Issue with using default tag with pointer type

[hnimtadd]

TL;DR: When using the mod:"default" tag with a field of type *string, if the field is nil, the library assigns it to a pointer referencing an empty string ("") instead of the defined default value.

---

Context:

Hi team, thank you for the great work on this library!

While integrating it into my project, I encountered an unexpected behavior and am seeking your advice.

Here’s a simplified example to illustrate the issue:

package main

import (
	"context"
	"fmt"
	"log"

	"github.com/go-playground/mold/v4/modifiers"
)

var conform = modifiers.New()

type User struct {
	Name *string `mod:"default=abc"`
}

func main() {
	user := User{}
	fmt.Println("Name:", user.Name) // Output: nil (as expected)

	err := conform.Struct(context.Background(), &user)
	if err != nil {
		log.Panic(err)
	}

	fmt.Println("Conformed Name:", *user.Name) // Output: "" (unexpected, expected "abc")
}

---

Problem:

The Name field in the User struct is of type *string and is configured with the mod:"default=abc" tag.

However, if the field is initially nil, the library assigns it to a pointer referencing an empty string ("") instead of a pointer referencing the expected default value ("abc").

---

Expected Behavior:

If the Name field is nil, it should be assigned to a pointer referencing the default value ("abc") as defined by the mod:"default=abc" tag

---

Investigation and proposal:

I traced the issue to the following section of code in the modifiers/multi.go file:

	case reflect.Ptr:
		fl.Field().Set(reflect.New(fl.Field().Type().Elem()))
	}

This code initializes a pointer field to point to an empty value (e.g., an empty string).

However, it does not account for the configured default value defined in the mod:"default=..." tag. As a result, the pointer field ends up pointing to an empty string instead of the specified default value.

---

To fix this, I suggest updating the logic to use the configured default value when initializing the pointer. Here’s a proposed modification:

case reflect.Ptr:
	value := fl.Param() // Retrieve the configured default value
	fl.Field().Set(reflect.ValueOf(&value)) // Set the field to point to the default value
}

---

Please let me know if this behavior is intentional or if there is a recommended way to achieve the expected behavior. Thanks in advance!

[deankarn]

@hnimtadd given your thorough example it does appear to be a bug, but I’ll also have to check it’s been a while since I’ve touched/looked at this code 😅

It might be a while until I can check thought, I’m in the middle of moving and everything including my computers are still packed so may be a week or two before I can.

if that long goes by and I haven’t please poke me to remind me 🙏

[hnimtadd]

Hi @deankarn , would you mind if I create a PR, and then you could review it? Since I have already implemented it in my vendor code.

[deankarn]

Yes by all means a PR would be helpful

[hnimtadd]

Hi @deankarn, could you grant me permission to create a PR in this repository?

I am unable to push my code to any feature branch. 🙇

[deankarn]

@hnimtadd you need to fork the repository, make the changes push to your branch and the make upstream PR

[hnimtadd]

Hi @deankarn. The PR is ready here

[hnimtadd]

Hi @deankarn , is there any new update on this?

[deankarn]

This should be working in Release https://github.com/go-playground/mold/releases/tag/v4.5.1