是否可以在映射中插入一个结构,其中键由插入的值所拥有?
在C中使用哈希映射时,这是我以前做过的事情。
伪代码示例:
struct MyStruct {
pub map: BTreeMap<&String, StructThatContainsString>,
// XXX ^ Rust wants lifetime specified here!
}
struct StructThatContainsString {
id: String,
other_data: u32,
}
fn my_fn() {
let ms = MyStruct { map: BTreeMap::new() };
let item = StructThatContainsString {
id: "Some Key".to_string(),
other_data: 0,
}
ms.insert(&item.id, item);
}
如何正确处理这种情况?
String
?set
而不是map
,然后将整个struct
存储为关键,但在比较时只使用其中一个值(看起来它可以工作,但如果你想比较其他的struct
可能适得其反上下文)。它不适用于普通引用:
let item = StructThatContainsString {
id: "Some Key".to_string(),
other_data: 0,
}
ms.insert(&item.id, item);
item
被移入地图,因此不能有任何待处理的借用/引用。
此外,像get_mut()
这样的方法会变得危险或不可能,因为它会让你修改具有杰出参考的项目。
假设想要这样做的原因是为了节省空间,显而易见的选择是:
struct OnlyKey {
id: String,
}
struct OnlyValue {
other_data: u32,
}
这可以通过适当的方法清理,以分开/重新组合各个部件。Rc
这样的东西作为价值的关键部分。如果Rc<T>
,Ord
实施BTreeMap
(T
所需)。
struct StructThatContainsString {
id: Rc<String>,
other_data: u32,
}
使用结构的单个成员作为映射中的键可以通过使用具有零开销包装器结构的集合(原则上)来完成,该结构仅用于覆盖实现。
Ord, Eq, PartialEq, PartialOrd
控制它在集合中的顺序。Borrow
所以BTreeSet.get(..)
可以采用用于排序的类型,而不是整个结构。这是一个工作示例:
use ::std::collections::BTreeSet;
#[derive(Debug)]
pub struct MyItem {
id: String,
num: i64,
}
mod my_item_ord {
use super::MyItem;
#[derive(Debug)]
pub struct MyItem_Ord(pub MyItem);
use ::std::cmp::{
PartialEq,
Eq,
Ord,
Ordering,
};
use ::std::borrow::Borrow;
impl PartialEq for MyItem_Ord {
fn eq(&self, other: &Self) -> bool {
return self.0.id.eq(&other.0.id);
}
}
impl PartialOrd for MyItem_Ord {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
return self.0.id.partial_cmp(&other.0.id);
}
}
impl Eq for MyItem_Ord {}
impl Ord for MyItem_Ord {
fn cmp(&self, other: &Self) -> Ordering {
return self.0.id.cmp(&other.0.id);
}
}
impl Borrow<str> for MyItem_Ord {
fn borrow(&self) -> &str {
return &self.0.id;
}
}
}
fn main() {
use my_item_ord::MyItem_Ord;
let mut c: BTreeSet<MyItem_Ord> = BTreeSet::new();
c.insert(MyItem_Ord(MyItem { id: "Zombie".to_string(), num: 21, }));
c.insert(MyItem_Ord(MyItem { id: "Hello".to_string(), num: 1, }));
c.insert(MyItem_Ord(MyItem { id: "World".to_string(), num: 22, }));
c.insert(MyItem_Ord(MyItem { id: "The".to_string(), num: 11, }));
c.insert(MyItem_Ord(MyItem { id: "Brown".to_string(), num: 33, }));
c.insert(MyItem_Ord(MyItem { id: "Fox".to_string(), num: 99, }));
for i in &c {
println!("{:?}", i);
}
// Typical '.get()', too verbose needs an entire struct.
println!("lookup: {:?}", c.get(&MyItem_Ord(MyItem { id: "Zombie".to_string(), num: -1, })));
// ^^^^^^^ ignored
// Fancy '.get()' using only string, allowed because 'Borrow<str>' is implemented.
println!("lookup: {:?}", c.get("Zombie"));
println!("done!");
}
为了避免必须手动定义这些,可以将其包装到宏中:
///
/// Macro to create a container type to be used in a 'BTreeSet' or ordered types
/// to behave like a map where a key in the struct is used for the key.
///
/// For example, data in a set may have a unique identifier which
/// can be used in the struct as well as a key for it's use in the set.
///
///
/// ```
/// // Defines 'MyTypeOrd', a container type for existing struct,
/// // using MyType.uuid is used as the key.
/// container_order_by_member_impl(MyTypeOrd, MyType, uuid);
/// ```
///
/// See: http://stackoverflow.com/questions/41035869
#[macro_export]
macro_rules! container_type_order_by_member_struct_impl {
($t_ord:ident, $t_base:ty, $t_member:ident) => {
/// Caller must define the struct, see: container_type_order_by_member_impl
// pub struct $t_ord(pub $t_base);
impl PartialEq for $t_ord {
fn eq(&self, other: &Self) -> bool {
return (self.0).$t_member.eq(&(other.0).$t_member);
}
}
impl PartialOrd for $t_ord {
fn partial_cmp(&self, other: &Self) -> Option<::std::cmp::Ordering> {
return (self.0).$t_member.partial_cmp(&(other.0).$t_member);
}
}
impl Eq for $t_ord {}
impl Ord for $t_ord {
fn cmp(&self, other: &Self) -> ::std::cmp::Ordering {
return (self.0).$t_member.cmp(&(other.0).$t_member);
}
}
impl ::std::borrow::Borrow<str> for $t_ord {
fn borrow(&self) -> &str {
return &(self.0).$t_member;
}
}
}
}
/// Macro that also defines structs.
#[macro_export]
macro_rules! container_type_order_by_member_impl {
(pub $t_ord:ident, $t_base:ty, $t_member:ident) => {
pub struct $t_ord(pub $t_base);
container_type_order_by_member_struct_impl!($t_ord, $t_base, $t_member);
};
($t_ord:ident, $t_base:ty, $t_member:ident) => {
struct $t_ord(pub $t_base);
container_type_order_by_member_struct_impl!($t_ord, $t_base, $t_member);
};
}