Structs
Heap structs
Structs are allocated on the stack. To allocate a struct on the heap
and get a reference to it, use the &
prefix:
The type of p
is &Point
. It's a reference to Point
.
References are similar to Go pointers and C++ references.
see also Stack and Heap
Default field values
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
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:
Short struct literal syntax
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:
Trailing struct literal arguments
V doesn't have default function arguments or named arguments, for that trailing struct literal syntax can be used instead:
As you can see, both the struct name and braces can be omitted, instead of:
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 writebutton := 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:
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.
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:
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
:
Note that new_information
is a factory function. Now when we want to use this struct
outside the module:
Methods
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.
With embedding, the struct Button
will automatically get all the fields and methods from
the struct Size
, which allows you to do:
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:
or assign values:
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.