Go struct

last modified April 11, 2024

In this article we show how to work with structures in Golang.

$ go version
go version go1.22.2 linux/amd64

We use Go version 1.22.2.

The struct

A struct is a user-defined type that contains a collection of fields. It is used to group related data to form a single unit. A Go struct can be compared to a lightweight class without the inheritance feature.

Go struct definition

A struct is defined with the type keyword.

type User struct {
    name       string
    occupation string
    age        int
}

A new type is created with the type keyword. It is followed by the name of the type (User). The struct keyword indicates that we are creating a struct. Inside the curly brackets, we have a list of fields. Each field has a name and a type.

Go initialize struct

We show how to initialize struct types in Go.

u := User{"John Doe", "gardener", 34}

A new User struct is created. The struct fields are initialized with the values provided between the curly brackets. In this case, the order of the fields is relevant.

u := User{
    name:       "John Doe",
    occupation: "gardener",
    age:        34,
}

We can provide both the field names and values. In this case, the order is not important. Note that the last comma is mandatory.

u := User{}

If we omit the values in the curly brackets, they are initialized to zero values.

Go struct simple example

The following is a Go struct simple example.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    age        int
}

func main() {

    u := User{"John Doe", "gardener", 34}

    fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)
}

We define a User struct with three fields.

type User struct {
    name       string
    occupation string
    age        int
}

We declare the User struct.

u := User{"John Doe", "gardener", 34}

We initialize the User struct.

fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)

We print the contents of the User struct.

$ go run main.go
John Doe is 34 years old and he is a gardener

Go struct access fields

The struct fields are accessed with the dot operator.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    age        int
}

func main() {

    u := User{}
    u.name = "John Doe"
    u.occupation = "gardener"
    u.age = 34

    fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)
}

We create an empty User struct. We initialize the fields with values and read them using the dot operator.

Go anonymous struct

It is possible to create anonymous structs in Go. Anonymous structs do not have a name. They are created only once.

main.go

package main

import "fmt"

func main() {

    u := struct {
        name       string
        occupation string
        age        int
    }{
        name:       "John Doe",
        occupation: "gardener",
        age:        34,
    }

    fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)
}

An anonymous struct is created only with the struct keyword. The declaration of the struct is followed by its initialization.

Go nested structs

Go structs can be nested.

main.go

package main

import "fmt"

type Address struct {
    city    string
    country string
}

type User struct {
    name    string
    age     int
    address Address
}

func main() {

    p := User{
        name: "John Doe",
        age:  34,
        address: Address{
            city:    "New York",
            country: "USA",
        },
    }

    fmt.Println("Name:", p.name)
    fmt.Println("Age:", p.age)
    fmt.Println("City:", p.address.city)
    fmt.Println("Country:", p.address.country)
}

In the code example, the Address struct is nested inside the User struct.

fmt.Println("City:", p.address.city)
fmt.Println("Country:", p.address.country)

To access the fields of the nested struct, we access first the inner struct with the dot operator; then we access the respective fields.

$ go run nested.go
Name: John Doe
Age: 34
City: New York
Country: USA

Go struct promoted fields

The fields of a nested anonymous struct are promoted; that is, they are accessed without referring to the nested struct.

main.go

package main

import "fmt"

type Address struct {
    city    string
    country string
}

type User struct {
    name string
    age  int
    Address
}

func main() {
    p := User{
        name: "John Doe",
        age:  34,
        Address: Address{
            city:    "New York",
            country: "USA",
        },
    }

    fmt.Println("Name:", p.name)
    fmt.Println("Age:", p.age)
    fmt.Println("City:", p.city)
    fmt.Println("Country:", p.country)
}

In the code example, we have a nested Address struct.

type User struct {
    name string
    age  int
    Address
}

The User struct has a nested anonymous Address struct. The field does not have a name.

fmt.Println("City:", p.city)
fmt.Println("Country:", p.country)

The city and country fields are promoted. They are accessed by directly referring to the parent struct.

$ go run main.go
Name: John Doe
Age: 34
City: New York
Country: USA

Go struct functions fields

The struct fields can be functions.

main.go

package main

import "fmt"

type Info func(string, string, int) string

type User struct {
    name       string
    occupation string
    age        int
    info       Info
}

func main() {

    u := User{
        name:       "John Doe",
        occupation: "gardener",
        age:        34,
        info: func(name string, occupation string, age int) string {

            return fmt.Sprintf("%s is %d years old and he is a %s\n", name, age, occupation)
        },
    }

    fmt.Printf(u.info(u.name, u.occupation, u.age))
}

In the code example, we have the User struct. Its info field is a function called Info.

Go struct pointer

A pointer to the struct can be created with the & operator or the new keyword. A pointer is dereferenced with the * operator.

main.go

package main

import "fmt"

type Point struct {
    x int
    y int
}

func main() {

    p := Point{3, 4}

    p_p := &p

    (*p_p).x = 1
    p_p.y = 2

    fmt.Println(p)
}

In the code example, we create a pointer to the Point struct.

p_p := &p

The & operator returns a pointer to the Point structure.

(*p_p).x = 1
p_p.y = 2

A pointer is dereferenced with the * operator. Go also allows to use the dot operator directly.

Alternatively, we can create a pointer to a struct with the new keyword.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    age        int
}

func main() {

    u := new(User)
    u.name = "Richard Roe"
    u.occupation = "driver"
    u.age = 44

    fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)
}

The example creates a new pointer to the User struct with the new keyword.

Go struct constructor

There are no built-in constructors in Go. Programmers sometimes create constructor functions as a best practice.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    age        int
}

func newUser(name string, occupation string, age int) *User {

    p := User{name, occupation, age}
    return &p
}

func main() {

    u := newUser("Richard Roe", "driver", 44)

    fmt.Printf("%s is %d years old and he is a %s\n", u.name, u.age, u.occupation)
}

In the code example, we have the newUser constructor function, which creates new User structs. The function returns a pointer to the newly created struct.

Go struct is a value type

Go structs are value types. When we assign a struct variable to another struct variable, a new copy of the struct is created. Likewise, when we pass a struct to another function, the function receives a new copy of the struct.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    age        int
}

func main() {

    u1 := User{"John Doe", "gardener", 34}

    u2 := u1

    u2.name = "Richard Roe"
    u2.occupation = "driver"
    u2.age = 44

    fmt.Printf("%s is %d years old and he is a %s\n", u1.name, u1.age, u1.occupation)
    fmt.Printf("%s is %d years old and he is a %s\n", u2.name, u2.age, u2.occupation)
}

In the code example, we assign a struct to another struct. Changing the fields of the new struct does not affect the original struct.

$ go run main.go
John Doe is 34 years old and he is a gardener
Richard Roe is 44 years old and he is a driver

The two structs are distinct entities.

Comparing Go structs

Go structs are equal if all their corresponding fields are equal.

main.go

package main

import "fmt"

type Point struct {
    x int
    y int
}

func main() {

    p1 := Point{3, 4}
    p2 := Point{3, 4}

    if p1 == p2 {

        fmt.Println("The structs are equal")
    } else {

        fmt.Println("The structs are not equal")
    }
}

In the code example, we compare two Point structs.

$ go run main.go
The structs are equal

Go export struct

Named structs that start with a capital letter are exported and accessible from outside their packages. Similarly, struct fields that start with a capital letter are exported. Struct names and fields starting with a small letter are visible only inside their package.

$ go mod init exporting

We create a new Go module with the go mod init command.

go.mod
main
└── main.go
model
├── address.go
└── user.go

This is the project structure.

model/user.go

package model

type User struct {
    Name       string
    Occupation string
    age        int
}

The Name and Occupation fields of the User struct are exported, while the age field is not.

model/address.go

package model

type address struct {
    city    string
    country string
}

The address struct is not exported. We cannot refer to it in the main.go file.

main/main.go

package main

import (
    "exporting/model"
    "fmt"
)

func main() {

    u := model.User{Name: "John Doe", Occupation: "gardener"}

    fmt.Printf("%s is a %s\n", u.Name, u.Occupation)
}

In the main.go file, we import the Name and Occupation fields from the exporting/model package.

Go create a slice of structs

In the following example, we create a slice of structs.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    country    string
}

func main() {

    users := []User{}
    users = append(users, User{"John Doe", "gardener", "USA"})
    users = append(users, User{"Roger Roe", "driver", "UK"})
    users = append(users, User{"Paul Smith", "programmer", "Canada"})
    users = append(users, User{"Lucia Mala", "teacher", "Slovakia"})
    users = append(users, User{"Patrick Connor", "shopkeeper", "USA"})
    users = append(users, User{"Tim Welson", "programmer", "Canada"})
    users = append(users, User{"Tomas Smutny", "programmer", "Slovakia"})

    for _, user := range users {

        fmt.Println(user)
    }
}

A User type is defined. Then we create an empty slice of User structs. We add elements to the slice with append.

Go filter slice of structs

In the next example, we filter a slice of Go structures.

main.go

package main

import "fmt"

type User struct {
    name       string
    occupation string
    country    string
}

func main() {

    users := []User{

        {"John Doe", "gardener", "USA"},
        {"Roger Roe", "driver", "UK"},
        {"Paul Smith", "programmer", "Canada"},
        {"Lucia Mala", "teacher", "Slovakia"},
        {"Patrick Connor", "shopkeeper", "USA"},
        {"Tim Welson", "programmer", "Canada"},
        {"Tomas Smutny", "programmer", "Slovakia"},
    }

    var programmers []User

    for _, user := range users {

        if isProgrammer(user) {
            programmers = append(programmers, user)
        }
    }

    fmt.Println("Programmers:")
    for _, u := range programmers {

        fmt.Println(u)
    }
}

func isProgrammer(user User) bool {

    return user.occupation == "programmer"
}

In the code example, we define a slice of users. We create a new slice that contains only programmers.

type User struct {
    name       string
    occupation string
    country    string
}

The User struct has three fields.

users := []User{

    {"John Doe", "gardener", "USA"},
    {"Roger Roe", "driver", "UK"},
    {"Paul Smith", "programmer", "Canada"},
    {"Lucia Mala", "teacher", "Slovakia"},
    {"Patrick Connor", "shopkeeper", "USA"},
    {"Tim Welson", "programmer", "Canada"},
    {"Tomas Smutny", "programmer", "Slovakia"},
}

This is the original slice of User structures.

var programmers []User

The filtered users/programmers are stored in the programmers slice.

for _, user := range users {

    if isProgrammer(user) {
        programmers = append(programmers, user)
    }
}

We go over the users slice and add a current user to the programmers slice only if the user satisfies the isProgrammer predicate.

func isProgrammer(user User) bool {

    return user.occupation == "programmer"
}

The IsProgrammer predicate returns true for all users whose occupation field equals to "programmer".

$ go run main.go
Programmers:
{Paul Smith programmer Canada}
{Tim Welson programmer Canada}
{Tomas Smutny programmer Slovakia}

Source

The Go Programming Language Specification

In this article we have covered structs in Golang.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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Go struct

The struct

Go struct definition

Go initialize struct

Go struct simple example

Go struct access fields

Go anonymous struct

Go nested structs

Go struct promoted fields

Go struct functions fields

Go struct pointer

Go struct constructor

Go struct is a value type

Comparing Go structs

Go export struct

Go create a slice of structs

Go filter slice of structs

Source

Author