Go: embedded type's field initialization in derived type

Question

Here is the code that works:

package main

import (
    "fmt"
)

type Base struct {
    Field  int
}

type Derived struct {
    Base
}

func main() {
    d := &Derived{}
    d.Field = 10
    fmt.Println(d.Field)
}

And here's the code that fails to compile with ./main.go:17: unknown Derived field 'Field' in struct literal

package main

import (
    "fmt"
)

type Base struct {
    Field  int
}

type Derived struct {
    Base
}

func main() {
    d := &Derived{
        Field: 10,
    }
    fmt.Println(d.Field)
}

What exactly is going on here? Sorry if it's obvious, but I just don't understand.

Stop using terms like Base and Derived with Go. Go doesn't have inheritance and you'll just cause yourself grief by trying to think in terms of inheritance in a language which instead embraces composition. — Dave C
– Dave C, Commented Oct 2, 2015 at 15:16

AndrewN · Accepted Answer · 2015-10-02 11:35:12Z

5

From the language specification:

Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in composite literals of the struct.

So that's why it doesn't work.

Here are two possible ways to work around that limitation, each illustrated in the following function:

func main() {
    d := &Derived{
        Base{Field: 10},
    }

    e := new(Derived)
    e.Field = 20

    fmt.Println(d.Field)
    fmt.Println(e.Field)
}

answered Oct 2, 2015 at 11:35

AndrewN

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Comments

nessuno · Accepted Answer · 2015-10-02 11:35:02Z

1

To initialize composed objects you have to initialize the embedded field, like any other object:

package main

import (
    "fmt"
)

type Base struct {
    Field int
}

type Derived struct {
    Base
}

func main() {
    d := &Derived{
        Base{10},
    }
    fmt.Println(d.Field)
}

answered Oct 2, 2015 at 11:35

nessuno

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