Composition via Struct Embedding

Snippet

Composition via Struct Embedding

Go uses struct embedding to achieve composition. When a type is embedded without a field name, its methods and fields are 'promoted' to the outer struct, allowing them to be accessed directly.

snippet.go
go
package main
 
import "fmt"
 
type User struct {
    ID   int
    Name string
}
 
func (u User) Greet() {
    fmt.Printf("Hello, I am %s\n", u.Name)
}
 
type Admin struct {
    User  // Embedded field
    Level int
}
 
func main() {
    a := Admin{
        User:  User{ID: 1, Name: "Alice"},
        Level: 10,
    }
    a.Greet() // Promoted method
    fmt.Println("ID:", a.ID) // Promoted field
}

Breakdown

type Admin struct { User ... }

Embeds the User struct into Admin. This is an anonymous field.

a.Greet()

Calls the Greet method from User directly on the Admin instance due to promotion.

a.ID

Accesses the ID field of User directly through the Admin instance.

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Composition via Struct Embedding

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