[rust-011]Rust的Iterator和生命周期

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主要为了解释这段代码：

impl Condition {/// Returns an iterator over only the variable tests, along with/// their indices./// 这个语法的解释可以参考这里 https://blog.katona.me/2019/12/29/Rust-Lifetimes-and-Iterators/fn variables(&self) -> impl Iterator<Item = (usize, VariableID)> + '_ {self.0.iter().enumerate().filter_map(|(i, test)| match test {ConditionTest::Variable(id) => Some((i, *id)),ConditionTest::Constant(_) => None,})}
}

这里有三个问题：

• Iterator
• -> impl
• + '_

1.Iterator

参考文档 https://doc.rust-lang.org/book/ch13-02-iterators.html

最简单的用法

fn main(){let v1 = vec![1, 2, 3];//常规操作for val in v1.iter() {println!("Got: {}", val);}//求和，注意需要有::<i32>，否则会报错println!("sum = {}", v1.iter().sum::<i32>());//这里有map，由于iter是懒惰求值，必须有collect()才能生成有值的v2let v2: Vec<_> = v1.iter().map(|x| x + 1).collect();println!("v2 = {:#?}", v2)
}

实现Iterator trait:

struct Counter {count: u32,
}impl Counter {fn new() -> Counter {Counter { count: 0 }}
}impl Iterator for Counter {type Item = u32;fn next(&mut self) -> Option<Self::Item> {if self.count < 5 {self.count += 1;//Some是Option<T>枚举Some(self.count)} else {//None是Option<T>枚举None}}
}fn main() {let mut counter = Counter::new();assert_eq!(counter.next(), Some(1));assert_eq!(counter.next(), Some(2));assert_eq!(counter.next(), Some(3));assert_eq!(counter.next(), Some(4));assert_eq!(counter.next(), Some(5));assert_eq!(counter.next(), None);
}

2.在返回值实现Iterator trait

参考文献：https://doc.rust-lang.org/nightly/edition-guide/rust-2018/trait-system/impl-trait-for-returning-complex-types-with-ease.html

trait Trait {}impl Trait for i32 {}fn returns_a_trait_object() -> impl Trait {5
}fn main(){let x = returns_a_trait_object();
}

和

//第一种情况，返回值实现了Iterator特质，其中，Iterator的Item是str
fn to_words<'a>(text: &'a str) -> impl Iterator<Item = &'a str> {//实际上这是split返回的Iteratortext.split(' ')
}#[derive(Debug)]
struct AStruct{x:i32,y:i32,
}impl AStruct{fn new() -> Self{Self{x:1,y:2}}fn get_iter_str<'a>(&self, text:&'a str) -> impl Iterator<Item = &'a str> {text.split(' ')}}fn main() {let text = "word1 word2 word3";for i in to_words(text){println!("{}", i);}println!("{}", to_words("abc def ghi jkl").take(2).count());let ast = AStruct::new();println!("ast count = {:?}", ast.get_iter_str("sdff").count());
}

3.关于+'_

参考文献

• https://blog.katona.me/2019/12/29/Rust-Lifetimes-and-Iterators/ 完整的例子介绍
• https://doc.rust-lang.org/edition-guide/rust-2018/trait-system/impl-trait-for-returning-complex-types-with-ease.html#return-position 返回值实现trait
• https://doc.rust-lang.org/nightly/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html 匿名lifetime '_

fn variables(&self) -> impl Iterator<Item = (usize, VariableID)> + '_ {self.0.iter().enumerate().filter_map(|(i, test)| match test {ConditionTest::Variable(id) => Some((i, *id)),ConditionTest::Constant(_) => None,})}

函数variables是Condition的成员函数。函数的返回值必须实现了Iterator特质，Iterator的type是tuple (usize, VariableID)，后面的"+'_"表示，返回值的生命周期是Condition对象的生命周期。

 

 

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