The proper ownership for "caching proxy" in Rust? - rust

Question

I'd like to use Factory to build an object from the String and have multiple impls: 1) actual building and 2) caching (stores in-memory in HashMap). The problem is that in case #1 it have to pass the ownership and in case #2 HashMap owns the value and a reference can be returned only.
use std::collections::HashMap;
// product interface
pub trait TProduct {
fn get_title(&self) -> &String;
}
// and concrete impls
pub struct ConcreteProduct1 {
}
impl TProduct for ConcreteProduct1 {
// ...
}
pub struct ConcreteProduct2 {
}
impl TProduct for ConcreteProduct2 {
// ...
}
// factory interface
pub trait TProductFactory {
fn product_from_text(&mut self, text: String) -> Box<dyn TProduct>;
// QUESTION: should it be Box (required for ProductFactory) or &Box (required for ProductCachingProxy)?
}
// actual building factory
pub struct ProductFactory {
}
impl TProductFactory for ProductFactory {
fn product_from_text(&mut self, text: String) -> Box<dyn TProduct> {
//...
// depending on some conditions
Box::new(ConcreteProduct1::from_text(text)); // has to pass the ownership
// or
Box::new(ConcreteProduct2::from_text(text)); // has to pass the ownership
//...
}
}
// caching proxy
trait TProductCache {
fn put(&mut self, text: &String, product: Box<dyn TProduct>);
fn get(&self, text: &String) -> Option<&Box<dyn TProduct>>;
fn clear(&mut self);
}
struct InMemoryProductCache {
map: HashMap<String, Box<dyn TProduct>>
}
impl InMemoryProductCache {
fn new() -> Self {
return InMemoryProductCache {
map: HashMap::new()
}
}
}
impl TProductCache for InMemoryProductCache {
fn put(&mut self, text: &String, product: Box<dyn TProduct>) {
self.map.insert(text.to_string(), product);
}
fn get(&self, text: &String) -> Option<&Box<dyn TProduct>> {
return match self.map.get(text) {
Some(boxed_product) => Some(boxed_product), // have to pass a reference to let it still own the value
None => None
}
}
fn clear(&mut self) {
self.map.clear();
}
}
struct ProductCachingProxy {
product_factory: Box<dyn TProductFactory>,
cache: Box<dyn TProductCache>
}
impl ProductCachingProxy {
fn new_for_factory(product_factory: Box<dyn TProductFactory>, cache: Box<dyn TProductCache>) -> Self {
return ProductCachingProxy {
product_factory,
cache
}
}
}
impl TProductFactory for ProductCachingProxy {
fn product_from_text(&mut self, text: String) -> &Box<dyn TProduct> { // can't pass ownership
let boxed_product = match self.cache.get(&text) {
Some(found_boxed_product) => found_boxed_product,
_ => {
// delegate creation to wrapped TProductFactory impl (`product_factory`)
let boxed_product = self.product_factory.product_from_text(text.clone());
// ... and put to the cache
self.cache.put(&text, boxed_product);
&boxed_product
}
};
return boxed_product;
}
}
// QUESTION: should it be Box (required for ProductFactory) or &Box (required for ProductCachingProxy) to be returned from TProductFactory.fn product_from_text(&mut self, text: String) -> Box<dyn TProduct>; ?
If caching proxy to return a Box, how can it be created from a reference without copying/cloning (TProductCache.get(..))?

Answer 1

Replace Box with Rc (or Arc if you use threads). It provides shared ownership and suites both your cases with single signature. Another option is to use Cow that is a enum of owned and borrowed states.