我在Rust中创建了一个数据 struct ，我想为它创建迭代器.不可变迭代器非常简单.我现在有这个，它很好用:
// This is a mock of the "real" EdgeIndexes class as
// the one in my real program is somewhat complex, but
// of identical type
struct EdgeIndexes;
impl Iterator for EdgeIndexes {
type Item = usize;
fn next(&mut self) -> Option<Self::Item> {
Some(0)
}
fn size_hint(&self) -> (usize, Option<usize>) {
(0, None)
}
}
pub struct CGraph<E> {
nodes: usize,
edges: Vec<E>,
}
pub struct Edges<'a, E: 'a> {
index: EdgeIndexes,
graph: &'a CGraph<E>,
}
impl<'a, E> Iterator for Edges<'a, E> {
type Item = &'a E;
fn next(&mut self) -> Option<Self::Item> {
match self.index.next() {
None => None,
Some(x) => Some(&self.graph.edges[x]),
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.index.size_hint()
}
}
我想创建一个迭代器，它还返回可变引用.我try 过这样做，但找不到编译的方法:
pub struct MutEdges<'a, E: 'a> {
index: EdgeIndexes,
graph: &'a mut CGraph<E>,
}
impl<'a, E> Iterator for MutEdges<'a, E> {
type Item = &'a mut E;
fn next(&mut self) -> Option<&'a mut E> {
match self.index.next() {
None => None,
Some(x) => self.graph.edges.get_mut(x),
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.index.size_hint()
}
}
编译此文件会导致以下错误:
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
--> src/lib.rs:54:24
|
54 | Some(x) => self.graph.edges.get_mut(x),
| ^^^^^^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 51:5...
--> src/lib.rs:51:5
|
51 | / fn next(&mut self) -> Option<&'a mut E> {
52 | | match self.index.next() {
53 | | None => None,
54 | | Some(x) => self.graph.edges.get_mut(x),
55 | | }
56 | | }
| |_____^
note: ...so that reference does not outlive borrowed content
--> src/lib.rs:54:24
|
54 | Some(x) => self.graph.edges.get_mut(x),
| ^^^^^^^^^^^^^^^^
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 48:6...
--> src/lib.rs:48:6
|
48 | impl<'a, E> Iterator for MutEdges<'a, E> {
| ^^
= note: ...so that the expression is assignable:
expected std::option::Option<&'a mut E>
found std::option::Option<&mut E>
我不确定如何解释这些错误，以及如何更改代码以允许MutEdges个返回可变引用.