How Can I Differentiate Between Empty and Missing Fields When Unmarshalling JSON in Go?

Distinguishing Void Values from Unspecified Fields in Go Unmarshaling

Unmarshalling JSON data in Golang can be straightforward, but differentiating between void values and unspecified field values can be a challenge. This article addresses this issue, providing a solution to distinguish between the two.

In the provided example, we have a slice of Category structs, where each category has a Name and Description field. When unmarshaling JSON data into this slice, we encounter a scenario where both category B and category C have empty Description fields. However, we want to know if there's a way to discern whether category B's Description is specified as an empty string or simply not present in the JSON data.

The key is to declare the Description field as a pointer to a string:

type Category struct {
    Name        string
    Description *string
}

By using a pointer, if a JSON field is present with an empty string value, it will be set to a pointer pointing to an empty string (*""). Conversely, if the field is not present in JSON, the pointer will be left nil ().

The updated output exemplifies this differentiation:

[{Name:A Description:0x1050c150} {Name:B Description:<nil>} {Name:C Description:0x1050c158}]

Here, category A's Description is non-empty, category B's Description is nil (indicating an unspecified field), and category C's Description is an empty string (indicating a specified empty value).

This technique allows you to distinguish between void values (nil) and unspecified fields (nil pointers), enabling you to tailor your program's behavior accordingly.

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