How to Pass Nested Structures By Reference in Reflection Using Value.Addr()?

Nested Structures and Pass-by-Reference in Reflection

In Go, understanding nested structures and how to pass them by reference in reflection is crucial. Consider a scenario where you have the nested Client and Contact structures:

<code class="go">type Client struct {
    Id                int
    Age               int
    PrimaryContact    Contact
    Name              string
}

type Contact struct {
    Id        int
    ClientId  int
    IsPrimary bool
    Email     string
}</code>

When you introspect the PrimaryContact field of the Client struct, you may encounter a "reflect.Value.Set using unaddressable value" panic. This is because PrimaryContact is passed by value, not by reference. To resolve this issue, we need to pass PrimaryContact by reference using reflection.

Solution Using Value.Addr()

1. Pass the pointer to the Client struct: To set field values of a struct using reflection, you must pass it as a pointer. In this case, &client is the pointer to the Client struct.
2. Get pointer value of the PrimaryContact field: Use Value.Addr() to get the pointer value of the PrimaryContact field. This addressable value can be used for setting the field values of the nested struct.

Code:

<code class="go">package main

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 main() {
    client := Client{}

    v := reflect.ValueOf(&client)
    primaryContact := v.FieldByName("PrimaryContact").Addr()

    primaryContact.FieldByName("Id").SetInt(123)
    primaryContact.FieldByName("ClientId").SetInt(456)
    primaryContact.FieldByName("IsPrimary").SetBool(true)
    primaryContact.FieldByName("Email").SetString("example@example.com")

    fmt.Printf("%+v\n", client)
}</code>

Output:

{Id:0 Age:0 PrimaryContact:{Id:123 ClientId:456 IsPrimary:true Email:example@example.com} Name:}

Key Points:

• To pass a nested struct by reference, use reflect.Value.Addr().
• Set field values using reflect.Value.SetInt(), reflect.Value.SetString(), etc.
• Iterate over the struct fields to set values for all nested structures.

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