Structs and Interfaces

Go Struct Embedding and Composition

Go has no inheritance. Reuse fields and methods by embedding one struct in another.

Intermediate
4 min read
1 runnable example

What you'll learn

Embedding a struct
Promoted fields and methods
Composition

Run code →

type Dog struct

Dog
- AnimalName, Describe()
- Breed

Build types from smaller types

Go has no inheritance. Instead, a struct can embed another struct, and the fields and methods of the embedded type become available on the outer one. This is called composition.

Dog embeds Animal, so dog.Name and dog.Describe() work as if they were declared on Dog. The embedded value can still be reached by its type name, as in dog.Animal.Name. Service gets a Log method by embedding Logger.

Key points

Dog embeds Animal and gets Name and Describe.
The embedded value is still reachable: dog.Animal.
Embedding is composition, not inheritance.

Runnable Go example

Run code →

main.go

package main

import "fmt"

type Animal struct {
	Name string
}

func (a Animal) Describe() string {
	return a.Name + " is an animal"
}

type Dog struct {
	Animal // embedded: Dog gets Name and Describe
	Breed  string
}

func (d Dog) Speak() string {
	return d.Name + " says woof"
}

type Logger struct{ Prefix string }

func (l Logger) Log(message string) { fmt.Println(l.Prefix + message) }

type Service struct {
	Logger
	Name string
}

func main() {
	dog := Dog{Animal: Animal{Name: "Rex"}, Breed: "Collie"}
	fmt.Println(dog.Describe())
	fmt.Println(dog.Speak())
	fmt.Println(dog.Animal.Name, dog.Breed)

	service := Service{Logger: Logger{Prefix: "[api] "}, Name: "users"}
	service.Log("started " + service.Name)
}

Get a Run button for this example Result of running the code

Rex is an animal
Rex says woof
Rex Collie
[api] started users

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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