How to extract type parameters using reflection-Golang-php.cn

How to extract type parameters using reflection

Question content

Context: I am writing a generic automapper that takes two types of structures and checks if each field of said structure has a given specified tags, then copies the values from the source structure to the target structure, assuming they have matching tags and types. Whenever a struct field is another (nested) struct, I want the automapper function to perform a recursive call that automatically maps down the rabbit hole.

Problem: I can only pass the concrete type of the root structure. Once I get into the generic function using reflection, trying to extract the nested struct type seems impossible. Although I can pass value.interface() as a parameter, I still need to pass the type parameter.

Here is some simplified code to show the problem.

type Alpha struct { Nested Beta `automap:"nested"` } type Beta struct { Info string `automap:"info"` } type Foo struct { Nested Bar `automap:"nested"` } type Bar struct { Info string `automap:"info"` } func TestAutoMap(t *testing.T) { b := Beta{Info: "Hello from Beta!"} a := Alpha{Nested: b} f, err := AutoMap[Alpha, Foo](a) if err != nil { fmt.Println(err) t.Fail() } fmt.Println("f.nested.info:", f.Nested.Info) } func AutoMap[S, T any](source S) (target T, err error) { targetStruct := reflect.ValueOf(&target).Elem() sourceStruct := reflect.ValueOf(&source).Elem() // .Type and .Kind directly did not work. nestedSourceType := ??? // I want this to be type Beta. nestedTargetType := ??? // I want this to be type Bar. sourceInterface := sourceStruct.Interface() t, err := AutoMap[nestedSourceType, nestedTargetType](sourceInterface) if err != nil { return target, err } target = t return target, nil }

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Solution

Following the advice of @mkopriva, I would like to share a simple solution to the problem I encountered.

Feel free to correct or improve it, but keep in mind that I intentionally did not include the various checks and assertions below.

(go playground example)

type Alpha struct { NestedOnce Beta } type Beta struct { NestedTwice Gamma } type Gamma struct { Info string } type Foo struct { NestedOnce Bar } type Bar struct { NestedTwice Baz } type Baz struct { Info string } func TestAutoMap(t *testing.T) { g := Gamma{"Hello from Gamma!"} b := Beta{g} a := Alpha{b} f, err := AutoMap[Foo](a) if err != nil { fmt.Println(err) t.Fail() } else { fmt.Println("Foo.NestedOnce.NestedTwice.Info:", f.NestedOnce.NestedTwice.Info) } } func AutoMap[T any](source any) (target T, err error) { // Peel off 'any' from the function parameter type. sourceStruct := reflect.ValueOf(&source).Elem().Elem() targetStruct := reflect.ValueOf(&target).Elem() err = autoMap(sourceStruct, targetStruct) return target, err } func autoMap(s, t reflect.Value) error { sourceField := s.Field(0) targetField := t.Field(0) if sourceField.Kind() == reflect.Struct { err := autoMap(sourceField, targetField) if err != nil { return err } return nil } targetField.Set(sourceField) return nil }

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