The problem that all values are the same occurs after golang for range traverses and assigns values to the dictionary

for range After traversing and assigning values to the dictionary, there is a problem that all values are the same

Let's look at this first （ Very classic ） The wrong sample ：( For convenience json Package printout structure , I used the export field starting with an uppercase letter )

m := make(map[string]*student)
stus := []student{
    
    {
    Name: " name 1", Age: 1},
    {
    Name: " name 2", Age: 2},
    {
    Name: " name 3", Age: 3},
}
for _, stu := range stus {
    
    m[stu.Name] = &stu
}
for k, v := range m {
    
    fmt.Println(k, ":", v.Name)
}

Output results ：

 name 3 :  name 3
 name 1 :  name 3
 name 2 :  name 3

• If you print in a loop stu, You will find that every time you traverse stu The output of is normal , That is, it does traverse the elements at the corresponding position of the array
• But when traversing to the next element , The corresponding value of the previous element in the dictionary will also become the value of this traversal element
• Last all keys mapping The value of is the same —— That is, the last element traversed

for range Is to create a copy of each element and then map the copy , It does not return a reference to an element , And the same traversal pointer is always used , This is no problem in circular reading . But if you loop through the hash table , Use the traversal pointer as the mapping of each key , Then the end of the loop will point to the last element , Cause all values to be the same . If we add printing

for _, stu := range stus {
    
    m[stu.Name] = &stu
    fmt.Println(m)
}

Output

map[ name 1:0xc00000c060  name 2:0xc00000c060  name 3:0xc00000c060]

It also shows these key Corresponding to the same memory address . Then the values in the natural dictionary are all the same .
There are two solutions ：

• Use array index instead of traversal pointer

for i, v := range stus {
    
    m[v.Name] = &stus[i]
}

Output

 name 1 :  name 1
 name 2 :  name 2
 name 3 :  name 3

• Create a new variable

for _, stu := range stus {
    
    st := stu
    m[stu.Name] = &st
}

The output is

 name 1 :  name 1
 name 2 :  name 2
 name 3 :  name 3

Reference resources

1. https://studygolang.com/articles/9701
2. https://draveness.me/golang/docs/part2-foundation/ch05-keyword/golang-for-range/