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如何附加到空接口（已验证为 *[] 结构）？

func main() {
  var mySlice []myStruct // myStruct can be any struct (dynamic)
  decode(&mySlice, "...")
}

func decode(dest interface{}, src string) {
  // assume dest has been verified to be *[]struct
  var modelType reflect.Type = getStructType(dest)
  rows, fields := getRows(src)
  for _, row := range rows {
    // create new struct of type modelType and assign all fields
    model := reflect.New(modelType)
    for field := fields {
      fieldValue := getRowValue(row, field)
      model.Elem().FieldByName(field).Set(fieldValue)
    }
    castedModelRow := model.Elem().Interface()
      
    // append model to dest; how to do this?
    // dest = append(dest, castedModelRow)
  }
}

我尝试过的事情：

这简直是​​恐慌：反射：在 ptr 值上调用 reflect.Append（因为我们通过 &mySlice 而不是 mySlice）

dest = reflect.Append(reflect.ValueOf(dest), reflect.ValueOf(castedModelRow))

这有效，但不会将值设置回 dest... 在 main func 中，调用 decode 函数后 len(mySlice) 保持为 0。

func decode(dest interface{}, src string) {
  ...
  result := reflect.MakeSlice(reflect.SliceOf(modelType), rowCount, rowCount)
  for _, row : range rows {
    ...
    result = reflect.Append(result, reflect.ValueOf(castedModelRow))
  }
  dest = reflect.ValueOf(result)
}
4

2 回答 2

2

这是修复问题中decode显示的第二个功能的方法。该声明

dest = reflect.ValueOf(result)

修改局部变量dest，而不是调用者的值。使用以下语句修改调用者的切片：

reflect.ValueOf(dest).Elem().Set(result)

问题中的代码在 reflect.MakeSlice 中创建的元素之后附加了解码的元素。结果切片具有len(rows)零值，后跟len(rows)解码值。通过更改修复

result = reflect.Append(result, reflect.ValueOf(castedModelRow))

至：

result.Index(i).Set(model)

decode这是问题中第二个函数的更新版本：

func decode(dest interface{}, src string) {
    var modelType reflect.Type = getStructType(dest)
    rows, fields := getRows(src)
    result := reflect.MakeSlice(reflect.SliceOf(modelType), len(rows), len(rows))
    for i, row := range rows {
        model := reflect.New(modelType).Elem()
        for _, field := range fields {
            fieldValue := getRowValue(row, field)
            model.FieldByName(field).Set(fieldValue)
        }
        result.Index(i).Set(model)
    }
    reflect.ValueOf(dest).Elem().Set(result)
}

在操场上运行它。

于 2021-11-07T02:49:13.283 回答
1

You were very close with your original solution. You had to de-reference the pointer before calling the append operation. This solution would be helpful if your dest already had some existing elements and you don't want to lose them by creating a newSlice.

tempDest := reflect.ValueOf(dest).Elem()
tempDest = reflect.Append(tempDest, reflect.ValueOf(model.Interface()))

Similar to how @I Love Reflection pointed out, you finally need to set the new slice back to the pointer.

reflect.ValueOf(dest).Elem().Set(tempDest)

Overall Decode:

var modelType reflect.Type = getStructType(dest)
rows, fields := getRows(src)
tempDest := reflect.ValueOf(dest).Elem()
for _, row := range rows {
    model := reflect.New(modelType).Elem()
    for _, field := range fields {
        fieldValue := getRowValue(row, field)
        model.FieldByName(field).Set(fieldValue)
    }
    tempDest = reflect.Append(tempDest, reflect.ValueOf(model.Interface()))
}
reflect.ValueOf(dest).Elem().Set(tempDest)
于 2021-11-07T03:14:24.150 回答