Why Can\'t I Modify a Simple Type\'s Value Using a Pointer Receiver in Go?

How to Modify a Simple Type's Value via a Pointer Receiver in Go?

In Go, pointer receivers in methods allow you to modify the original value of a struct. However, this doesn't apply to simple types. This question delves into why such modifications don't work and presents solutions.

Problem

Consider this code:

package main

import (
    "fmt"
    "strconv"
)

type FooInt int
func (fi *FooInt) FromString(i string) {
    num, _ := strconv.Atoi(i)
    tmp := FooInt(num)
    fi = &tmp
}

func main() {
    var fi *FooInt
    fi.FromString("5")
    fmt.Printf("%v\n", fi) // Prints <nil>
}</nil>

Here, you'd expect the function FromString to set the value of fi to 5. But it prints , indicating a nil pointer.

Understanding the Behavior

In Go, function/method arguments, including the receiver, are copies of the original values. As a result, modifications within the function only affect the copy.

Even for pointers, the receiver is a copy of the original pointer, not the actual pointer itself. Thus, modifications made to the receiver's value won't affect the original pointer.

Solutions

To resolve this, there are several options:

1. Return the Modified Pointer:

   • Modify the function to return the modified pointer and assign it to the original pointer.

   func (fi *FooInt) FromString(i string) *FooInt {
       num, _ := strconv.Atoi(i)
       tmp := FooInt(num)
       return &tmp
   }
   
   func main() {
       var fi *FooInt
       fi = fi.FromString("5")
       fmt.Printf("%v %v\n", fi, *fi) // Prints 0xc0000b4020 5
   }

2. Pass a Pointer to Modify:

   • Pass a non-nil pointer of the desired type to the function, and modify its value.

   func (fi *FooInt) FromString(i string, p **FooInt) {
       num, _ := strconv.Atoi(i)
       tmp := FooInt(num)
       *p = &tmp
   }
   
   func main() {
       var fi *FooInt
       fi.FromString("5", &fi)
       fmt.Printf("%v %v\n", fi, *fi) // Prints 0xc0000b4020 5
   }

3. Ensure Non-Nil Receiver:

   • Initialize the receiver to a non-nil value before calling the method, so the pointed value can be modified.

   func (fi *FooInt) FromString(i string) {
       num, _ := strconv.Atoi(i)
       *fi = FooInt(num)
   }
   
   func main() {
       fi := new(FooInt) // Initialize to non-nil
       fi.FromString("5")
       fmt.Printf("%v %v\n", fi, *fi) // Prints 0xc0000b4020 5
   }

By adopting one of these solutions, you can effectively modify the value of simple types using pointer receivers.

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