Why Does Golang's Loop Behavior Differ When Using References vs. Copies of Loop Variables?

Understanding Differing Loop Behavior in Golang: Return by Reference vs. Copy

The Issue

In the provided code, two loop variations exhibit different behaviors when accessing elements from a slice. Loop1 returns "update" repeatedly, while Loop2 prints the expected sequence of "delete," "update," and "create."

Loop Variable Reference

The key to understanding this difference lies in the way the loop variable (cmd) is used in the closure (func()). In Loop1, a reference to the loop variable is stored in the closure. This means that any subsequent changes to cmd will affect all the closures in the map.

When the second loop executes, the value of cmd has already been updated to "update," the last element in the cmds slice. Therefore, all the closures in the map refer to this last value, resulting in the repeated "update" output.

Loop Variable Copying

In Loop2, however, a copy of the loop variable is stored in the closure. This creates a detached variable that is not affected by subsequent changes to the original cmd. Each iteration of the loop assigns a different value to cmd2, which the closure then references.

As a result, the second loop correctly prints each element of the cmds slice.

Best Practice

To avoid such reference issues, it's generally recommended to use the index of the slice instead of the loop variable when accessing elements within a closure. This way, each closure can access the correct element regardless of changes to the loop variable.

Use Case for Returned Element

The second value returned by the range loop (a copy of the element) can be useful when you want to pass the value to a separate goroutine or thread without worrying about concurrent access to the original variable. This makes it convenient for tasks that require sharing data without fear of corrupting the source.

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