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<!DOCTYPE html>
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<div class="nav-entry is-group-header">
<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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</div>
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<div class="nav-entry is-group-header">
<a class="nav-entry-link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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</div>
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<div class="nav-entry is-group-header">
<a class="nav-entry-link" href="other-online-resources.html">Other online resources</a>
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<div class="nav-entry">
<a class="nav-entry-link" href="other-online-resources.html#v-contributing-guide"> V contributing guide</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-language-documentation"> V language documentation</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-online-playground"> V online playground</a>
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<a class="nav-entry-link" href="other-online-resources.html#awesome-v"> Awesome V</a>
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<a class="nav-entry-link" href="other-online-resources.html#the-v-language-discord"> The V language Discord</a>
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<article>
<div class="container">
<div class="content">
<div class="content-container">
<main class="v-doc">
<h1 class="main">Attributes</h1>
<p>V has several attributes that modify the behavior of functions and structs.</p>
<p>An attribute is a compiler instruction specified inside <code>[]</code> right before a
function/struct/enum declaration and applies only to the following declaration.</p>
<div class="language-v">// @[flag] enables Enum types to be used as bitfields
@[flag]
enum BitField {
read
write
other
}
fn main() {
assert 1 == int(BitField.read)
assert 2 == int(BitField.write)
mut bf := BitField.read
assert bf.has(.read | .other) // test if *at least one* of the flags is set
assert !bf.all(.read | .other) // test if *all* of the flags are set
bf.set(.write | .other)
assert bf.has(.read | .write | .other)
assert bf.all(.read | .write | .other)
bf.toggle(.other)
assert bf == BitField.read | .write
assert bf.all(.read | .write)
assert !bf.has(.other)
empty := BitField.zero()
assert empty.is_empty()
assert !empty.has(.read)
assert !empty.has(.write)
assert !empty.has(.other)
mut full := empty
full.set_all()
assert int(full) == 7 // 0x01 + 0x02 + 0x04
assert full == .read | .write | .other
mut v := full
v.clear(.read | .other)
assert v == .write
v.clear_all()
assert v == empty
assert BitField.read == BitField.from('read')!
assert BitField.other == BitField.from('other')!
assert BitField.write == BitField.from(2)!
assert BitField.zero() == BitField.from('')!
}
</div>
<div class="language-v">// @[_allow_multiple_values] allows an enum to have multiple duplicate values.
// Use it carefully, only when you really need it.
@[_allow_multiple_values]
enum ButtonStyle {
primary = 1
secondary = 2
success = 3
blurple = 1
grey = 2
gray = 2
green = 3
}
fn main() {
assert int(ButtonStyle.primary) == 1
assert int(ButtonStyle.blurple) == 1
assert int(ButtonStyle.secondary) == 2
assert int(ButtonStyle.gray) == 2
assert int(ButtonStyle.grey) == 2
assert int(ButtonStyle.success) == 3
assert int(ButtonStyle.green) == 3
assert ButtonStyle.primary == ButtonStyle.blurple
assert ButtonStyle.secondary == ButtonStyle.grey
assert ButtonStyle.secondary == ButtonStyle.gray
assert ButtonStyle.success == ButtonStyle.green
}
</div>
<p>Struct field deprecations:</p>
<div class="language-v">module abc
// Note that only *direct* accesses to Xyz.d in *other modules*, will produce deprecation notices/warnings:
pub struct Xyz {
pub mut:
a int
d int @[deprecated: 'use Xyz.a instead'; deprecated_after: '2999-03-01']
// the tags above, will produce a notice, since the deprecation date is in the far future
}
</div>
<p>Function/method deprecations:</p>
<p>Functions are deprecated before they are finally removed to give users time to migrate their code.
Adding a date is preferable in most cases. An immediate change, without a deprecation date, may be
used for functions that are found to be conceptually broken and obsoleted by much better
functionality. Other than that setting a date is advisable to grant users a grace period.</p>
<p>Deprecated functions cause warnings, which cause errors if built with <code>-prod</code>. To avoid immediate
CI breakage, it is advisable to set a future date, ahead of the date when the code is merged. This
gives people who actively developed V projects, the chance to see the deprecation notice at least
once and fix the uses. Setting a date in the next 30 days, assumes they would have compiled their
projects manually at least once, within that time. For small changes, this should be plenty
of time. For complex changes, this time may need to be longer.</p>
<p>Different V projects and maintainers may reasonably choose different deprecation policies.
Depending on the type and impact of the change, you may want to consult with them first, before
deprecating a function.</p>
<div class="language-v">// Calling this function will result in a deprecation warning
@[deprecated]
fn old_function() {
}
// It can also display a custom deprecation message
@[deprecated: 'use new_function() instead']
fn legacy_function() {}
// You can also specify a date, after which the function will be
// considered deprecated. Before that date, calls to the function
// will be compiler notices - you will see them, but the compilation
// is not affected. After that date, calls will become warnings,
// so ordinary compiling will still work, but compiling with -prod
// will not (all warnings are treated like errors with -prod).
// 6 months after the deprecation date, calls will be hard
// compiler errors.
@[deprecated: 'use new_function2() instead']
@[deprecated_after: '2021-05-27']
fn legacy_function2() {}
</div>
<div class="language-v">// This function's calls will be inlined.
@[inline]
fn inlined_function() {
}
// This function's calls will NOT be inlined.
@[noinline]
fn function() {
}
// This function will NOT return to its callers.
// Such functions can be used at the end of or blocks,
// just like exit/1 or panic/1. Such functions can not
// have return types, and should end either in for{}, or
// by calling other `[noreturn]` functions.
@[noreturn]
fn forever() {
for {}
}
// The following struct must be allocated on the heap. Therefore, it can only be used as a
// reference (`&Window`) or inside another reference (`&OuterStruct{ Window{...} }`).
// See section "Stack and Heap"
@[heap]
struct Window {
}
// Calls to following function must be in unsafe{} blocks.
// Note that the code in the body of `risky_business()` will still be
// checked, unless you also wrap it in `unsafe {}` blocks.
// This is useful, when you want to have an `[unsafe]` function that
// has checks before/after a certain unsafe operation, that will still
// benefit from V's safety features.
@[unsafe]
fn risky_business() {
// code that will be checked, perhaps checking pre conditions
unsafe {
// code that *will not be* checked, like pointer arithmetic,
// accessing union fields, calling other `[unsafe]` fns, etc...
// Usually, it is a good idea to try minimizing code wrapped
// in unsafe{} as much as possible.
// See also [Memory-unsafe code](#memory-unsafe-code)
}
// code that will be checked, perhaps checking post conditions and/or
// keeping invariants
}