Go: How can I "unpack" a struct?

huangapple
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英文:

Go: How can I "unpack" a struct?

问题

我有一个结构体：

  1. type mystruct struct {
  2.     Foo string
  3.     Bar int
  4. }

我想从这个结构体创建以下形式的SQL插入语句：

  1. m := mystruct{"Hello", 1}
  2. query := "INSERT INTO mytbl (foo, bar) VALUES (?, ?)"
  3. res, err := db.Exec(query, m.Foo, m.Bar)

现在我的问题是：如何根据结构体（或m本身）动态生成最后一行代码？我可以使用reflect获取结构体的字段名，但我不知道如何为db.Exec()调用创建[]interface{}切片。这是我尝试过的代码：（http://play.golang.org/p/GR1Bb61NFH）

  1. package main
  3. import (
  4.     "fmt"
  5.     "reflect"
  6. )
  8. type mystruct struct {
  9.     Foo string
  10.     Bar int
  11. }
  13. func main() {
  14.     m := mystruct{"Foo", 1}
  15.     fmt.Println(readNames(m))
  16.     x := unpackStruct(m)
  17.     fmt.Printf("%#v\n", x)
  18. }
  20. func unpackStruct(a interface{}) []interface{} {
  22.     // 将a转换为m类型
  24.     // 不起作用，来自“反射定律”
  25.     s := reflect.ValueOf(&t).Elem()
  26.     typeOfT := s.Type()
  27.     for i := 0; i < s.NumField(); i++ {
  28.         f := s.Field(i)
  29.         fmt.Printf("%d: %s %s = %v\n", i,
  30.             typeOfT.Field(i).Name, f.Type(), f.Interface())
  31.     }
  33.     // 这是我想要的原则：
  34.     m := mystruct{"Hello", 2}
  35.     var ret []interface{}
  36.     ret = make([]interface{}, s.NumField())
  37.     ret[0] = m.Foo
  38.     ret[1] = m.Bar
  39.     return ret
  40. }
  42. // 运行良好：
  43. func readNames(a interface{}) []string {
  44.     s := reflect.TypeOf(a)
  45.     lenStruct := s.NumField()
  46.     ret := make([]string, lenStruct)
  48.     for i := 0; i < lenStruct; i++ {
  49.         ret[i] = s.Field(i).Name
  50.     }
  51.     return ret
  52. }
英文:

I have a struct:

  1. type mystruct struct {
  2.     Foo string
  3.     Bar int
  4. }

I'd like to create SQL insert statements from the struct which have the following form:

  1. m := mystruct{ &quot;Hello&quot; , 1 }
  2. query := &quot;INSERT INTO mytbl ( foo, bar ) VALUES ( ?,? )&quot;
  3. res,err := db.Exec(query, m.Foo, m.Bar)

Now my question is: how can I make the last line dynamically from the struct (or m) itself? I am able to get the struct names using reflect, but I don't know how to create the []interface{} slice for the db.Exec() call. This is what I have tried: (http://play.golang.org/p/GR1Bb61NFH)

  1. package main
  3. import (
  4. 	&quot;fmt&quot;
  5. 	&quot;reflect&quot;
  6. )
  8. type mystruct struct {
  9. 	Foo string
  10. 	Bar int
  11. }
  13. func main() {
  14. 	m := mystruct{&quot;Foo&quot;, 1}
  15. 	fmt.Println(readNames(m))
  16. 	x := unpackStruct(m)
  17. 	fmt.Printf(&quot;%#v\n&quot;, x)
  18. }
  20. func unpackStruct(a interface{}) []interface{} {
  22. 	// &quot;convert&quot; a to m t
  24. 	// doesn&#39;t work, from &#39;laws of reflection&#39;
  25. 	s := reflect.ValueOf(&amp;t).Elem()
  26. 	typeOfT := s.Type()
  27. 	for i := 0; i &lt; s.NumField(); i++ {
  28. 		f := s.Field(i)
  29. 		fmt.Printf(&quot;%d: %s %s = %v\n&quot;, i,
  30. 			typeOfT.Field(i).Name, f.Type(), f.Interface())
  31. 	}
  33. 	// this is in principle what I want:
  34. 	m := mystruct{&quot;Hello&quot;, 2}
  35. 	var ret []interface{}
  36. 	ret = make([]interface{}, s.NumField())
  37. 	ret[0] = m.Foo
  38. 	ret[1] = m.Bar
  39. 	return ret
  40. }
  42. // works fine:
  43. func readNames(a interface{}) []string {
  44. 	s := reflect.TypeOf(a)
  45. 	lenStruct := s.NumField()
  46. 	ret := make([]string, lenStruct)
  48. 	for i := 0; i &lt; lenStruct; i++ {
  49. 		ret[i] = s.Field(i).Name
  50. 	}
  51. 	return ret
  52. }

答案1

得分: 2

如果你的问题是获取字段的值，以下代码片段应该能帮到你：

  1. s := reflect.ValueOf(a)
  2. ret := make([]interface{}, s.NumField())
  3. for i := 0; i < s.NumField(); i++ {
  4.     ret[i] = s.Field(i).Interface()
  5. }
英文:

If getting the values of the fields is your issue, this code snippet should help:

  1. s := reflect.ValueOf(a)
  2. ret := make([]interface{}, s.NumField())
  3. for i := 0; i &lt; s.NumField(); i++ {
  4.     ret[i] = s.Field(i).Interface()
  5. }

答案2

得分: 0

如果创建[]interface{}值是你的问题，使用reflect的切片创建机制应该能很好地解决：

  1. slc := reflect.MakeSlice(InterfaceType, len, cap) // 请参考下面的链接来创建InterfaceType
  2. slc.Index(0).Set(TargetValue)
  3. return slc.Interface()

（这是上述提到的链接）。

修改上述代码以循环遍历结构体中的值而不仅仅是第0个索引应该不会太麻烦。

英文:

If creating the []interface{} value is your problem, using reflect's slice creation mechanisms should work nicely:

  1. slc := reflect.MakeSlice(InterfaceType, len, cap) // See the link below for creating InterfaceType
  2. slc.Index(0).Set(TargetValue)
  3. return slc.Interface()

(Here's the above-mentioned link).

Modifying the above code to loop over the values in the struct instead of just the 0th index shouldn't be too bad.

huangapple
  • 本文由 发表于 2013年9月5日 14:04:53
  • 转载请务必保留本文链接：https://go.coder-hub.com/18628852.html
  • go
  • reflection
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