Structs
struct Point {
x int
y int
}
mut p := Point{
x: 10
y: 20
}
println(p.x) // Struct fields are accessed using a dot
// Alternative literal syntax
p = Point{10, 20}
assert p.x == 10
Heap structs
Structs are allocated on the stack. To allocate a struct on the heap
and get a reference to it, use the & prefix:
struct Point {
x int
y int
}
p := &Point{10, 10}
// References have the same syntax for accessing fields
println(p.x)
The type of p is &Point. It's a reference to Point.
References are similar to Go pointers and C++ references.
struct Foo {
mut:
x int
}
fa := Foo{1}
mut a := fa
a.x = 2
assert fa.x == 1
assert a.x == 2
// fb := Foo{ 1 }
// mut b := &fb // error: `fb` is immutable, cannot have a mutable reference to it
// b.x = 2
mut fc := Foo{1}
mut c := &fc
c.x = 2
assert fc.x == 2
assert c.x == 2
println(fc) // Foo{ x: 2 }
println(c) // &Foo{ x: 2 } // Note `&` prefixed.
see also Stack and Heap
Default field values
struct Foo {
n int // n is 0 by default
s string // s is '' by default
a []int // a is `[]int{}` by default
pos int = -1 // custom default value
}
All struct fields are zeroed by default during the creation of the struct.
Array and map fields are allocated.
In case of reference value, see here.
It's also possible to define custom default values.
Required fields
struct Foo {
n int @[required]
}
You can mark a struct field with the [required] attribute, to tell V that
that field must be initialized when creating an instance of that struct.
This example will not compile, since the field n isn't explicitly initialized:
_ = Foo{}
Short struct literal syntax
struct Point {
x int
y int
}
mut p := Point{
x: 10
y: 20
}
p = Point{
x: 30
y: 4
}
assert p.y == 4
//
// array: first element defines type of array
points := [Point{10, 20}, Point{20, 30}, Point{40, 50}]
println(points) // [Point{x: 10, y: 20}, Point{x: 20, y: 30}, Point{x: 40,y: 50}]
Omitting the struct name also works for returning a struct literal or passing one
as a function argument.
Struct update syntax
V makes it easy to return a modified version of an object:
struct User {
name string
age int
is_registered bool
}
fn register(u User) User {
return User{
...u
is_registered: true
}
}
mut user := User{
name: 'abc'
age: 23
}
user = register(user)
println(user)
Trailing struct literal arguments
V doesn't have default function arguments or named arguments, for that trailing struct
literal syntax can be used instead:
@[params]
struct ButtonConfig {
text string
is_disabled bool
width int = 70
height int = 20
}
struct Button {
text string
width int
height int
}
fn new_button(c ButtonConfig) &Button {
return &Button{
width: c.width
height: c.height
text: c.text
}
}
button := new_button(text: 'Click me', width: 100)
// the height is unset, so it's the default value
assert button.height == 20
As you can see, both the struct name and braces can be omitted, instead of:
new_button(ButtonConfig{text:'Click me', width:100})
This only works for functions that take a struct for the last argument.
[!NOTE]
Note the [params] tag is used to tell V, that the trailing struct parameter
can be omitted entirely, so that you can write button := new_button().
Without it, you have to specify at least one of the field names, even if it
has its default value, otherwise the compiler will produce this error message,
when you call the function with no parameters:
error: expected 1 arguments, but got 0.
Access modifiers
Struct fields are private and immutable by default (making structs immutable as well).
Their access modifiers can be changed with
pub and mut. In total, there are 5 possible options:
struct Foo {
a int // private immutable (default)
mut:
b int // private mutable
c int // (you can list multiple fields with the same access modifier)
pub:
d int // public immutable (readonly)
pub mut:
e int // public, but mutable only in parent module
__global:
// (not recommended to use, that's why the 'global' keyword starts with __)
f int // public and mutable both inside and outside parent module
}
Private fields are available only inside the same module, any attempt
to directly access them from another module will cause an error during compilation.
Public immutable fields are readonly everywhere.
Anonymous structs
V supports anonymous structs: structs that don't have to be declared separately
with a struct name.
struct Book {
author struct {
name string
age int
}
title string
}
book := Book{
author: struct {
name: 'Samantha Black'
age: 24
}
}
assert book.author.name == 'Samantha Black'
assert book.author.age == 24
Static type methods
V now supports static type methods like User.new(). These are defined on a struct via
fn [Type name].[function name] and allow to organize all functions related to a struct:
struct User {}
fn User.new() User {
return User{}
}
user := User.new()
This is an alternative to factory functions like fn new_user() User {} and should be used
instead.
[!NOTE]
Note, that these are not constructors, but simple functions. V doesn't have constructors or
classes.
[noinit] structs
V supports [noinit] structs, which are structs that cannot be initialised outside the module
they are defined in. They are either meant to be used internally or they can be used externally
through factory functions.
For an example, consider the following source in a directory sample:
module sample
@[noinit]
pub struct Information {
pub:
data string
}
pub fn new_information(data string) !Information {
if data.len == 0 || data.len > 100 {
return error('data must be between 1 and 100 characters')
}
return Information{
data: data
}
}
Note that new_information is a factory function. Now when we want to use this struct
outside the module:
import sample
fn main() {
// This doesn't work when the [noinit] attribute is present:
// info := sample.Information{
// data: 'Sample information.'
// }
// Use this instead:
info := sample.new_information('Sample information.')!
println(info)
}
Methods
struct User {
age int
}
fn (u User) can_register() bool {
return u.age > 16
}
user := User{
age: 10
}
println(user.can_register()) // "false"
user2 := User{
age: 20
}
println(user2.can_register()) // "true"
V doesn't have classes, but you can define methods on types.
A method is a function with a special receiver argument.
The receiver appears in its own argument list between the fn keyword and the method name.
Methods must be in the same module as the receiver type.
In this example, the can_register method has a receiver of type User named u.
The convention is not to use receiver names like self or this,
but a short, preferably one letter long, name.
Embedded structs
V supports embedded structs.
struct Size {
mut:
width int
height int
}
fn (s &Size) area() int {
return s.width * s.height
}
struct Button {
Size
title string
}
With embedding, the struct Button will automatically get all the fields and methods from
the struct Size, which allows you to do:
mut button := Button{
title: 'Click me'
height: 2
}
button.width = 3
assert button.area() == 6
assert button.Size.area() == 6
print(button)
output :
Button{
Size: Size{
width: 3
height: 2
}
title: 'Click me'
}
Unlike inheritance, you cannot type cast between structs and embedded structs
(the embedding struct can also have its own fields, and it can also embed multiple structs).
If you need to access embedded structs directly, use an explicit reference like button.Size.
Conceptually, embedded structs are similar to mixins
in OOP, NOT base classes.
You can also initialize an embedded struct:
mut button := Button{
Size: Size{
width: 3
height: 2
}
}
or assign values:
button.Size = Size{
width: 4
height: 5
}
If multiple embedded structs have methods or fields with the same name, or if methods or fields
with the same name are defined in the struct, you can call methods or assign to variables in
the embedded struct like button.Size.area().
When you do not specify the embedded struct name, the method of the outermost struct will be
targeted.