06 / 20 · Day 2

Day 2 · Concept 06

Structs & methods

Structs hold data. Methods attach to types. There are no classes, and yet Go's type system is more flexible than most class-based ones, because methods can attach to any named type (even type Celsius float64) and composition (struct embedding) replaces inheritance.

1 · The intuition

Go has no class keyword. A type is either a primitive (int, string), a composite (struct, map, slice), or a named type (type Foo X). Methods attach to any named type. Inheritance doesn't exist: you compose by embedding one type into another and inherit its method set.

The rule of three. A struct definition declares its fields. A constructor function (idiomatic: NewFoo(...)) creates instances. Methods are functions with a "receiver" argument that lives between func and the function name.

2 · Try it: struct + method

go main.go · the User type

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package main

import "fmt"

// Type definition — capitalised fields are exported.
type User struct {
    Name  string
    Age   int
    email string  // lowercase = package-private
}

// Constructor — idiomatic name is "NewUser" returning *User
func NewUser(name string, age int) *User {
    return &User{Name: name, Age: age}
}

// Method on User (value receiver). u is a copy.
func (u User) Greet() string {
    return "Hello, " + u.Name + "!"
}

// Method on *User (pointer receiver). u points to the original.
func (u *User) SetEmail(e string) {
    u.email = e
}

func main() {
    alice := NewUser("Alice", 30)
    fmt.Println(alice.Greet())

    alice.SetEmail("[email protected]")
    fmt.Printf("%+v
", alice)
}

3 · Value vs pointer receivers

Two ways to attach a method: func (u User) F() or func (u *User) F(). The first gets a copy; the second gets a pointer. The choice matters for mutation and for sharing.

go main.go · value vs pointer receiver

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package main

import "fmt"

type Counter struct{ n int }

// Value receiver — works on a copy
func (c Counter) IncrCopy() { c.n++ }

// Pointer receiver — mutates the original
func (c *Counter) Incr() { c.n++ }

func main() {
    c := Counter{}
    c.IncrCopy()
    c.IncrCopy()
    fmt.Println(c.n)  // 0  — the copy was mutated, not c

    c.Incr()
    c.Incr()
    fmt.Println(c.n)  // 2  — pointer receiver actually mutates
}

When to use which. Pointer receiver: the method mutates, or the struct is large (avoid copy cost). Value receiver: small immutable data. Consistency rule: within a single type, all methods should use the same receiver kind. Mixing causes subtle interface-satisfaction bugs.

4 · Embedding: composition over inheritance

Embedding lets one struct include another anonymously. The embedded type's fields and methods become "promoted": accessible directly on the outer struct.

go main.go · embedding

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package main

import "fmt"

type Animal struct {
    Name string
}

func (a Animal) Greet() string { return "Hi, I am " + a.Name }

type Dog struct {
    Animal  // embedded — anonymous field
    Breed string
}

func main() {
    d := Dog{Animal: Animal{Name: "Rex"}, Breed: "Labrador"}

    fmt.Println(d.Name)     // promoted from Animal
    fmt.Println(d.Greet())  // promoted method
    fmt.Println(d.Breed)
}

Embedding is not inheritance. Dog is not a subtype of Animal. You can't pass a Dog where an Animal is expected unless an interface mediates. But the syntactic affordance (d.Greet()) feels like inheritance and saves a lot of forwarding code.

5 · Methods on non-struct types

You can attach methods to any named type, not just structs. This is the trick behind time.Duration, http.Status, and dozens of standard library types.

go main.go · method on a named type

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package main

import "fmt"

type Celsius float64

func (c Celsius) Fahrenheit() float64 {
    return float64(c)*9/5 + 32
}

type ID int

func (i ID) String() string {
    return fmt.Sprintf("ID-%06d", i)
}

func main() {
    body := Celsius(37)
    fmt.Println(body.Fahrenheit())  // 98.6

    user := ID(42)
    fmt.Println(user)               // ID-000042  (Println calls String())
}

6 · Struct tags: the metadata layer

Tags are string literals after struct fields. They're metadata read at runtime via reflection. The standard use is JSON, XML, ORM mapping.

go main.go · struct tags for JSON

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package main

import (
    "encoding/json"
    "fmt"
)

type User struct {
    Name      string `json:"name"`
    Age       int    `json:"age,omitempty"`
    SecretKey string `json:"-"`
}

func main() {
    u := User{Name: "Alice", Age: 30, SecretKey: "shhh"}
    b, _ := json.Marshal(u)
    fmt.Println(string(b))
}

The three common tag rules. json:"name": rename the field. json:"age,omitempty": omit if zero. json:"-": never serialize. The encoding/json package reads these via reflection.

7 · From the wild

From Docker's container engine: classic struct + methods + embedding.

go docker · container/container.go (paraphrased)

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type Container struct {
    StreamConfig *stream.Config

    State          *State
    Root           string
    BaseFS         containerfs.ContainerFS
    RWLayer        layer.RWLayer
    ID             string
    Created        time.Time
    Managed        bool
    Path           string
    Args           []string
    Config         *containertypes.Config
    NetworkSettings *network.Settings
    HostConfig     *containertypes.HostConfig

    AppliedVolumesFrom map[string]struct{}
    LogPath            string
    Name               string
    Driver             string

    sync.Mutex   // <-- embedded mutex, promotes Lock()/Unlock() to Container
}

// State management methods all live on *Container — they mutate.
func (c *Container) IsRunning() bool {
    c.Lock()
    defer c.Unlock()
    return c.State.Running
}

From the wild: github.com/moby/moby · Apache 2.0

8 · Coming from another language?

If you know…	The bridge
Python	`struct` ≈ `@dataclass`. No `__init__`: use a `NewFoo` function. No inheritance: use embedding.
Java / C#	≈ a class with all-final fields and no constructor. Methods are written outside the type definition. Embedding ≠ `extends`; it's syntactic composition.
JavaScript / TS	≈ `interface` + a "class" that implements it. Tags ≈ `@serializable` decorators.
Rust	≈ `struct` + `impl`. The pointer/value receiver distinction is Rust's `&self` vs `self` vs `&mut self` simplified.
C++	≈ a POD struct with member functions. Embedding ≠ public inheritance; it's more like composition with method-name promotion.

9 · Common mistakes

Mixing value and pointer receivers on one type. Pick one and stick to it. The official advice: if any method needs *T, use *T for all.
Returning a struct by value when it has a mutex. Copying a sync.Mutex produces an unlocked copy: silent bug. go vet catches this.
Forgetting that struct equality requires comparable fields. A struct with a slice field can't be compared with ==.
Capitalisation mismatches. name won't marshal to JSON unless you tag it; Name will, even without a tag.
Methods on slice types: fine. Methods on map types: also fine. Embedding via pointer (*Animal): works, but the embedded value can be nil; check for it.

10 · Exercises (~15 min)

Build a Stack[int]. Struct with a slice field. Methods: Push, Pop, Peek, Len. Use pointer receiver for mutators, value receiver for Len.
Add JSON tags. Take your Stack from #1 and add tags so it serializes as {"items":[1,2,3]}. Test with json.Marshal.
Embed a Logger. Define type Logger struct{ prefix string } with Logf method. Embed it into Service. Call service.Logf(...) directly.
Method on a named int. type Status int. Add a String() string method that returns "active", "pending", etc. Use it with fmt.Println(Status(1)).

11 · When it clicks

You write NewX() by reflex; the constructor returns *X.
You can predict whether a method mutates from its receiver signature alone.
You reach for embedding when you'd reach for inheritance in another language.
You add String() string to your custom types so they print nicely.

Next · Day 3 / Concept 07

Control flow

One loop keyword, no parentheses, defer is the surprise. switch is a power tool.

Next concept

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