标准库Option源码学习
基础结构
#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
#[rustc_diagnostic_item = "Option"]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum Option<T> {
/// No value
#[lang = "None"]
#[stable(feature = "rust1", since = "1.0.0")]
None,
/// Some value `T`
#[lang = "Some"]
#[stable(feature = "rust1", since = "1.0.0")]
Some(#[stable(feature = "rust1", since = "1.0.0")] T),
}
学习点
#[lang = "..."]属性表示这是语言项- 泛型参数T支持任何类型
- 自动派生多个trait(
Copy，PartialEq等)
2. 核心方法:map和map_or
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
match self {
Some(x) => Some(f(x)),
None => None,
}
}
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
match self {
Some(t) => f(t),
None => default,
}
}
学习点
- map保留Some/None结构
- map_or提供默认值
- 使用FnOnce因为闭包最多被调用一次
3.解引用实现
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Deref> Deref for Option<T> {
type Target = Option<T::Target>;
fn deref(&self) -> &Self::Target {
match self {
Some(t) => Some(t.deref()),
None => &None,
}
}
}
学习点
- 为
Option<Box<T>>等类型提供自动解引用 - 实现
Deref而不是直接实现方法，保持一致性
4. and_then和or_else
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
match self {
Some(x) => f(x),
None => None,
}
}
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
match self {
Some(x) => Some(x),
None => f(),
}
}
学习点
and_then用于链式操作or_else提供回退逻辑- 闭包
F只在需要时调用
5. transpose方法
#[inline]
#[stable(feature = "transpose_result", since = "1.33.0")]
pub fn transpose(self) -> Result<Option<T, E>>
{
match self {
Some(Ok(ok)) => Ok(Some(ok)),
Some(Err(err)) => Err(err),
None => Ok(None),
}
}
学习点
- 在
Option和Result之间转换 - 保持错误传播语义
6. zip和zip_with
#[stable(feature = "option_zip_option", since = "1.46.0")]
pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
match (self, other) {
(Some(a), Some(b)) => Some((a, b)),
_ => None,
}
}
#[unstable(feature = "option_zip", issue = "70086")]
pub fn zip_with<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
match self {
Some(x) => f(x),
None => None,
}
}
学习点
zip组合两个Option值zip_with提供更灵活的转换- 使用元组模式匹配处理组合逻辑
7. 性能优化：Option<&T>
// 编译器对 Option<&T> 有特殊优化
// size_of::<Option<&T>>() == size_of::<&T>()
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Option<&T> {
#[inline]
pub fn copied(self) -> Option<T>
where
T: Copy,
{
self.map(|&t| t)
}
#[inline]
pub fn cloned(self) -> Option<T>
where
T: Clone,
{
self.map(|t| t.clone())
}
}
学习点
- 零成本抽象
- 为引用类型提供特化实现
Copy和Clone的区别处理