英文:
Append to golang slice passed as empty interface
问题
如何向空接口(已验证为* []struct)追加内容?
func main() {
var mySlice []myStruct // myStruct可以是任何结构体(动态的)
decode(&mySlice, "...")
}
func decode(dest interface{}, src string) {
// 假设已验证dest为* []struct
var modelType reflect.Type = getStructType(dest)
rows, fields := getRows(src)
for _, row := range rows {
// 创建类型为modelType的新结构体并分配所有字段
model := reflect.New(modelType)
for field := fields {
fieldValue := getRowValue(row, field)
model.Elem().FieldByName(field).Set(fieldValue)
}
castedModelRow := model.Elem().Interface()
// 将model追加到dest;如何实现?
// dest = append(dest, castedModelRow)
}
}
我尝试过的方法:
这样做会导致恐慌:reflect: call of reflect.Append on ptr Value(因为我们传递的是&mySlice而不是mySlice)
dest = reflect.Append(reflect.ValueOf(dest), reflect.ValueOf(castedModelRow))
这个方法可以工作,但在调用decode函数后,main函数中的len(mySlice)仍然为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)
}
英文:
How to append to empty interface (that has been verified to be a *[]struct)?
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)
}
}
Things I've tried:
This simply panics: reflect: call of reflect.Append on ptr Value (as we pass &mySlice instead of mySlice)
dest = reflect.Append(reflect.ValueOf(dest), reflect.ValueOf(castedModelRow))
This works but doesn't set the value back to dest... in main func, len(mySlice) remains 0 after decode function is called.
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)
}
答案1
得分: 2
以下是修复问题中第二个decode
函数的方法。语句
dest = reflect.ValueOf(result)
修改的是局部变量dest
,而不是调用者的值。使用以下语句来修改调用者的切片:
reflect.ValueOf(dest).Elem().Set(result)
问题中的代码在reflect.MakeSlice
创建的元素之后追加了解码后的元素。结果切片中有len(rows)
个零值,然后是len(rows)
个解码后的值。通过将
result = reflect.Append(result, reflect.ValueOf(castedModelRow))
改为:
result.Index(i).Set(model)
来进行修复。
以下是问题中第二个decode
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)
}
英文:
Here's how to fix the second decode
function shown in the question. The statement
dest = reflect.ValueOf(result)
modifies local variable dest
, not the caller's value. Use the following statement to modify the caller's slice:
reflect.ValueOf(dest).Elem().Set(result)
The code in the question appends decoded elements after the elements created in reflect.MakeSlice. The resulting slice has len(rows)
zero values followed by len(rows)
decoded values. Fix by changing
result = reflect.Append(result, reflect.ValueOf(castedModelRow))
to:
result.Index(i).Set(model)
Here's the update version of the second decode
function in the question:
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)
}
答案2
得分: 1
你的原始解决方案非常接近。在调用追加操作之前,你需要对指针进行解引用。如果你的目标(dest)已经有一些现有元素,并且你不想通过创建一个newSlice
来丢失它们,这个解决方案将非常有帮助。
tempDest := reflect.ValueOf(dest).Elem()
tempDest = reflect.Append(tempDest, reflect.ValueOf(model.Interface()))
就像@I Love Reflection指出的那样,最后你需要将新的切片设置回指针。
reflect.ValueOf(dest).Elem().Set(tempDest)
总体解码:
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)
英文:
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)
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