Structs and Methods

Go is not an object-oriented language in the traditional sense — it has no classes, no inheritance, and no constructors. Instead, Go uses structs to group data and methods to add behaviour to types. This approach is simpler than class-based OOP while remaining powerful and flexible.

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Defining a Struct

A struct is a composite type that groups named fields:

type User struct {
    ID        int
    Name      string
    Email     string
    Active    bool
    CreatedAt time.Time
}

Creating Struct Instances

// Named fields (recommended)
u := User{
    ID:    1,
    Name:  "Alice",
    Email: "alice@example.com",
    Active: true,
}

// Positional (fragile — not recommended)
u := User{1, "Alice", "alice@example.com", true, time.Now()}

// Zero-value struct
var u User   // all fields set to zero values

Accessing Fields

fmt.Println(u.Name)     // Alice
u.Active = false         // modify a field

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Struct Tags

Struct tags are metadata attached to fields, commonly used for JSON encoding, database mapping, and validation:

type User struct {
    ID    int    `json:"id" db:"user_id"`
    Name  string `json:"name" db:"user_name"`
    Email string `json:"email,omitempty"`
}

Tag	Purpose
json:"name"	JSON field name
json:",omitempty"	Omit field if zero value
json:"-"	Exclude from JSON
db:"column_name"	Database column name
validate:"required"	Validation rules

JSON Encoding Example

u := User{ID: 1, Name: "Alice"}
data, _ := json.Marshal(u)
fmt.Println(string(data))
// {"id":1,"name":"Alice"}

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Methods

A method is a function with a receiver — it belongs to a specific type:

func (u User) FullName() string {
    return u.Name
}

// Usage:
u := User{Name: "Alice"}
fmt.Println(u.FullName())   // Alice

Value Receivers vs Pointer Receivers

Receiver	Syntax	Can Modify?	Copy?
Value	func (u User)	No	Works on a copy
Pointer	func (u *User)	Yes	Works on the original

// Value receiver — cannot modify u
func (u User) Greeting() string {
    return "Hello, " + u.Name
}

// Pointer receiver — can modify u
func (u *User) Deactivate() {
    u.Active = false
}

When to Use Pointer Receivers