I always had a question “Should I use value for type in struct? Or using pointer?”
After reading many different projects on the web, it seems that they all have different practices.
Until I recently encountered a problem that gave me some ideas.

In our projects, we often start by defining the Request incoming Struct
Then use json.Unmarshal to convert the incoming json string into Struct for subsequent operations

Previously, the default value for fields was false, so we pass by value.
In this case, the default value is true and the problem happened.

Take the first look at the code and execution results

package main

import (
	"encoding/json"
	"fmt"
)

type John struct {
	Name   string `json:"name"`
	CanRun bool   `json:"canRun"`
	CanFly bool   `json:"canFly"`
}

type Doe struct {
	Name   string `json:"name"`
	CanRun *bool  `json:"canRun"`
	CanFly *bool  `json:"canFly"`
}

func main() {
	var param = []byte(`{"Name":"John Doe", "canRun":true}`)
	defult := true

	var john = new(John)
	json.Unmarshal(param, &john)
	jsondata, _ := json.Marshal(john)
	fmt.Println(string(jsondata))

	var doe = new(Doe)
	json.Unmarshal(param, &doe)

	doejsondata, _ := json.Marshal(doe)
	fmt.Println(string(doejsondata))
	if doe.CanFly == nil {
		doe.CanFly = &defult
	}
	doejsondata, _ = json.Marshal(doe)
	fmt.Println(string(doejsondata))
}

In the example, there are two Structs.
John, whose Type is a Bollinger pass by value, and Doe, whose Type is a Bollinger pass by pointer.

If the CanFly field is “optional” and the default value is “true”

The param in the example code play as a request without CanFly parameter

1. Assign the variables of type “John” and “Doe”
2. Use json.Unmarshal to convert the incoming json string to Struct
3. Convert to json strings via json.Marshal for easy reading and printing on the screen

The first line of the output is “John”, which using the boolean value as type.
When calling new(John) will automatically bring in the zero value .
And the default zero value of boolean is false.

And Name and CanRnu are overwritten with zero value because they are passed in.
But because CanFly is optional and not passed in by the user, it is still false
We can’t identify the false is from user or from zero value.

If you use “pointer” as the type in Struct, you can see that the second line of CanFly is null.
So you can set the default value to “true” later on.

This is what I learned about the default value problem when converting json strings to Struct, and how to solve it with a pointer.