Loop and pointer problem in Go language: Why are v.name 'blogs'?

Go language loops and pointer traps: Detailed explanation of the reason why v.name is all "blog"

When using for...range loops and pointers in Go language, some confusing problems are prone to occur. This article uses an example to deeply analyze the mechanism of for...range loops and the use of pointers, and explains why the code output results are all "blogs".

The following code snippet shows this problem:

 type student struct {
    name string
    age int
}

func main() {
    m := make(map[string]*student)
    stus := []student{
        {name: "pprof.cn", age: 18},
        {name: "test", age: 23},
        {name: "Blog", age: 28},
    }

    for _, stu := range stu {
        m[stu.name] = &stu // The problem is here}
    for k, v := range m {
        fmt.Println(k, "=>", v.name)
    }
}

Run this code and the output will all be博客. This is because not every iteration of stu variable in for...range loop creates a new variable. It is a loop variable whose memory address remains unchanged throughout the loop. Therefore, &stu always points to the same memory address. Each iteration assigns the same address to the value in the map, and finally all pointers in the map point to the last stu variable, namely {name: "博客", age: 28} .

Workaround: Create a new copy of the variable

To avoid this problem, you need to create a copy of stu in each iteration, rather than directly using its address:

 for _, stu := range stu {
    s := stu // Create a copy of stu m[stu.name] = &s
}

By creating a new variable s , m[stu.name] = &s now points to the memory address of s , and s is a new variable in each iteration, thus solving the problem that pointer points to the same memory address, and the final output will correctly reflect each student's name.

Understanding the mechanism of for...range loops and the features of pointers is crucial for writing efficient and correct Go code. The key to avoiding this kind of trap is to make sure that each pointer points to a different memory address when using pointers in a loop.

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