What are structs in Go?

Question

Accepted Answer

A **struct** is a composite type that groups related fields together — Go's way to define custom data types and model entities. Since Go has no classes, structs (plus methods) are how you do object-oriented-style programming.

## Defining and using a struct

```go
type Person struct {
    Name string     // exported (capitalized) → accessible from other packages
    Age  int
    email string    // unexported (lowercase) → package-private
}

// creating instances
p1 := Person{Name: "Ann", Age: 30}        // named fields (preferred — clear, order-independent)
p2 := Person{"Bob", 25, "bob@x.com"}       // positional (fragile — order matters)
var p3 Person                              // zero value: {"", 0, ""}

p1.Name          // access fields with dot notation
p1.Age = 31      // mutate
```

## Field visibility via capitalization

```text
Capitalized field/type/func name → EXPORTED (public, usable outside the package)
lowercase name → unexported (private to the package)
```

Go uses capitalization, not keywords, for visibility — a distinctive convention.

## Methods on structs (Go's OOP)

```go
// a method is a function with a RECEIVER (the struct it operates on)
func (p Person) Greet() string {           // value receiver
    return "Hi, I'm " + p.Name
}
func (p *Person) Birthday() {                // pointer receiver — can MODIFY the struct
    p.Age++
}

p1.Greet()       // "Hi, I'm Ann"
p1.Birthday()    // modifies p1 (Go auto-takes the address)
```

Methods attach behavior to structs via a *receiver*. Pointer receivers (`*Person`) let methods modify the struct; value receivers get a copy.

## Structs are value types

```go
p1 := Person{Name: "Ann"}
p2 := p1            // COPIES the entire struct (independent)
p2.Name = "Bob"
fmt.Println(p1.Name)   // "Ann" — unchanged (it was a copy)

// pass a pointer to share/modify
func update(p *Person) { p.Name = "X" }
```

Assigning or passing a struct **copies** it (value semantics) — to share or modify, use a pointer.

## Struct tags (metadata)

```go
type User struct {
    Name  string `json:"name"`           // tags guide encoding/decoding
    Email string `json:"email,omitempty"`
}
```

Tags attach metadata (used by JSON, DB, validation libraries) read via reflection.

## Why it matters

Structs are the core way to model data and build custom types in Go — and combined with methods, they're how Go does object-oriented-style programming (it has no classes).

Understanding struct definition, the **capitalization-based visibility** convention (unique to Go), **value semantics** (assignment copies — so use pointers to share/modify, a common gotcha), methods with value vs pointer receivers, and struct tags (for JSON/DB mapping) is fundamental to nearly all Go code.

Structs appear everywhere — domain models, configuration, API payloads, data structures — making them essential, everyday knowledge for working effectively in Go.