How Do Pointers to Pointers Work in Go?

Pointers to Pointers in Go

In programming, pointers to pointers can provide a level of indirection that is useful in certain scenarios. Consider the following Go code:

package main

import "fmt"

func main() {
    var num int

    fmt.Println(&num) // address of num
    makePointer(&num)
}

func makePointer(firstPointer *int) {
    fmt.Println(firstPointer)  // address of num
    fmt.Println(&firstPointer) // address of firstPointer
    makePointerToAPointer(&firstPointer)
}

func makePointerToAPointer(secondPointer **int) {
    fmt.Println(secondPointer)  // address of firstPointer
    fmt.Println(&secondPointer) // address of secondPointer
}

This code demonstrates how pointers to pointers work. The integer variable num has an address stored in &num. The makePointer function takes the address of &num as an argument, which is stored in firstPointer. The makePointerToAPointer function takes the address of firstPointer as an argument, which is stored in secondPointer.

Practical Applications

While it may seem convoluted, pointers to pointers do have use cases in production code:

• Function Parameters: Pointers to pointers can be useful in function parameters, allowing the function to modify the pointer itself, not just the value it points to. For example, in the Go compiler's internals, some functions take a **Node parameter to enable manipulating an object's pointer.
• External Pointers: Pointers to pointers can also be used in situations where a library or external code operates on a pointer that needs to be modified. For instance, HTML parsing functions may require a **html.Node pointer to handle HTML pages that may or may not be parsed.
• Additional Return Values: In languages without multiple return values, pointers to pointers can be used to provide an additional return value. In Objective-C, many methods take an NSError** parameter to optionally return an error.

Pointers to pointers offer flexibility and indirection in programming, enabling certain operations and code structures that would otherwise be more difficult or impossible to implement.

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