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<a class="link subtopic" href="statements-&-expressions.html#in-operator"> In operator</a>
<a class="link subtopic" href="statements-&-expressions.html#for-loop"> For loop</a>
<a class="link subtopic" href="statements-&-expressions.html#defer"> Defer</a>
<a class="link subtopic" href="statements-&-expressions.html#goto"> Goto</a>
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<a class="link active" href="structs.html">Structs</a>
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<a class="link subtopic" href="structs.html#required-fields"> Required fields</a>
<a class="link subtopic" href="structs.html#short-struct-literal-syntax"> Short struct literal syntax</a>
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<a class="link subtopic" href="structs.html#noinit-structs"> [noinit] structs</a>
<a class="link subtopic" href="structs.html#methods"> Methods</a>
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<a class="link" href="unions.html">Unions</a>
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<a class="link" href="functions-2.html">Functions 2</a>
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<a class="link" href="builtin-functions.html">Builtin functions</a>
<a class="link subtopic" href="builtin-functions.html#println"> println</a>
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<a class="link subtopic" href="type-declarations.html#custom-error-types"> Custom error types</a>
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<a class="link" href="concurrency.html">Concurrency</a>
<a class="link subtopic" href="concurrency.html#spawning-concurrent-tasks"> Spawning Concurrent Tasks</a>
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<a class="link subtopic" href="testing.html#running-tests"> Running tests</a>
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<a class="link subtopic" href="memory-management.html#control"> Control</a>
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<a class="link" href="sizeof-and-__offsetof.html">sizeof and __offsetof</a>
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<a class="link" href="limited-operator-overloading.html">Limited operator overloading</a>
<a class="link subtopic" href="limited-operator-overloading.html#implicitly-generated-overloads"> Implicitly generated overloads</a>
<a class="link subtopic" href="limited-operator-overloading.html#restriction"> Restriction</a>
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<a class="link" href="performance-tuning.html">Performance tuning</a>
<a class="link subtopic" href="performance-tuning.html#tuning-operations-details"> Tuning operations details</a>
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<a class="link" href="atomics.html">Atomics</a>
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<a class="link" href="global-variables.html">Global Variables</a>
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<a class="link" href="cross-compilation.html">Cross compilation</a>
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<a class="link" href="debugging.html">Debugging</a>
<a class="link subtopic" href="debugging.html#c-backend-binaries-(default)"> C Backend binaries (Default)</a>
<a class="link subtopic" href="debugging.html#native-backend-binaries"> Native Backend binaries</a>
<a class="link subtopic" href="debugging.html#javascript-backend"> Javascript Backend</a>
<div class="items">
<a class="link" href="v-and-c.html">V and C</a>
<a class="link subtopic" href="v-and-c.html#calling-c-from-v"> Calling C from V</a>
<a class="link subtopic" href="v-and-c.html#calling-v-from-c"> Calling V from C</a>
<a class="link subtopic" href="v-and-c.html#passing-c-compilation-flags"> Passing C compilation flags</a>
<a class="link subtopic" href="v-and-c.html#pkgconfig"> pkgconfig</a>
<a class="link subtopic" href="v-and-c.html#including-c-code"> Including C code</a>
<a class="link subtopic" href="v-and-c.html#c-types"> C types</a>
<a class="link subtopic" href="v-and-c.html#c-declarations"> C Declarations</a>
<a class="link subtopic" href="v-and-c.html#export-to-shared-library"> Export to shared library</a>
<a class="link subtopic" href="v-and-c.html#translating-c-to-v"> Translating C to V</a>
<a class="link subtopic" href="v-and-c.html#working-around-c-issues"> Working around C issues</a>
<div class="items">
<a class="link" href="other-v-features.html">Other V Features</a>
<a class="link subtopic" href="other-v-features.html#inline-assembly"> Inline assembly</a>
<a class="link subtopic" href="other-v-features.html#hot-code-reloading"> Hot code reloading</a>
<a class="link subtopic" href="other-v-features.html#cross-platform-shell-scripts-in-v"> Cross-platform shell scripts in V</a>
<a class="link subtopic" href="other-v-features.html#vsh-scripts-with-no-extension"> Vsh scripts with no extension</a>
<div class="items">
<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
<div class="items">
<a class="link" href="appendix-ii-operators.html">Appendix II: Operators</a>
</div>
</section>
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<article>
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<div class="content">
<div class="content-container">
<main>
<h1 class="main">Structs</h1>
<div class="language-v">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
</div>
<h2 id="heap-structs">Heap structs <a href="structs.html#heap-structs" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Structs are allocated on the stack. To allocate a struct on the heap
and get a <a href="references.html">reference</a> to it, use the <code>&</code> prefix:</p>
<div class="language-v">struct Point {
x int
y int
}
p := &Point{10, 10}
// References have the same syntax for accessing fields
println(p.x)
</div>
<p>The type of <code>p</code> is <code>&Point</code>. It's a <a href="references.html">reference</a> to <code>Point</code>.
References are similar to Go pointers and C++ references.</p>
<div class="language-v">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.
</div>
<p>see also <a href="memory-management.html#stack-and-heap">Stack and Heap</a></p>
<h2 id="default-field-values">Default field values <a href="structs.html#default-field-values" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">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
}
</div>
<p>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 <a href="structs-with-reference-fields.html">here</a>.</p>
<p>It's also possible to define custom default values.</p>
<h2 id="required-fields">Required fields <a href="structs.html#required-fields" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">struct Foo {
n int @[required]
}
</div>
<p>You can mark a struct field with the <code>[required]</code> <a href="attributes.html">attribute</a>, to tell V that
that field must be initialized when creating an instance of that struct.</p>
<p>This example will not compile, since the field <code>n</code> isn't explicitly initialized:</p>
<div class="language-v">_ = Foo{}
</div>
<p><a id='short-struct-initialization-syntax'></a></p>
<h2 id="short-struct-literal-syntax">Short struct literal syntax <a href="structs.html#short-struct-literal-syntax" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">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}]
</div>
<p>Omitting the struct name also works for returning a struct literal or passing one
as a function argument.</p>
<h2 id="struct-update-syntax">Struct update syntax <a href="structs.html#struct-update-syntax" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V makes it easy to return a modified version of an object:</p>
<div class="language-v">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)
</div>
<h2 id="trailing-struct-literal-arguments">Trailing struct literal arguments <a href="structs.html#trailing-struct-literal-arguments" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V doesn't have default function arguments or named arguments, for that trailing struct
literal syntax can be used instead:</p>
<div class="language-v">@[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
</div>
<p>As you can see, both the struct name and braces can be omitted, instead of:</p>
<div class="language-v">new_button(ButtonConfig{text:'Click me', width:100})
</div>
<p>This only works for functions that take a struct for the last argument.</p>
<blockquote class="tip">
<p>[!NOTE]
Note the <code>[params]</code> tag is used to tell V, that the trailing struct parameter
can be omitted <em>entirely</em>, so that you can write <code>button := new_button()</code>.
Without it, you have to specify <em>at least</em> 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:
<code>error: expected 1 arguments, but got 0</code>.</p>
</blockquote>
<h2 id="access-modifiers">Access modifiers <a href="structs.html#access-modifiers" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Struct fields are private and immutable by default (making structs immutable as well).
Their access modifiers can be changed with
<code>pub</code> and <code>mut</code>. In total, there are 5 possible options:</p>
<div class="language-v">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
}
</div>
<p>Private fields are available only inside the same <a href="modules.html">module</a>, any attempt
to directly access them from another module will cause an error during compilation.
Public immutable fields are readonly everywhere.</p>
<h2 id="anonymous-structs">Anonymous structs <a href="structs.html#anonymous-structs" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V supports anonymous structs: structs that don't have to be declared separately
with a struct name.</p>
<div class="language-v">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
</div>
<h2 id="static-type-methods">Static type methods <a href="structs.html#static-type-methods" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V now supports static type methods like <code>User.new()</code>. These are defined on a struct via
<code>fn [Type name].[function name]</code> and allow to organize all functions related to a struct:</p>
<div class="language-v">struct User {}
fn User.new() User {
return User{}
}
user := User.new()
</div>
<p>This is an alternative to factory functions like <code>fn new_user() User {}</code> and should be used
instead.</p>
<blockquote class="tip">
<p>[!NOTE]
Note, that these are not constructors, but simple functions. V doesn't have constructors or
classes.</p>
</blockquote>
<h2 id="noinit-structs"><code>[noinit]</code> structs <a href="structs.html#noinit-structs" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V supports <code>[noinit]</code> 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 <em>factory functions</em>.</p>
<p>For an example, consider the following source in a directory <code>sample</code>:</p>
<div class="language-v">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
}
}
</div>
<p>Note that <code>new_information</code> is a <em>factory</em> function. Now when we want to use this struct
outside the module:</p>
<div class="language-v">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)
}
</div>
<h2 id="methods">Methods <a href="structs.html#methods" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">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"
</div>
<p>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 <code>fn</code> keyword and the method name.
Methods must be in the same module as the receiver type.</p>
<p>In this example, the <code>can_register</code> method has a receiver of type <code>User</code> named <code>u</code>.
The convention is not to use receiver names like <code>self</code> or <code>this</code>,
but a short, preferably one letter long, name.</p>
<h2 id="embedded-structs">Embedded structs <a href="structs.html#embedded-structs" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V supports embedded structs.</p>
<div class="language-v">struct Size {
mut:
width int
height int
}
fn (s &Size) area() int {
return s.width * s.height
}
struct Button {
Size
title string
}
</div>
<p>With embedding, the struct <code>Button</code> will automatically get all the fields and methods from
the struct <code>Size</code>, which allows you to do:</p>
<div class="language-v">mut button := Button{
title: 'Click me'
height: 2
}
button.width = 3
assert button.area() == 6
assert button.Size.area() == 6
print(button)
</div>
<p>output :</p>
<pre><code>Button{
Size: Size{
width: 3
height: 2
}
title: 'Click me'
}
</code></pre>
<p>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).</p>
<p>If you need to access embedded structs directly, use an explicit reference like <code>button.Size</code>.</p>
<p>Conceptually, embedded structs are similar to <a href="https://en.wikipedia.org/wiki/Mixin">mixin</a>s
in OOP, <em>NOT</em> base classes.</p>
<p>You can also initialize an embedded struct:</p>
<div class="language-v">mut button := Button{
Size: Size{
width: 3
height: 2
}
}
</div>
<p>or assign values:</p>
<div class="language-v">button.Size = Size{
width: 4
height: 5
}
</div>
<p>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 <code>button.Size.area()</code>.
When you do not specify the embedded struct name, the method of the outermost struct will be
targeted.</p>
</main>
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