How to use struct self in member method closure
Solution 1:
Split your data and methods into smaller components, then you can take disjoint borrows to various components on self
:
fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }
#[derive(Default)]
struct BaseUrl(Option<String>);
impl BaseUrl {
fn get(&mut self) -> &str {
self.0.get_or_insert_with(|| get_env_url())
}
}
#[derive(Default)]
struct App {
base_url: BaseUrl,
access_token: Option<String>,
}
impl App {
fn new() -> App {
App::default()
}
fn get_access_token(&mut self) -> &str {
let base_url = &mut self.base_url;
self.access_token
.get_or_insert_with(|| fetch_access_token(base_url.get()))
}
}
fn main() {}
You can go further and do this for both values:
fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }
#[derive(Default)]
struct BaseUrl(Option<String>);
impl BaseUrl {
fn get(&mut self) -> &str {
self.0.get_or_insert_with(|| get_env_url())
}
}
#[derive(Default)]
struct AccessToken(Option<String>);
impl AccessToken {
fn get(&mut self, base_url: &str) -> &str {
self.0.get_or_insert_with(|| fetch_access_token(base_url))
}
}
#[derive(Default)]
struct App {
base_url: BaseUrl,
access_token: AccessToken,
}
impl App {
fn new() -> App {
App::default()
}
fn get_access_token(&mut self) -> &str {
let base_url = self.base_url.get();
self.access_token.get(base_url)
}
}
fn main() {}
Which lets you see that you can abstract out common functionality:
fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }
#[derive(Default)]
struct StringCache(Option<String>);
impl StringCache {
fn get<F>(&mut self, f: F) -> &str
where
F: FnOnce() -> String,
{
self.0.get_or_insert_with(f)
}
}
#[derive(Default)]
struct App {
base_url: StringCache,
access_token: StringCache,
}
impl App {
fn new() -> App {
App::default()
}
fn get_access_token(&mut self) -> &str {
let base_url = self.base_url.get(get_env_url);
self.access_token.get(|| fetch_access_token(base_url))
}
}
fn main() {}
And then you realize the abstraction can be made generic:
fn fetch_access_token(_base_url: &str) -> String { String::new() }
fn get_env_url() -> String { String::new() }
#[derive(Default)]
struct Cache<T>(Option<T>);
impl<T> Cache<T> {
fn get<F>(&mut self, f: F) -> &T
where
F: FnOnce() -> T,
{
self.0.get_or_insert_with(f)
}
}
#[derive(Default)]
struct App {
base_url: Cache<String>,
access_token: Cache<String>,
}
impl App {
fn new() -> App {
App::default()
}
fn get_access_token(&mut self) -> &str {
let base_url = self.base_url.get(get_env_url);
self.access_token.get(|| fetch_access_token(base_url))
}
}
fn main() {}
See also:
- Borrowing references to attributes in a struct
- Why is it discouraged to accept a reference to a String (&String), Vec (&Vec) or Box (&Box) as a function argument?
- The Rust Programming Language chapter on closures, which creates this caching struct as part of the exercises.
Solution 2:
The closure passed to the get_or_insert_with
method in Option<T>
is of type FnOnce
- it thus consumes or moves the captured variables. In this case self
is captured because of the usage of self.get_base_url()
in the closure. However, since self
is already borrowed, the closure cannot consume or move the value of self
for unique access.
This can be circumvented by using the get_or_insert
method, but it will require you to perform the potentially expensive operation of fetching the access token every time get_access_token
is called regardless of whether access_token
is None
or not.