How to Set Default Values for Nested Structures Using Reflection by Reference?

Passing Nested Structures by Reference for Reflection

This question explores the challenge of looping through nested structures using reflection and setting default values for fields. The primary issue encountered is that passing nested structures by value prevents field updates from being reflected in the original structure.

To address this, we must pass structures by reference using reflection. The solution involves the following steps:

1. Access the pointer value of the nested structure:

   • Retrieve the reflect.Value of the structure and indirect it to obtain its pointer value.

2. Recursively iterate over fields:

   • Use a recursive function to iterate over each field of the structure, and for each struct field, use its pointer value to set default values recursively.

Here's a working implementation:

<code class="go">import (
    "fmt"
    "reflect"
)

type Client struct {
    Id                int
    Age               int
    PrimaryContact    Contact
    Name              string
}

type Contact struct {
    Id        int
    ClientId  int
    IsPrimary bool
    Email     string
}

func SetDefault(s interface{}) error {
    return setDefaultValue(reflect.ValueOf(s).Elem())
}

func setDefaultValue(v reflect.Value) error {
    switch v.Kind() {
        case reflect.Int:
            v.SetInt(42)
        case reflect.String:
            v.SetString("Foo")
        case reflect.Bool:
            v.SetBool(true)
        case reflect.Struct:
            // Recursive call using the pointer value of the nested struct
            err := setDefaultValue(v.Addr())
            if err != nil {
                return err
            }
        default:
            return errors.New("Unsupported kind: " + v.Kind().String())
    }

    return nil
}

func main() {
    a := Client{}
    err := SetDefault(&a)
    if err != nil {
        fmt.Println("Error: ", err)
    } else {
        fmt.Printf("%+v\n", a)
    }
}</code>

This code recursively sets default values for all primitive fields and nested structures by passing them by reference using reflection. The sample output is:

{Id:42 Age:42 PrimaryContact:{Id:42 ClientId:42 IsPrimary:true Email:Foo} Name:Foo}

Using this technique, you can effectively loop through nested structures using reflection and set default values while ensuring that changes are propagated back to the original structure.

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