Loop Variable Captured by Closure

Why This Happens

In Go versions before 1.22, the loop variable is a single variable reused across iterations. If a closure or goroutine captures it, all of them see the final value of the variable. This is fixed in Go 1.22+ where each iteration gets its own variable, but is still a common issue in older codebases.

The Problem

func main() {
    values := []int{1, 2, 3}
    for _, v := range values {
        go func() {
            fmt.Println(v) // prints 3, 3, 3
        }()
    }
    time.Sleep(time.Second)
}

The Fix

func main() {
    values := []int{1, 2, 3}
    for _, v := range values {
        v := v // create new variable for each iteration
        go func() {
            fmt.Println(v) // prints 1, 2, 3
        }()
    }
    time.Sleep(time.Second)
}

Step-by-Step Fix

1. 1

   Identify the captured variable

   Find closures or goroutines inside loops that reference the loop variable.

2. 2

   Create a local copy

   Add v := v at the start of the loop body to create a new variable per iteration.

3. 3

   Or pass as parameter

   Alternatively, pass the variable as a function argument: go func(v int) { ... }(v).