When and Why Would You Use Pointers to Pointers in Programming?

Pointers to Pointers in Programming: A Practical Use Case

In computer programming, a pointer to a pointer, also known as a double pointer (eg. int)**, is a variable that stores the address of another pointer variable. This concept initially raises the question of its practical application.

To delve into the use of double pointers, let's refer to the example provided in the code snippet:

package main

import "fmt"

func main() {
    var num int

    fmt.Println(&num) //  0x...0
    makePointer(&num)
}

func makePointer(firstPointer *int) {
    fmt.Println(firstPointer)  //  0x...0
    fmt.Println(&firstPointer) //  0x...1

    makePointerToAPointer(&firstPointer)
}

func makePointerToAPointer(secondPointer **int) {
    fmt.Println(secondPointer)  //  0x...1
    fmt.Println(&secondPointer) //  0x...2
}

In this example, we declare a double pointer named secondPointer and pass it to the function makePointerToAPointer. Inside this function, we can manipulate the value pointed to by the firstPointer pointer. This allows us to effectively change the value of the original num variable.

Practical Use Cases

Double pointers find their applications in various scenarios:

• Parameter Passing: In function parameters, double pointers are useful when you want to pass a pointer that can be modified by the function. This is especially important in languages that lack multiple return values. For example, a function may take a **Node parameter (a double pointer to a node in a tree structure) to indicate that a caller can pass in a pointer to an existing node in the tree, or the function can create a new node and have the caller's pointer point to it.
• Error Handling: In languages that don't support multiple return values, double pointers can be used to provide an additional error value. Objective-C is an example where it's common to use NSError** as the last parameter of a function for optional error reporting.

Example Use Cases

Double pointers have been employed in real-world applications:

• Tree Data Structures: Double pointers are used in tree data structures to maintain a hierarchy of nodes. For instance, the Go compiler internally utilizes **Node pointers to facilitate tree manipulation operations.
• HTML Parsing: In HTML parsing, functions that can optionally parse an HTML page into a tree of nodes may use double pointers to store the parsed tree for reuse.

By utilizing double pointers, programmers can achieve essential functionalities that go beyond simple pointer operations and provide flexibility in parameter handling and error management.

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