CODEWITHSUNDEEP
vectorrustborrow-checker
VP.
VP.

Reputation: 16715

How to make a vector of boxed trait objects from another vector of them?

I am making a bot. When the bot receives a message it needs to check all the commands if they trigger on the message and if yes - perform an action.

So I have a vector of commands (Command trait) in main struct:

struct Bot {
    cmds: Vec<Box<Command>>,
}

Everything is good until I try to make a list of triggered commands and to use them later in (&self mut) method:

let mut triggered: Vec<Box<command::Command>>;
for c in &self.cmds {
    if c.check(&message) {
        triggered.push(c.clone());
    }
}

Error:

bot.rs:167:44: 167:56 error: mismatched types:
 expected `Box<Command>`,
    found `&Box<Command>`
(expected box,
    found &-ptr) [E0308]

What am I doing wrong here? I tried a lot but nothing helps. Initially I was doing the following:

for c in &self.cmds {
    if c.check(&message) {
        c.fire(&message, self);
    }
}

but it gave me:

bot.rs:172:46: 172:50 error: cannot borrow `*self` as mutable because `self.cmds` is also borrowed as immutable [E0502]
bot.rs:172        

                 c.fire(&message, self);

So I stackoverflowed it and came to solution above.

Upvotes: 0

Views: 652

Answers (1)

malbarbo
malbarbo

Reputation: 11177

What am I doing wrong here? I tried a lot but nothing helps. Initially I was doing the following:

for c in &self.cmds {
    if c.check(&message) {
        c.fire(&message, self);
    }
}

If the fire function does not need access to other commands, an option is to temporarily replace self.cmd with an empty vector: 

trait Command {
    fn check(&self, message: &str) -> bool;
    fn fire(&mut self, bot: &Bot);
}

struct Bot {
    cmds: Vec<Box<Command>>,
}

impl Bot {
    fn test(&mut self, message: &str) {
        use std::mem;
        // replace self.cmds with a empty vector and returns the
        // replaced vector
        let mut cmds = mem::replace(&mut self.cmds, Vec::default());
        for c in &mut cmds {
            if c.check(message) {
                c.fire(self);
            }
        }
        // put back cmds in self.cmds
        mem::replace(&mut self.cmds, cmds);
    }
}

There are other answers that use this approach.

If fire does need access to some fields of Bot, you can pass only the needed fields instead of self:

c.fire(self.others)

Upvotes: 2

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