How to Return Nil String in Go?

Question

I have a function which returns a string under certain circumstances, namely when the program runs in Linux or MacOS, otherwise the return value should be nil in order to omit some OS-specific checks further in code.

func test() (response string) {
    if runtime.GOOS != "linux" {  
        return nil
    } else {
        /* blablabla*/
    }
}

however when I try to compile this code I get an error:

test.go:10:3: cannot use nil as type string in return argument.

If I return just an empty string like return "", I cannot compare this return value with nil further in code.

So the question is how to return a correct nil string value?

Possible duplicate of How to return int or nil in golang?

Jonathan Hall
– Jonathan Hall

2018-09-10 12:18:24 +00:00
Commented Sep 10, 2018 at 12:18 — Jonathan Hall
– Jonathan Hall, Commented Sep 10, 2018 at 12:18

icza · Accepted Answer · 2018-09-10 15:05:07Z

82

If you can't use "", return a pointer of type *string; or–since this is Go–you may declare multiple return values, such as: (response string, ok bool).

Using *string: return nil pointer when you don't have a "useful" string to return. When you do, assign it to a local variable, and return its address.

func test() (response *string) {
    if runtime.GOOS != "linux" {
        return nil
    } else {
        ret := "useful"
        return &ret
    }
}

Using multiple return values: when you have a useful string to return, return it with ok = true, e.g.:

return "useful", true

Otherwise:

return "", false

This is how it would look like:

func test() (response string, ok bool) {
    if runtime.GOOS != "linux" {
        return "", false
    } else {
        return "useful", true
    }
}

At the caller, first check the ok return value. If that's true, you may use the string value. Otherwise, consider it useless.

Also see related questions:

How do I represent an Optional String in Go?

Alternatives for obtaining and returning a pointer to string: How do I do a literal *int64 in Go?

edited Sep 10, 2018 at 15:05

answered Sep 10, 2018 at 10:23

icza

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2 Comments

sotona Over a year ago

well, looks like returning string pointer instead of the string itself is the best way to avoid additional entities

icza Over a year ago

@sotona Depends on how (and how often) you intend to use this function. Multi-return values are quite common in Go, and working with string pointers is more cumbersome. I'd go with multi-return values in most of the time.

Nic · Accepted Answer · 2018-09-10 13:16:43Z

12

Go has built-in support for multiple return values:

This feature is used often in idiomatic Go, for example to return both result and error values from a function.

In your case it could be like this:

func test() (response string, err error) {
    if runtime.GOOS != "linux" {  
        return "", nil
    } else {
        /* blablabla*/
    }
}

And then:

response, err := test()
if err != nil { 
    // Error handling code
    return;
}

// Normal code

If you want to ignore the error, simply use _:

response, _ := test()
// Normal code

edited Sep 10, 2018 at 13:16

answered Sep 10, 2018 at 10:24

Nic

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Comments

user2047029 · Accepted Answer · 2018-09-10 10:25:14Z

1

Go allows multiple return types. So use this to your advantage and return an error or any other type. Check this out for more info: http://golangtutorials.blogspot.com/2011/06/return-values-from-go-functions.html?m=1

answered Sep 10, 2018 at 10:25

user2047029

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