Created
March 23, 2021 11:22
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Implement As method to convert interface type to the right data structure as it is first write. That means, if it use struct A, it only valid to output target as A. If we re-declare struct A, it will not match since it is not the real A in the first declaration.
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| package main_test | |
| import ( | |
| "fmt" | |
| "reflect" | |
| "testing" | |
| ) | |
| type AnimalName string | |
| const ( | |
| C AnimalName = "crocodile" | |
| L AnimalName = "lion" | |
| ) | |
| type Crocodile struct { | |
| CanineTooth bool | |
| } | |
| type Lion struct { | |
| CanineTooth bool | |
| } | |
| type Animal struct { | |
| Type AnimalName | |
| Feature interface{} | |
| } | |
| // As return Feature data as type variable. | |
| // Before convert to the out variable, it validate whether current value in Feature | |
| // is match with the AnimalName data structure. | |
| func (a *Animal) As(out interface{}) error { | |
| outVal := reflect.ValueOf(out) | |
| outType := outVal.Type() | |
| if outType.Kind() != reflect.Ptr || outVal.IsNil() { | |
| return fmt.Errorf("not type of pointer") | |
| } | |
| trueDataStructureVal := reflect.ValueOf(a.Feature) | |
| targetType := outType.Elem() | |
| if !reflect.TypeOf(a.Feature).AssignableTo(targetType) { | |
| return fmt.Errorf("target has type %s but the true structure is %s", | |
| targetType.String(), trueDataStructureVal.Type().String(), | |
| ) | |
| } | |
| // set if assignable | |
| outVal.Elem().Set(trueDataStructureVal) | |
| return nil | |
| } | |
| func TestTypeConversion(t *testing.T) { | |
| animal := Animal{ | |
| Type: L, | |
| Feature: Lion{CanineTooth: true}, | |
| } | |
| var lion Lion | |
| err := animal.As(&lion) | |
| if err != nil { | |
| t.Fatalf(err.Error()) | |
| return | |
| } | |
| var crocodile Crocodile | |
| err = animal.As(&crocodile) | |
| if err == nil { | |
| t.Fatalf("should error because is not type of %T", crocodile) | |
| return | |
| } | |
| type Lion struct { | |
| CanineTooth bool | |
| } | |
| var likeLion Lion | |
| err = animal.As(&likeLion) | |
| if err == nil { | |
| t.Fatalf("should error because is not type of %T", likeLion) | |
| return | |
| } | |
| } |
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