Why Does Go's `range` Function Reuse Memory Addresses, and How Can This Be Avoided?

Reusing Memory Addresses in Go: Understanding the Range Function

When working with slices in Go, it's important to understand how the range function interacts with your data. A recent issue encountered in a project raised questions about the behavior of pointers within slices. Specifically, why did a particular method return incorrect memory addresses, and how was an alternative solution able to resolve this issue?

The original method, ToModelList, intended to convert a slice of Region objects (Regions) into a list of Model interfaces. However, the first pointer to the Region was repeatedly duplicated in the output. To fix this, a modified version of the method was introduced.

But how did this slight modification make a difference? The key lies in understanding the range function. In the first version of ToModelList, item was the loop variable. While its value changed, its address remained constant. As a result, the same address was assigned to multiple elements in the output slice.

In the revised version, the loop syntax was modified: for idx, := range *coll. This time, we used as a placeholder for the unused loop variable, allowing us to access the actual item using the index idx. By accessing the item indirectly via *coll, we ensured that a new address was used for each iteration, resolving the memory reuse issue.

To illustrate this further, consider the following code:

func main() {
    coll := []int{5, 10, 15}
    for i, v := range coll {
        fmt.Printf("Always the same: %v\n", &v)
        fmt.Println("Increments by 4 bytes each iteration: %v\n", &coll[i])
    }
}

In this example, the first loop variable v always refers to the same memory address, while the second loop variable &coll[i] increments by 4 bytes with each iteration. This demonstrates the difference between using a loop variable and accessing the element directly.

Understanding the inner workings of the range function is essential for working with slices in Go. The examples provided highlight the potential pitfalls and how the loop syntax can impact your results.

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