets run the program go run main.go

output

OS running on my system

Because sysos variable is blank, we can see only the "OS running on my system" in output above.

Now we can fill the blank variable by introuducing an init() function that initialize the value of sysos to the os of system

maig.go //refer the package withimport

   package main

   import (
        "fmt"
        "runtime"
        )

    var sysos string

    func init() {
        sysos = runtime.GOOS
    }
    func main() {
        fmt.Println("OS running on my system", sysos)
    }

In this program we assinged runtime.GOOS(imported runtime package) to sysos variable inside init() function. at runtime, sysos will be holding a value of the current system OS name

output

    OS running on my system windows

Currently this program running on windows OS, so we can see in output "OS running on my system windows"

This illustrate that how init() works.

Another typical use case of init() when importing a package

a.go // refere to packageimport package

    package packageimprt

    import "fmt"

    var AState string

    func init() {
    	AState = "WelCome in file a.go"
    	fmt.Println(AState)
    }

b.go // refere to packageimport package

    package packageimprt

    import (
        "fmt"
    )

    var BState string

    func init() {
    	BState = "Resume in file b.go"
    	fmt.Println(BState)
    }

Above two file a.go and b.go are in a package named "packageimport" and having a global variable AState, BState in a.go and b.go respectively, these variables are initialized to random string in init() function in both the file.

Now we have another package called "mainimport" that uses/calls "packageimport" package for accessing the globle variable AState and BState.

main.go //refer "mainimport" package

    package main

    import (
        "fmt"

        _ "github.com/shysudo/initfunc/packageimport"
    )

    var Main string

    func init() {
        Main = "Running from main.go"
        fmt.Println("Init func inside main package")
    }

    func main() {
        fmt.Println("Main func")
        fmt.Println(Main)
    }

output

    WelCome in file a.go
    Resume in file b.go
    Init func inside main package
    Main func
    Running from main.go

From the output from above we can figure out the order of init() func execution/call. In main.go file the package "github.com/shysudo/initfunc/packageimport" is imported, so init() function from this imported package execute/runs and runs init() function from main.go.

The execution/run of the order of init() function from imported pacakge runs based on the alphabetical order of the files in the imported package i.e init() function in a.go execute first "WelCome in file a.go" and init() function in b.go execute later "Resume in file b.go"