golang struct attribute as nil

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I have a struct as

type Result struct {
    Foo *string
}

I would like to get a json object like

{"Foo": null}

How do I achieve this ?

I have tried a few things: approach 1:

var res1 *Result
json.Unmarshal(nil, &res1)

approach2

var res1 Result
res1.Foo = nil

I get a res1 struct with Foo as nil

Thanks for help!

Edit: var res1 *Result -> var res1 Result

go json
1 Answers

Basic types in Go are not nillable. string is a basic type.

In order for Foo to be nillable, one solution is to make it a string pointer. That would look like this:

type Result struct {
    Foo *string
}

If you don't like nil, you can also add a boolean field explaining if Foo is present or not:

type Result struct {
    Foo string
    IsPresent bool // true if Foo is present. false otherwise.
}

But you would need to write a custom JSON deserializer for this. So just making Foo a pointer is what I would do.

Edit after question was changed to mention OP already is using *string:

To go from the JSON string { "Foo" : null } to the Result Go struct listed above in my answer, the json package Unmarshal function can be used:

var r Result

err := json.Unmarshal([]byte(`{ "Foo" : null }`), &r)
if err != nil { /* ... */ }

// now f contains the desired data

To convert from the Result struct to the above JSON string, the json library Marshal function can be used:

var r Result // Foo will be zero'd to nil

jsonStr, err := json.Marshal(&r)
if err != nil { /* ... */ }

// now jsonStr contains `{ "Foo" : null }`

Here are the above two code blocks running in Go Playground.

phonaputer0
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