An interview question about common pitfalls in golang for range

The following code runs , What will be output ？

package main

import "fmt"

type student struct {
    
	Name string
	Age  int
}

func pase_student() {
    
	m := make(map[string]*student)
	stus := []student{
    
		{
    Name: "A", Age: 1},
		{
    Name: "B", Age: 2},
		{
    Name: "C", Age: 3},
	}
	for _, stu := range stus {
    
		m[stu.Name] = &stu
	}

	for k, v := range m {
    
		fmt.Printf("%v,%v,%v\n", k, v.Name, v.Age)
	}
}


func main() {
    
	pase_student()
}

Please think about it. , The answer is at the end of the article
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answer

A,C,3
B,C,3
C,C,3

Why? ？

Because in golang Of for range in , Temporary variable stu Will be reused , Then copy the new value , Without opening up another space . So in the title m[stu.Name] = &stu, It can lead to stu The value in will be the last copied value , namely stus Last value .

We can write a test program , Verify this “ pit ”：

func main() {
    

	a := []int{
    1, 2, 3, 4}

	for x, n := range a {
    
		fmt.Printf("%d: %d\n", x, n)
		fmt.Printf("%p\n", &n)
	}
}

Output

0: 1
0xc0000160c8
1: 2
0xc0000160c8
2: 3
0xc0000160c8
3: 4
0xc0000160c8

You can see , Every time &n It's all fixed

How to modify ？

The first 1 Kind of , Using value , Instead of Pointers . Can be m := make(map[string]*student) Change to m := make(map[string]student) , The complete code is as follows ：

func pase_student() {
    
	m := make(map[string]student)
	stus := []student{
    
		{
    Name: "A", Age: 1},
		{
    Name: "B", Age: 2},
		{
    Name: "C", Age: 3},
	}
	for _, stu := range stus {
    
		m[stu.Name] = stu
	}

	for k, v := range m {
    
		fmt.Printf("%v,%v,%v\n", k, v.Name, v.Age)
	}
}

The first 2 Kind of , no need for range Switch to classic for

func pase_student() {
    
	m := make(map[string]student)
	stus := []student{
    
		{
    Name: "A", Age: 1},
		{
    Name: "B", Age: 2},
		{
    Name: "C", Age: 3},
	}
	for i := 0; i < len(stus); i++ {
    
		m[stus[i].Name] = stus[i]
	}

	for k, v := range m {
    
		fmt.Printf("%v,%v,%v\n", k, v.Name, v.Age)
	}
}

Other methods can also .

therefore , In actual project development , be used for range Be sure to pay attention to this when assigning values “ pit ”