Trait implementation with specific struct as parameter

Question

I have a trait for readable and writable streams (like TcpStream):

pub trait MyTraitPool<T: Read + Write> {
    fn acquire(&self, &String) -> T;
    fn free(&self, T);
}

I want to implement that trait with TcpStream as T, so I would like to write

struct MyPool;

impl<T> MyTraitPool<T> for MyPool
    where T: Read + Write {

    fn acquire(&self, addr: &String) -> T {
        TcpStream::connect(addr.as_str()).unwrap()
    }

    fn free(&self, con: T) {
        con.shutdown(Shutdown::Both);
    }
}

I get the error "expected type parameter, found struct std::net::TcpStream" in the acquire method. As for the free method, I know that shutdown is TcpStream-specific, but I would like to have an implementation specific for TcpStreams at that point and hence be able to call TcpStream-specific methods. So how do I go about doing so?

As a side note: the implementation is just an example for what kind of things I would like to do, not for how the code operates later!

I'm getting different issues: "expected type parameter, found enum std::result::Result" (are you missing an unwrap in the code above?) and "no method named shutdown found for type T in the current scope" (assuming you are using only std::net::{TcpStream, Shutdown} and std::io::{Read, Write}). — ljedrz
– ljedrz, Commented Feb 1, 2017 at 13:42
@ljedrz the implementation above is only a sample, so I did not check for the return types or error management etc. (I wanted to keep the purpose simple and readable). The second problem is actually mentioned in the text above ;) — Maru
– Maru, Commented Feb 1, 2017 at 14:23
Ah yes, I wasn't reading carefully enough :). Nevertheless, it is always a good idea to provide code that reproduces the exact issue you are having upon compilation (MCVE) - it makes it much easier to find the solution applicable to your specific case. — ljedrz
– ljedrz, Commented Feb 1, 2017 at 14:30

oli_obk · Accepted Answer · 2017-02-01 13:49:37Z

Instead of making the MyTraitPool trait generic, you'd make your MyPool generic and you'd create an intermediate MyStream trait to offer you all methods you need:

trait MyStream: Read + Write {
    fn connect(&str) -> Self;
    fn shutdown(self);
}
impl MyStream for TcpStream {
    fn connect(addr: &str) -> Self { TcpStream::connect(addr) }
    fn shutdown(self) { TcpStream::shutdown(self, Shutdown::Both); }
}
impl<T: MyStream> MyTraitPool for MyPool<T> {
    fn acquire(&self, addr: &str) -> T {
        MyStream::connect(addr)
    }
    fn free(&self, con: T) {
        con.shutdown()
    }
}

Streams which do not actually need to shutdown, would simply leave the implementation empty.

That looks like a good idea and I managed to make it compile, thank you a lot! I think I learned something new today :)

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