Go 中 for range 的使用易错点

看下面这段程序：

package main

import "fmt"

type Person struct {
	Name string
	Age  int
}

func main() {
	jim := Person{
		Name: "jim",
		Age:  20,
	}
	lily := Person{
		Name: "lily",
		Age:  18,
	}
	tom := Person{
		Name: "tom",
		Age:  21,
	}
	people := []Person{jim, lily, tom}

	namePersonMap := make(map[string]*Person)
	// look at this
	for _, p := range people {
		namePersonMap[p.Name] = &p
	}

	fmt.Println(namePersonMap)
	for k, v := range namePersonMap {
		fmt.Println(k, v)
	}
}

程序想做的事情很简单：将Person的切片转化为Person.Name和*Person的映射。 然后程序的输出却大大出乎我意料：

map[jim:0xc0000a6020 lily:0xc0000a6020 tom:0xc0000a6020]
jim &{tom 21}
lily &{tom 21}
tom &{tom 21}

可以看出，所得map的value都一样，就是for循环中最后一轮迭代用到的Person对象–tom的指针。

这是为什么呢？

因为在上述程序的 for range 表达式中，用到的p并不是people切片中的元素本身，而是元素的拷贝。并且p会在迭代中重复使用，也就是说：“装载” p 变量的内存地址在第一轮迭代中放置了jim的拷贝，在第二轮迭代中放置了lily的拷贝，在第三轮迭代中放置了tom的拷贝。所以，最终得到的namePersonMap中的value都是一致的，即为指向“tom的拷贝”的指针。

正确的写法是使用切片的index来做循环，这样就不会对切片中元素做拷贝了：

for i := range people {
    namePersonMap[people[i].Name] = &people[i]
}

输出是：

map[jim:0xc00019a000 lily:0xc00019a018 tom:0xc00019a030]
lily &{lily 18}
tom &{tom 21}
jim &{jim 20}

得到了我们想要的结果。

详细的解释可以参看官方spec的For statements with range clause一节。重点是这几句话：

For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block. The iteration values are assigned to the respective iteration variables as in an assignment statement. The iteration variables may be declared by the “range” clause using a form of short variable declaration (:=). In this case their types are set to the types of the respective iteration values and their scope is the block of the “for” statement; they are re-used in each iteration. If the iteration variables are declared outside the “for” statement, after execution their values will be those of the last iteration.

所以，以后在使用 for range 时，还得多长一个心眼，尤其是这种涉及到指针的操作，一不小心就会造成难以排查的bug。

---

延伸阅读：https://www.ardanlabs.com/blog/2017/06/for-range-semantics.html 文中提到：for _, p := range people 是基于 value semantics 的写法，而for i := range people 是基于 pointer semantics 的写法。作者建议：

I want you to decide what semantic each type is using and be consistent. The more consistent you are with the semantic for a piece of data, the better off your code base will be.

使用上的不一致，往往容易产生bug。