英文:
Why the value of `votes` is changed after db.Prepare?
问题
我正在使用http://github.com/Go-SQL-Driver/MySQL。
我想从数据库中获取一个名为votes的值，例如"0000"，并将其更新为"1000"。在db.Prepare()之前，它正常工作。但是在此之后，votes的值发生了变化。除了db.Prepare()之外，我没有对它做任何操作。我的代码如下：
func Vote(_type, did int, username string) (isSucceed bool) {
db := lib.OpenDb()
defer db.Close()
stmt, err := db.Prepare(
`SELECT votes
FROM users
WHERE username = ?`)
lib.CheckErr(err)
res := stmt.QueryRow(username)
stmt.Close()
var votes Votes
res.Scan(&votes)
fmt.Println(votes)//输出：[48 48 48 48]
fmt.Println(string(votes))//输出：0000
isSucceed = votes.add(VoteType(_type), 1)
fmt.Println(votes)//输出：[49 48 48 48]
fmt.Println(string(votes))//输出：1000
//v := []byte{[]byte(votes)[0], []byte(votes)[1], []byte(votes)[2], []byte(votes)[3]}
if isSucceed {
//更新用户的投票数
stmt, err := db.Prepare(
`UPDATE users
SET votes = ?
WHERE username = ?`)
lib.CheckErr(err)
fmt.Println(votes)//输出：[4 254 0 0]
fmt.Println(string(votes))//输出：[EOT]□[NUL][NUL]
//_, _ = stmt.Exec(v, username)
_, _ = stmt.Exec(votes, username)
stmt.Close()
//插入投票数据
stmt, err = db.Prepare(
`INSERT votes
SET did = ?, username = ?, date = ?`)
lib.CheckErr(err)
today := time.Now()
_, _ = stmt.Exec(did, username, today)
stmt.Close()
}
return
}
Votes类型定义如下：
type Votes []byte
type VoteType int
func (this *Votes) add(_type VoteType, num int) (isSucceed bool) {
if []byte(*this)[_type] > VOTE_MAX-1 { //超过最大值
isSucceed = false
} else {
[]byte(*this)[_type]++
isSucceed = true
}
return
}
最后，我将votes的值复制到v中，它正常工作。我无法理解为什么votes的值会发生变化。我的代码有什么问题吗？任何帮助将不胜感激。
英文:
I am using http://github.com/Go-SQL-Driver/MySQL
I want to get a value votes like "0000" from database and update it into "1000". Before db.Prepare() it works normally. But after it, the value of votes is changed. I didn't do anything with it except db.Prepare(). My code is
func Vote(_type, did int, username string) (isSucceed bool) {
db := lib.OpenDb()
defer db.Close()
stmt, err := db.Prepare(
`SELECT votes
FROM users
WHERE username = ?`)
lib.CheckErr(err)
res := stmt.QueryRow(username)
stmt.Close()
var votes Votes
res.Scan(&votes)
fmt.Println(votes)//output: [48 48 48 48]
fmt.Println(string(votes))//output: 0000
isSucceed = votes.add(VoteType(_type), 1)
fmt.Println(votes)//output: [49 48 48 48]
fmt.Println(string(votes))//output: 1000
//v := []byte{[]byte(votes)[0], []byte(votes)[1], []byte(votes)[2], []byte(votes)[3]}
if isSucceed {
//Update user votes
stmt, err := db.Prepare(
`UPDATE users
SET votes = ?
WHERE username = ?`)
lib.CheckErr(err)
fmt.Println(votes)//output: [4 254 0 0]
fmt.Println(string(votes))//output: [EOT]□[NUL][NUL]
//_, _ = stmt.Exec(v, username)
_, _ = stmt.Exec(votes, username)
stmt.Close()
//Insert the vote data
stmt, err = db.Prepare(
`INSERT votes
SET did = ?, username = ?, date = ?`)
lib.CheckErr(err)
today := time.Now()
_, _ = stmt.Exec(did, username, today)
stmt.Close()
}
return
}
the Votes type is :
type Votes []byte
type VoteType int
func (this *Votes) add(_type VoteType, num int) (isSucceed bool) {
if []byte(*this)[_type] > VOTE_MAX-1 { //beyond
isSucceed = false
} else {
[]byte(*this)[_type]++
isSucceed = true
}
return
}
Finally I copy the value from votes into v and it works well. I can not understand why the value of votes is changed. Is there anything wrong with my code? Any help would be appreciated.
答案1
得分: 0
我认为问题出在以下语句：
res.Scan(&votes)
应该改为：
res.Scan((*[]byte)(&votes))
你传递给Scan的Votes不会被识别为[]byte，除非你进行断言。
下面是一个清楚展示识别问题的示例：
package main
import "fmt"
type BYTES []byte
func test(v interface{}) {
b, ok := v.(*[]byte)
fmt.Println(b, ok)
}
func main() {
p := BYTES("hello")
test(&p)
test((*[]byte)(&p))
}
以上代码输出：
<nil> false
&[104 101 108 108 111] true
英文:
I think the problem lies in the statement:
res.Scan(&votes)
which should be:
res.Scan((*[]byte)(&votes))
the *Votes you pass on to Scan will not be identified as *[]byte unless you make an assertion.
Here is an example that clearly shows the identification problem:
package main
import "fmt"
type BYTES []byte
func test(v interface{}) {
b, ok := v.(*[]byte)
fmt.Println(b, ok)
}
func main() {
p := BYTES("hello")
test(&p)
test((*[]byte)(&p))
}
The code above prints:
<nil> false
&[104 101 108 108 111] true
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