How Does Go's `init()` Function Work in Package Initialization?

Init() in Go: A Precise Explanation

When working with the Go programming language, it's essential to understand how the init() function operates. This function plays a crucial role in the initialization process of packages and their dependencies.

As mentioned in Effective Go:

"init is called after all the variable declarations in the package have evaluated their initializers, and those are evaluated only after all the imported packages have been initialized."

This statement signifies that the init() function is executed after all the package's variables have been initialized and after all the imported packages have been initialized. By initializing variables, it means evaluating their initializers.

To clarify further, consider the following scenario:

var WhatIsThe = AnswerToLife()

func AnswerToLife() int { // 1
    return 42
}

func init() { // 2
    WhatIsThe = 0
}

func main() { // 3
    if WhatIsThe == 0 {
        fmt.Println("It's all a lie.")
    }
}

In this example:

1. AnswerToLife() will be called before init().
2. AnswerToLife() will return 42 and assign it to WhatIsThe.
3. init() will then be called, and it will reassign WhatIsThe to 0.
4. Finally, main() will be called, and it will print "It's all a lie."

This demonstrates that AnswerToLife() will be executed before init(), and init() will be executed before main(). Moreover, it highlights that init() will execute all the initialization logic, such as setting variable values, after all the package's dependencies and variables have been initialized.

It's important to note that init() is always called, regardless of whether there is a main() function or not. Hence, if you import a package that contains an init() function, it will be executed. Additionally, a package can have multiple init() functions, and they will be executed in the order they appear in the file.

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