Go - Pointers

go go/book/learninggo

A pointer is a variable that stores the memory address of another variable.

package main
 
import (
	"fmt"
	"reflect"
)
 
func main() {
    var x int = 42
    var y int = 50
    var p *int = &x // p now holds the address of x
 
    fmt.Println("p value: ", reflect.ValueOf(p), "p type: ", reflect.TypeOf(p))
 
    fmt.Println("Value of x:", x, "Address of x:", &x)      // 42
    fmt.Println("Value of p:", p)      // (same memory address)
    fmt.Println("p:", *p)      // dereferencing p 
 
    *p = 21                 // changing the value at the memory address p points to
 
    fmt.Println("Value of x after change:", x) // 21
 
    p = &y
 
    fmt.Println("p:", *p)      // dereferencing p 
}

Pointers with Structs

Structs

• Pointers are especially useful with structs, **allowing you to avoid copying large structs and enabling direct modification.**

package main
 
import "fmt"
 
// Define a struct called 'Person'
type Person struct {
	Name string
	Age  int
}
 
// Function that takes a 'Person' value (copy) and modifies it
func modifyPersonCopy(p Person) {
	p.Name = "Alice"
	p.Age = 30
}
 
// Function that takes a pointer to 'Person' and modifies it
func modifyPersonPointer(p *Person) {
	p.Name = "Alice"
	p.Age = 30
}
 
func main() {
	// Original 'Person' struct
	person := Person{Name: "John", Age: 25}
 
	// Copy the struct and attempt modification
	modifyPersonCopy(person)
	fmt.Println("After modifyPersonCopy:", person) // No change to 'person'
 
	// Pass a pointer to the struct and modify
	modifyPersonPointer(&person)
	fmt.Println("After modifyPersonPointer:", person) // Changes to 'person'
}

Go - Types & Structs > Pointer to Structs

Best Practices

• Use pointers to avoid copying large data structures.

• Remember that pointers can be nil. Always check for nil pointers before dereferencing them to avoid runtime errors.

// Function that takes a pointer to an integer
func printValue(val *int) {
	// Dereference the pointer to access the integer value
	fmt.Println("Value:", *val) // this line cause runtime Error
}
 
func main() {
	var ptr *int // nil pointer
 
	printValue(ptr) // Output: Pointer is nil, cannot dereference.
}

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Slice copy as the value but the underlying implementation points to the pointer of the array.