wiki/content/20200828181259-structs.md

---
date: 20200828
id: 463ce364-f426-4089-a0ca-ffe45f0461f2
title: Golang Structs
---

# Basics

Golang structs are what you expect

``` go
package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

func main() {
    fmt.Println(Vertex{1, 2})
}
```

# Struct fields

Struct fields are accessed using a dot

``` go
package main

import "fmt"

type Coorindate struct {
    X int
    Y int
}

func main() {
    coordinate := Coorindate{1, 67}
    coordinate.X = 26
    fmt.Println(coordinate.X)
}
```

# Struct literals

You can also newly allocate a struct by listing the values of its
fields. This is known as a "struct literal" (duh)

``` go
package main

import "fmt"

type Vertex struct {
    X, Y int
}

var (
    v1 = Vertex{1, 2}  // has type Vertex
    v2 = Vertex{X: 1}  // Y:0 is implicit
    v3 = Vertex{}      // X:0 and Y:0
    p  = &Vertex{1, 2} // has type *Vertex
)

func main() {
    fmt.Println(v1, p, v2, v3)
}
```

# Struct tags

[Struct tags](https://go.dev/wiki/Well-known-struct-tags) allow other
modules accessing struct members to format member values.

## Control encoding

``` go
package main

import (
    "encoding/json"
    "fmt"
    "log"
    "os"
    "time"
)

type User struct {
    Name          string    `json:"name"`
    Password      string    `json:"password"`
    PreferredFish []string  `json:"preferredFish"`
    CreatedAt     time.Time `json:"createdAt"`
}

func main() {
    u := &User{
        Name:      "Sammy the Shark",
        Password:  "fisharegreat",
        CreatedAt: time.Now(),
    }

    out, err := json.MarshalIndent(u, "", "  ")
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }

    fmt.Println(string(out))
}
```

## Remove empty fields

``` go
package main

import (
    "encoding/json"
    "fmt"
    "log"
    "os"
    "time"
)

type User struct {
    Name          string    `json:"name"`
    Password      string    `json:"password"`
    PreferredFish []string  `json:"preferredFish,omitempty"`
    CreatedAt     time.Time `json:"createdAt"`
}

func main() {
    u := &User{
        Name:      "Sammy the Shark",
        Password:  "fisharegreat",
        CreatedAt: time.Now(),
    }

    out, err := json.MarshalIndent(u, "", "  ")
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }

    fmt.Println(string(out))
}
```

## Ignore private fields

``` go
package main

import (
    "encoding/json"
    "fmt"
    "log"
    "os"
    "time"
)

type User struct {
    Name      string    `json:"name"`
    Password  string    `json:"-"`
    CreatedAt time.Time `json:"createdAt"`
}

func main() {
    u := &User{
        Name:      "Sammy the Shark",
        Password:  "fisharegreat",
        CreatedAt: time.Now(),
    }

    out, err := json.MarshalIndent(u, "", "  ")
    if err != nil {
        log.Println(err)
        os.Exit(1)
    }

    fmt.Println(string(out))
}
```