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How to define / edit a struct istance field with a method? (feddit.it)

submitted 1 week ago* (last edited 1 week ago) by [email protected] to c/[email protected]

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Let's say I have a struct Event which implements the method permalink:

struct Event {
    base_url: String,
    rel_permalink: String,
    permalink: String,
    title: String,
}

impl Event {
    fn permalink(&self) -> String {
        let permalink = format!("{}{}", self.base_url, self.rel_permalink);
        permalink
    }
}

The method takes 2 fields of the struct and the target would be to return the definition of another field.

Later I instantiate an event1: Event:

let event1 = Event {
                base_url: base_url,
                rel_permalink: rel_permalink.to_string(),
                title: node.value().name().to_string(),
                permalink = permalink(),
            };

Essentially I would like the field permalink to be the value returned by the method permalink, is something like this possible, is this correct? I couldn't find something similar in the docs..

Pheraps using an associated function as constructor would be a better way to handle a similar situation?

Thank you so much!

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[–] [email protected] 6 points 1 week ago* (last edited 1 week ago) (1 children)

It's quite simple. Just remove the permalink field! If you are calculating it then no need to store it in the struct.

If you do need the field to be there (ex you serialise it with serde), then create a method called new that takes everything but the permalink, construct your permalink there, then return a new object.

Your permalink method can now just be self.permalink.to_string()

P.S. in the second case I'd recommend to change the return type of self.permalink() to &str. It avoids unnecessary cloning of the string.

[–] BB_C 2 points 1 week ago (1 children)

It’s quite simple. Just remove the permalink field! If you are calculating it then no need to store it in the struct.

This is inefficient. It should be the other way around. Remove base_url and rel_permalink, and store permalink and the rel_permalink offset.

That way, you can get a zero cost &str for any of the three.

[–] [email protected] 2 points 1 week ago

Technically yes, but a newbie doesn't need that. I doubt it's an application that performance is so critical either.

Too much premature optimisation IMO

[–] [email protected] 3 points 1 week ago* (last edited 1 week ago) (1 children)

Hmm, yeah, I'd use a constructor method:

impl Event {
    pub fn new(base_url: String, rel_permalink: String, title: String) -> Self {
        let permalink = format!("{}{}", self.base_url, self.rel_permalink);

        Self {
                base_url,
                rel_permalink,
                title,
                permalink,
         };
    }

    // ...
}

Then you'd instantiate it like so:

let title = node.value().name().to_string();
let event = Event::new(base_url, rel_permalink.to_string(), title);

[–] [email protected] 1 points 1 week ago* (last edited 1 week ago) (1 children)

~~Is it really valid to call permalink() in that context, since it requires &self? There's no self in that context afaik. Can't test it now but it looks suspicious~~

EDIT: Invalid since the comment was updated

[–] [email protected] 2 points 1 week ago

Sorry, I posted the comment, then realized that problem. 😅