How to Correctly Convert a Slice of Strings to a Slice of String Pointers in Go?

Converting Slice of String to Slice of Pointer to String

In programming, it's sometimes necessary to convert a slice of string values to a slice of pointers to strings. This can be challenging when dealing with the nuances of references and memory allocation.

Original Problem

Consider the following example:

package main

import (
    "fmt"
)

func main() {
    values1 := []string{"a", "b", "c"}
    var values2 []*string
    for _, v := range values1 {
        fmt.Printf("%p | %T\n", v, v)
        values2 = append(values2, &v)
    }
    fmt.Println(values2)
}

In this example, we expect to create a slice of pointers (values2) that point to the string elements in values1. However, the output reveals that all the pointers point to the same address, indicating that they reference the same object rather than distinct strings:

%!p(string=a) => string
%!p(string=b) => string
%!p(string=c) => string
[0xc42000e1d0 0xc42000e1d0 0xc42000e1d0]

Understanding the Problem

This behavior occurs because the loop variable v is passed by value. In each iteration, it receives a copy of the current string in values1, and thus, all the pointers added to values2 point to the same copy of the string.

Proposed Solution

To resolve this issue, we must pass a pointer to the string element in values1 to the loop variable. One solution is to use the following approach:

for i, _ := range values1 {
    values2 = append(values2, &values1[i])
}

Here, the loop variable i is used to index into values1, and the address of the indexed element is appended to values2. This ensures that each pointer in values2 points to a unique string element in values1.

Impact on Memory Management

It's important to note that while the above solution resolves the pointer issue, it does have implications on memory management. The slice values2 contains pointers to the elements in values1, preventing values1 from being garbage collected until both values1 and values2 become unreachable. Additionally, modifying elements in values1 will indirectly modify elements in values2, as they both point to the same data.

Alternative Solution

Another solution to avoid the memory management implications is to create temporary local variables and append the addresses of those to values2:

for _, v := range values1 {
    v2 := v
    values2 = append(values2, &v2)
}

In this approach, each iteration creates a new local variable v2 and assigns it the value of the current string in values1. The address of v2 is then appended to values2. This ensures that each pointer in values2 points to an independent string object, and values1 can be garbage collected independently of values2.

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