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<!DOCTYPE html>
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<h1 class="main">Attributes</h1>
<p>V has several attributes that modify the behavior of functions and structs.</p>
<p><code>@[noalloc]</code> checks that a function's reachable code does not allocate, with an exception for
growing an existing mutable array parameter. <code>@[noalloc: strict]</code> also forbids that growth.
See <a href="https://github.com/vlang/v/blob/master/doc/noalloc.md">Allocation contracts</a> for conservative checks and foreign function contracts.</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.
Attributes with arguments support both <code>name: value</code> and call-style <code>name(value)</code> syntax.
Call-style attributes can also use named arguments.</p>
<p>The C backend emits <code>@[_constructor]</code> and <code>@[_destructor]</code> as GNU function attributes before the
function declarator, so definitions compile with GCC and Clang. When supported by the C compiler,
these hooks run before <code>main</code> and after it returns. They must not depend on V's runtime being
initialized. MSVC ignores these attributes.</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