Go for … range

今天写代码发现一个 bug：数据回写到 MySQL 的时候，第一条数据被重复写了，后面的数据没写进入。最后定位到：

for _, r := range recvs {
    ib = ib.Values(&lastId, &e.Fingerprint, &e.Version, &r)
}

操作数据库使用了 squirrel， Values 的参数是 interface{} ，所以我下意识的用了地址。我把取地址 & 去掉之后， 就正常了。

然后就开始怀疑， 难道 Go range 中的第二个参数每次的地址都是一样的？ 那不就是简单的赋值操作，做个浅拷贝？

写了一段测试代码：

package main

import "fmt"

func main () {
    arr := []int{1, 2, 3, 4, 5, 6}

    for i, a := range arr {
        fmt.Println(&a, &arr[i])
    }
}

输出：

0xc0000140f0 0xc000018090
0xc0000140f0 0xc000018098
0xc0000140f0 0xc0000180a0
0xc0000140f0 0xc0000180a8
0xc0000140f0 0xc0000180b0
0xc0000140f0 0xc0000180b8

还真是。查了一下官方文档¹

1. For an array, pointer to array, or slice value a, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to len(a)-1 and does not index into the array or slice itself. For a nil slice, the number of iterations is 0.
2. For a string value, the "range" clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type rune, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be 0xFFFD, the Unicode replacement character, and the next iteration will advance a single byte in the string.
3. The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced. If a map entry is created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is nil, the number of iterations is 0.
4. For channels, the iteration values produced are the successive values sent on the channel until the channel is closed. If the channel is nil, the range expression blocks forever.

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.

也就是说对于 array, slice 这种只是一个简单的赋值 := 然后变量复用。我本以为是一个迭代器，迭代器里引用了变量的地址。 没想到是个这么原始的实现。

既然这样的话，对于比较大的结构体，最好在数组中存放指针。如 []*BigStruct ，或者在遍历的时候，通过索引访问（使用第一个参数，忽略第二个参数）。