embedding_mechanics.go

// https://go.dev/play/p/_Gt8U6GFkPk
// go embedding field mechanics ⚙️
package main

import (
	"fmt"
	"unsafe"
)

type A struct {
	X int
	Y int
}

func (s A) Validate() {
	fmt.Println("a", s.X, s.Y)
}

func (s A) GetX() int { return s.X }

type B struct {
	X int
	A
}

func (s B) Validate() {
	fmt.Println("b", s.X, s.Y, "b.a", s.A.X, s.A.Y)
}

func main() {
	// on field assignment only outer struct field is updated
	b := B{X: 1, A: A{X: 2, Y: 3}}
	fmt.Printf("%#v\n", b)
	b.X = 5
	b.Y = 10
	fmt.Printf("%#v\n", b)
	
	// method that is not overwritten, will read overwritten field from inner struct
	fmt.Println("b.a.GetX()", b.A.GetX())

	// only outer struct method is called
	// it accesses outer struct field
	b.Validate()

	// only extra fields are physically stored in outer struct
	a := A{X: 2, Y: 3}
	fmt.Println("size b", unsafe.Sizeof(b), "size b.a", unsafe.Sizeof(b.A), "size of a", unsafe.Sizeof(a))
}