Golang struct inheritance not working as intended?

Question

When declaring a struct that inherits from a different struct:

type Base struct {
    a string
    b string
}

type Something struct {
    Base
    c string
}

Then calling functions specifying values for the inherited values gives a compilation error:

f(Something{
    a: "letter a",
    c: "letter c",
})

The error message is: unknown Something field 'a' in struct literal.

This seems highly weird to me. Is this really the intended functionality?

Thanks for the help!

Leo Correa · Accepted Answer · 2020-01-28 12:38:39Z

43

Golang doesn't provide the typical notion of inheritance. What you are accomplishing here is embedding.

It does not give the outer struct the fields of the inner struct but instead allows the outer struct to access the fields of the inner struct.

In order to create the outer struct Something you need to give its fields which include the inner struct Base

In your case:

Something{Base: Base{a: "letter a"}, c: "letter c"}

edited Jan 28, 2020 at 12:38

answered Jan 6, 2016 at 22:38

Leo Correa

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2 Comments

Kat Lim Ruiz Over a year ago

one good consequence is that if you do json.Marshal you will get the {a, b, c} structure you want

Finesse Over a year ago

What're the differences from type Something struct { Base Base c string }?

Grzegorz Żur · Accepted Answer · 2016-01-06 22:37:54Z

11

You need to explicitly create Base field like that

f(Something{
    Base: Base{a: "letter a"},
    c:    "letter c",
})

Go has no inheritance, it is just composition.

answered Jan 6, 2016 at 22:37

Grzegorz Żur

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2 Comments

evanmcdonnal Over a year ago

Composition is the ubiquitous computer science term, worth pointing out though that the actual Go feature being utilized here is embedding.

Grzegorz Żur Over a year ago

@evanmcdonnal True, but I intentionally used this wording, because of en.wikipedia.org/wiki/Composition_over_inheritance

evanmcdonnal · Accepted Answer · 2016-01-06 22:40:57Z

5

You have to actually instantiate the embedded struct as well. Just so you know this isn't inheritance technically, no such feature exists in Go. It's called embedding. It just hoists fields and methods from the embedded type to the embeddors scope. So anyway, the composite literal instantiation you're trying to do would look like this;

f(Something{
    Base: Base{a: "a", b: "b"},
    c:    "c",
})

answered Jan 6, 2016 at 22:40

evanmcdonnal

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Comments

Jinsong Li · Accepted Answer · 2023-09-28 13:36:29Z

3

Read the example from Go by Example: Struct Embedding, the trick part is that we need to explicitly initialize the embedded struct, but we can directly access the embedded fields.

type base struct {
    num int
}
type container struct {
    base
    str string
}
// initialize the embedding explicitly
co := container{
    base: base{
       num: 1,
    },
    str: "some name",
}
// however, the embedded field can be accessed directly
fmt.Printf("co={num: %v, str: %v}\n", co.num, co.str)
// this also works
fmt.Println("also num:", co.base.num)

answered Sep 28, 2023 at 13:36

Jinsong Li

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