Rust trait field lifetime

Question

I think this is something obvious I'm missing, but here goes..

use std::io;

pub trait Source<'a, T> {
    fn push(&self, t: T) -> io::Result<()>;

    fn link(&mut self, sink: &dyn Sink<'a, T>) -> io::Result<()>;
}

pub trait Sink<'a, T> {
    fn push(&self, t: T) -> io::Result<()>;

    fn link(&mut self, source: &dyn Source<T>) -> io::Result<()>;
}

pub struct SyncSource<'a, T> {
    sink: Option<&'a dyn Sink<'a, T>>,
}

impl<'a, T> SyncSource<'a, T> {
    pub fn new() -> SyncSource<'a, T> {
        SyncSource {
            sink: None,
        }
    }
}

impl<'a, T> Source<'a, T> for SyncSource<'a, T> {
    fn push(&self, t: T) -> io::Result<()> {
        match self.sink {
            Some(sink) => sink.push(t),
            None => Err(io::Error::new(io::ErrorKind::NotConnected, "no sink")),
        }
    }

    fn link(&mut self, sink: &dyn Sink<'a, T>) -> io::Result<()> {
        self.sink = Some(sink);
        Ok(())
    }
}

pub struct SyncSink<'a, T> {
    source: Option<&'a dyn Source<'a, T>>,
}

impl<'a, T> SyncSink<'a, T> {
    pub fn new() -> SyncSink<'a, T> {
        SyncSink {
            source: None,
        }
    }
}

impl<'a, T> Sink<'a, T> for SyncSink<'a, T> {
    fn push(&self, t: T) -> io::Result<()> {
        match self.source {
            Some(source) => source.push(t),
            None => Err(io::Error::new(io::ErrorKind::NotConnected, "no source")),
        }
    }

    fn link(&mut self, source: &dyn Source<T>) -> io::Result<()> {
        self.source = Some(source);
        Ok(())
    }
}

I read the rustlang book chapter about lifetimes but could not really understand what's wrong here. What I'm trying to do is build a basic pipe and filter architecture. A source knows its sink and a sink knows its source, thus I want to store references to the objects. Obviously, there's a lifetime issue here.

I first thought about introducing the lifetime 'a to say the source/sink shall live as long as the object it is linked to. This does not work. Now I'm thinking I may need a lifetime 'b which outlives 'a and somehow throw that in the mix, but as you can see, this is where I am confused.

What is the question? What is the problem? Does the code not compile? If so what is the error? Does it compile and crash? If so what is the message? Does it run and not do what you wanted? If so what does it do and what did you expect instead? — Jmb
Hi! , thanks for your question! I had some struggle looking for it, I had a similar question and found the answer here, could you improve the title of your question? Thanks in advance! — Esteban Borai

Prime_Aqasix Prime_Aqasix · Accepted Answer · 2020-04-22T04:42:40

You are almost there:

use std::io;

pub trait Source<'a, T> {
    fn push(&self, t: T) -> io::Result<()>;

    // Make sure the references themselves have the 'a lifetime marker
    fn link(&'a mut self, sink: &'a dyn Sink<'a, T>) -> io::Result<()>; 
}

pub trait Sink<'a, T> {
    fn push(&self, t: T) -> io::Result<()>;

    // Make sure the references themselves have the 'a lifetime marker
    fn link(&'a mut self, source: &'a dyn Source<'a, T>) -> io::Result<()>; 
}

pub struct SyncSource<'a, T> {
    sink: Option<&'a dyn Sink<'a, T>>,
}

impl<'a, T> SyncSource<'a, T> {
    pub fn new() -> SyncSource<'a, T> {
        SyncSource {
            sink: None,
        }
    }
}

impl<'a, T> Source<'a, T> for SyncSource<'a, T> {
    fn push(&self, t: T) -> io::Result<()> {
        match self.sink {
            Some(sink) => sink.push(t),
            None => Err(io::Error::new(io::ErrorKind::NotConnected, "no sink")),
        }
    }

    // Now match the lifetime definitions that is defined in the trait
    fn link(&'a mut self, sink: &'a dyn Sink<'a, T>) -> io::Result<()> {
        self.sink = Some(sink);
        Ok(())
    }
}

pub struct SyncSink<'a, T> {
    source: Option<&'a dyn Source<'a, T>>,
}

impl<'a, T> SyncSink<'a, T> {
    pub fn new() -> SyncSink<'a, T> {
        SyncSink {
            source: None,
        }
    }
}

impl<'a, T> Sink<'a, T> for SyncSink<'a, T> {
    fn push(&self, t: T) -> io::Result<()> {
        match self.source {
            Some(source) => source.push(t),
            None => Err(io::Error::new(io::ErrorKind::NotConnected, "no source")),
        }
    }

    // Now match the lifetime definitions that is defined in the trait
    fn link(&'a mut self, source: &'a dyn Source<'a, T>) -> io::Result<()> {
        self.source = Some(source);
        Ok(())
    }
}

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Rust trait field lifetime

1 Answers