How to set a field in a struct with an empty value?

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There’s no null in Rust (and no Null Pointer Exception, Rust is designed for safety).

You must either

1) use an option (i.e. a field of type Option<TcpStream>)

2) or better: return a result when building the struct

Here, the best option would probably be to connect from inside a function returning a Result<FistClient>, so that you don’t have to check whether your struct has a valid stream.

I would do something like this:

pub struct FistClient {
    addr: String,
    conn: TcpStream,
}

impl FistClient {
    pub fn new(ip: &str, port: &str) -> Result<Self> {
        let addr = format!("{}:{}", ip, port);
        let conn = TcpStream::connect(&addr)?;
        Ok(FistClient { addr, conn })
    }
}

Side note: It’s really preferable to not build your applications with calls to panic, even when you think you’re just building a dirty draft. Handle errors instead.

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