How Does Defer Capture Parameters in Go Closures?

Defer Capture in Go with or Without Parameters

In Go, the defer keyword can be used to defer the execution of a function until the surrounding function returns. When used with anonymous closures, however, the behavior can vary depending on how the closure captures its parameters.

Consider the following code:

import "fmt"

func main() {
    var whatever [5]struct{}

    for i := range whatever {
        fmt.Println(i)
    } // part 1

    for i := range whatever {
        defer func() { fmt.Println(i) }()
    } // part 2

    for i := range whatever {
        defer func(n int) { fmt.Println(n) }(i)
    } // part 3
}

Part 2: Closure Without Parameter

In part 2, the closure does not capture any parameters. This means that when the closure is executed, the variable i has the value that it had when the defer statement was executed. In this case, i will have the value of the last iteration of the for loop, which is 4. Consequently, the output of part 2 will be:

4 4 4 4 4

Part 3: Closure With Parameter

In part 3, the closure captures the parameter n, which is initialized to the value of i when the closure is created. This means that when the closure is executed, n will have the value that it had when the defer statement was executed. In this case, each closure captures a different value of i, resulting in the output:

4 3 2 1 0

Conclusion

The key difference between part 2 and part 3 is that part 2 captures the i variable while part 3 does not. This affects the behavior of the closure when it is executed, leading to different outputs.

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