


How to call a generic function in Go by passing a Type via reflection
I have a Go application that receives different JSON responses for the same API, so I tried to write a custom unmarshaller, It will try to process every possible JSON response until it finds the correct one.
To do this, I have created a generic function to unmarshal JSON, and I want to call it using the type obtained from reflection.
For example:
package main import ( "bytes" "encoding/json" "fmt" "reflect" ) type JSONHandler struct{} func (handler *JSONHandler) Unmarshal(data []byte) error { t := reflect.TypeOf(*handler) for i := 0; i < t.NumField(); i++ { fieldType := t.Field(i) fmt.Printf("Trying to unmarshal: %s\n", fieldType.Name) result, err := unmarshal[fieldType](data) // fieldType (variable of type reflect.StructField) is not a typecompiler (NotAType) if err == nil { fmt.Printf("result: %s\n", result) break } } return nil } func unmarshal[T interface{}](data []byte) (*T, error) { obj := new(T) reader := bytes.NewReader(data) decoder := json.NewDecoder(reader) decoder.DisallowUnknownFields() err := decoder.Decode(obj) if err != nil { return nil, err } return obj, nil } type User struct { Username *string `json:"username,omitempty"` Password *string `json:"password,omitempty"` } type UserError struct { Error *string `json:"error,omitempty"` Code *int `json:"code,omitempty"` } type UserOrError struct { User *User UserError *UserError JSONHandler } var userJson = `{ "username": "[email protected]", "password": "password" }` func main() { user := new(UserOrError) result := user.Unmarshal([]byte(userJson)) // { User: something, UserError: nil } }
However, the Go compiler gave me this error: fieldType (variable of type reflect.StructField) is not a type compiler (NotAType)
.
Is there a way to pass reflection parameters to a generic function?
Correct answer
This question is beyond the scope of Go's general functionality.
Use reflect.New to create a value given the reflect.Type of that value. p>
Go has no inheritance. The Unmarshal method in the question operates on types that have no fields. Fixed by passing the "handler" to a normal function.
// Unmarshal unmarshals data to each field type in handler // and returns the first successful result. The handler argument // must be a pointer to a struct. func Unmarshal(handler any, data []byte) (any, error) { t := reflect.TypeOf(handler).Elem() for i := 0; i < t.NumField(); i++ { fieldType := t.Field(i) v := reflect.New(fieldType.Type) decoder := json.NewDecoder(bytes.NewReader(data)) decoder.DisallowUnknownFields() err := decoder.Decode(v.Interface()) if err == nil { return v.Elem().Interface(), err } } return nil, errors.New("no matching type") }
Use the Marshal function like this:
type UserOrError struct { User *User UserError *UserError } var userJson = `{ "username": "<a href="https://www.php.cn/link/89fee0513b6668e555959f5dc23238e9" class="__cf_email__" data-cfemail="0e646166604e6b636f6762206d6163">[email protected]</a>", "password": "password" }` result, err := Unmarshal(&UserOrError{}, []byte(userJson)) // { User: something, UserError: nil } fmt.Printf("err=%v, result=%#v\n", err, result)
https://www.php.cn/link/c76e4b2fa54f8506719a5c0dc14c2eb9
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