[英]Implementing PartialEq on a boxed trait
我有一個Rust程序,它包含許多不同的結構,它們都實現了一個叫做ApplyAction
的特性。 另一個結構ActionList
包含一個實現ApplyAction
的盒裝對象向量。 我想創建一些單元測試,將ActionList
與彼此進行比較。
有幾個不同的SO問題在盒裝特征上處理PartialEq
,我已經用它們來實現一些實現。 但是,在下面的(簡化)代碼(和Playground )中, main()
的斷言失敗,因為傳遞給eq()
的對象的類型id不同。 為什么?
此外,對於這樣一個簡單的用例來說,這似乎非常復雜 - 有更簡單的方法嗎?
use std::any::TypeId;
use std::boxed::Box;
use std::fmt;
use std::mem::transmute;
#[derive(Debug, Eq, PartialEq)]
pub struct MyAction<T: fmt::Debug> {
label: T,
}
impl<T: fmt::Debug> MyAction<T> {
pub fn new(label: T) -> MyAction<T> {
MyAction { label: label }
}
}
pub trait ApplyAction<T: fmt::Debug + PartialEq>: fmt::Debug {
fn get_type(&self) -> TypeId;
fn is_eq(&self, other: &ApplyAction<T>) -> bool;
}
impl<T: fmt::Debug + Eq + 'static> ApplyAction<T> for MyAction<T> {
fn get_type(&self) -> TypeId {
TypeId::of::<MyAction<T>>()
}
fn is_eq(&self, other: &ApplyAction<T>) -> bool {
if other.get_type() == TypeId::of::<Self>() {
// Rust thinks that self and other are different types in the calls below.
let other_ = unsafe { *transmute::<&&ApplyAction<T>, &&Self>(&other) };
self.label == other_.label
} else {
false
}
}
}
impl<T: fmt::Debug + Eq + PartialEq + 'static> PartialEq for ApplyAction<T> {
fn eq(&self, other: &ApplyAction<T>) -> bool {
if other.get_type() == TypeId::of::<Self>() {
self.is_eq(other)
} else {
false
}
}
}
#[derive(Debug)]
pub struct ActionList<T: fmt::Debug> {
actions: Vec<Box<ApplyAction<T>>>,
}
impl<T: fmt::Debug + PartialEq> ActionList<T> {
pub fn new() -> ActionList<T> {
ActionList { actions: vec![] }
}
pub fn push<A: ApplyAction<T> + 'static>(&mut self, action: A) {
self.actions.push(Box::new(action));
}
}
impl<T: fmt::Debug + Eq + PartialEq + 'static> PartialEq for ActionList<T> {
fn eq(&self, other: &ActionList<T>) -> bool {
for (i, action) in self.actions.iter().enumerate() {
if **action != *other.actions[i] {
return false;
}
}
true
}
}
fn main() {
let mut script1: ActionList<String> = ActionList::new();
script1.push(MyAction::new("foo".to_string()));
let mut script2: ActionList<String> = ActionList::new();
script2.push(MyAction::new("foo".to_string()));
let mut script3: ActionList<String> = ActionList::new();
script3.push(MyAction::new("bar".to_string()));
assert_eq!(script1, script2);
assert_ne!(script1, script3);
}
在impl<...> PartialEq for ApplyAction<T>
你使用了TypeId::of::<Self>()
; 即未確定的特征對象的類型。 那不是你想要的; 但刪除if
並直接調用self.is_eq(other)
,你的代碼應該正常工作。
遺憾的是,您的示例需要大量代碼才能ApplyAction<T> for MyAction<T>
實現ApplyAction<T> for MyAction<T>
- 並且還需要您可能想要使用的其他每種操作類型。
我試圖消除這種開銷,並且夜間功能完全消失(否則只剩下一個小存根):
// see `default impl` below
#![feature(specialization)]
// Any::<T>::downcast_ref only works for special trait objects (`Any` and
// `Any + Send`); having a trait `T` derive from `Any` doesn't allow you to
// coerce ("cast") `&T` into `&Any` (that might change in the future).
//
// Implementing a custom `downcast_ref` which takes any
// `T: Any + ?Sized + 'static` as input leads to another problem: if `T` is a
// trait that didn't inherit `Any` you still can call `downcast_ref`, but it
// won't work (it will use the `TypeId` of the trait object instead of the
// underlying (sized) type).
//
// Use `SizedAny` instead: it's only implemented for sized types by default;
// that prevents the problem above, and we can implement `downcast_ref` without
// worrying.
mod sized_any {
use std::any::TypeId;
// don't allow other implementations of `SizedAny`; `SizedAny` must only be
// implemented for sized types.
mod seal {
// it must be a `pub trait`, but not be reachable - hide it in
// private mod.
pub trait Seal {}
}
pub trait SizedAny: seal::Seal + 'static {
fn get_type_id(&self) -> TypeId {
TypeId::of::<Self>()
}
}
impl<T: 'static> seal::Seal for T {}
impl<T: 'static> SizedAny for T {}
// `SizedAny + ?Sized` means it can be a trait object, but `SizedAny` was
// implemented for the underlying sized type.
pub fn downcast_ref<From, To>(v: &From) -> Option<&To>
where
From: SizedAny + ?Sized + 'static,
To: 'static,
{
if TypeId::of::<To>() == <From as SizedAny>::get_type_id(v) {
Some(unsafe { &*(v as *const From as *const To) })
} else {
None
}
}
}
use sized_any::*;
use std::boxed::Box;
use std::fmt;
// `ApplyAction`
fn foreign_eq<T, U>(a: &T, b: &U) -> bool
where
T: PartialEq + 'static,
U: SizedAny + ?Sized + 'static,
{
if let Some(b) = downcast_ref::<U, T>(b) {
a == b
} else {
false
}
}
pub trait ApplyAction<T: 'static>: fmt::Debug + SizedAny + 'static {
fn foreign_eq(&self, other: &ApplyAction<T>) -> bool;
}
// requires `#![feature(specialization)]` and a nightly compiler.
// could also copy the default implementation manually to each `impl` instead.
//
// this implementation only works with sized `A` types; we cannot make
// `ApplyAction<T>` inherit `Sized`, as that would destroy object safety.
default impl<T: 'static, A: PartialEq + 'static> ApplyAction<T> for A {
fn foreign_eq(&self, other: &ApplyAction<T>) -> bool {
foreign_eq(self, other)
}
}
impl<T: 'static> PartialEq for ApplyAction<T> {
fn eq(&self, other: &ApplyAction<T>) -> bool {
self.foreign_eq(other)
}
}
// `MyAction`
#[derive(Debug, Eq, PartialEq)]
pub struct MyAction<T: fmt::Debug> {
label: T,
}
impl<T: fmt::Debug> MyAction<T> {
pub fn new(label: T) -> MyAction<T> {
MyAction { label: label }
}
}
impl<T: fmt::Debug + PartialEq + 'static> ApplyAction<T> for MyAction<T> {}
// `ActionList`
#[derive(Debug)]
pub struct ActionList<T> {
actions: Vec<Box<ApplyAction<T>>>,
}
impl<T: 'static> ActionList<T> {
pub fn new() -> ActionList<T> {
ActionList { actions: vec![] }
}
pub fn push<A: ApplyAction<T> + 'static>(&mut self, action: A) {
self.actions.push(Box::<A>::new(action));
}
}
impl<T: 'static> PartialEq for ActionList<T> {
fn eq(&self, other: &ActionList<T>) -> bool {
if self.actions.len() != other.actions.len() {
return false;
}
for (i, action) in self.actions.iter().enumerate() {
if **action != *other.actions[i] {
return false;
}
}
true
}
}
// `main`
fn main() {
let mut script1: ActionList<String> = ActionList::new();
script1.push(MyAction::new("foo".to_string()));
let mut script2: ActionList<String> = ActionList::new();
script2.push(MyAction::new("foo".to_string()));
let mut script3: ActionList<String> = ActionList::new();
script3.push(MyAction::new("bar".to_string()));
assert_eq!(script1, script2);
assert_ne!(script1, script3);
}
也可以看看:
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