Go by Example: Struct Embedding

Go supports embedding of structs and interfaces to express a more seamless composition of types. This is not to be confused with //go:embed which is a go directive introduced in Go version 1.16+ to embed files and folders into the application binary.

 
package main
 
import "fmt"
 
type base struct {
    num int
}
 
func (b base) describe() string {
    return fmt.Sprintf("base with num=%v", b.num)
}

A container embeds a base. An embedding looks like a field without a name.

 
type container struct {
    base
    str string
}
 
func main() {

When creating structs with literals, we have to initialize the embedding explicitly; here the embedded type serves as the field name.

 
    co := container{
        base: base{
            num: 1,
        },
        str: "some name",
    }

We can access the base’s fields directly on co, e.g. co.num.

 
    fmt.Printf("co={num: %v, str: %v}\n", co.num, co.str)

Alternatively, we can spell out the full path using the embedded type name.

 
    fmt.Println("also num:", co.base.num)

Since container embeds base, the methods of base also become methods of a container. Here we invoke a method that was embedded from base directly on co.

 
    fmt.Println("describe:", co.describe())
 
    type describer interface {
        describe() string
    }

Embedding structs with methods may be used to bestow interface implementations onto other structs. Here we see that a container now implements the describer interface because it embeds base.

 
    var d describer = co
    fmt.Println("describer:", d.describe())
}
 
$ go run struct-embedding.go
co={num: 1, str: some name}
also num: 1
describe: base with num=1
describer: base with num=1

Next example: Generics.

by Mark McGranaghan and Eli Bendersky | source | license