How Can I Work Around Go's JSON Tag Restrictions Using Unexported Structs and Type Casting?

Using Type Casting within Unexported Structs

In Go, you can't define multiple JSON tags for the same field in a struct. However, there is a workaround using casting between unexported structs.

First, create two identically-structured structs with different JSON tags, as in your example:

type Foo struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

type Bar struct {
    Name string `json:"employee_name"`
    // Age omitted with backtick syntax
    Age  int    `json:"-"`
}

Now, make Bar unexported by starting its name with a lowercase letter:

type bar struct {
    Name string `json:"employee_name"`
    Age  int    `json:"-"`
}

To transform from Foo to bar, cast Foo to *bar, as shown below:

f := Foo{Name: "Sam", Age: 20}
b := (*bar)(unsafe.Pointer(&f))

// b now has the modified JSON tags

Cautions:

• This technique should be used cautiously and only within a controlled environment.
• Never expose the unexported bar type outside of the current package.
• Ensure that the types are always aligned, otherwise casting will panic.

Example:

package main

import "fmt"
import "unsafe"

type Foo struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

type bar struct {
    Name string `json:"employee_name"`
    Age  int    `json:"-"`
}

func main() {
    f := Foo{Name: "Sam", Age: 20}
    b := (*bar)(unsafe.Pointer(&f))

    fmt.Println(b.Name, b.Age) // Output: Sam 0

    // Changing f.Age affects b.Age
    f.Age = 30
    fmt.Println(b.Name, b.Age) // Output: Sam 30
}

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