Tricky case with value-to-references in for-range

Question

Look to the code - what do you think the output would be? It's return "Third" instead of "Second" and took me a while to understand why.

Do you know a reason?

I get concept of pass-by-value & pass-by-reference quite well but this case is a bit tricky for people coming from languages like Python. So I decided it worth to share.

package main

import "fmt"

type Record struct {
    Id int
    Name string
}

var records = []Record{
    Record{1, "First"},
    Record{2, "Second"},
    Record{3, "Third"},
}

func findRecod(id int) (foundRecord *Record) {
    for _, record := range records {
        if record.Id == id {
            foundRecord = &record
            // You think we can do a break here but imagine we need to do...
        }       
        // ...something more here
    }
    return foundRecord
}

func main() {
    foundRecord := findRecod(2)
    if foundRecord == nil {
        fmt.Println("Nothing found")
    } else {
        fmt.Println("Found: ", foundRecord.Name)
    }
}

Run it online to check: https://play.golang.org/p/Y_iAl6m7Ms

I've spent some time figuring out what's going on.

Answer 1

You are returning pointer to a record variable which is reused by each iteration of the loop. Last iteration sets the pointer to the third structure.

The reuse of variable has an enormous advantage. It does not allocate memory in every iteration of the loop. This saves a lot of garbage collection time.

This is well know behaviour described in FAQ.

To fix it, return a pointer to an element of the slice. It is safe as slice elements are referencable in Go.

package main

import "fmt"

type Record struct {
    Id int
    Name string
}

var records = []Record{
    Record{1, "First"},
    Record{2, "Second"},
    Record{3, "Third"},
}

func findRecod(id int) (foundRecord *Record) {
    for i, record := range records {
        if record.Id == id {
            foundRecord = &records[i]
            // You think we can do a break here but imagine we need to do...
        }       
        // ...something more here
    }
    return foundRecord
}

func main() {
    foundRecord := findRecod(2)
    if foundRecord == nil {
        fmt.Println("Nothing found")
    } else {
        fmt.Println("Found: ", foundRecord.Name)
    }
}

Playground