How are you able to create partially initialised structs?

Question

When creating a struct in Rust it seems like it's difficult to create one without having all of the fields set. For example with the following code

struct Connection {
    url: String,
    stream: TcpStream
}

You aren't able to set url without giving stream as well.

// Compilation error asking for 'stream'
let m = Connection { url: "www.google.com".to_string() }; 

How are you able to create these references that might be Option<None> until a later time?

The best I have found is using the Default trait, but I'd rather not have to create the TcpStream until a later time than when the struct is initialised. Am I able to do this with something like a Box?

Answer 1

One thing you can do is to wrap the TcpStream in an Option, i.e. Option<TcpStream>. When you first construct the struct, it'll be None, and when you initialize it you make it self.stream = Some(<initialize tcp stream>). Wherever you use the TCPStream, you'll have to check if it's Some, i.e. if it has already been initialized. If you can guarantee your behavior then you can just unwrap(), but it's probably better to make a check anyways.

struct Connection {
    url: String,
    stream: Option<TcpStream>
}

impl Connection {
    pub fn new() -> Connection {
        Connection {
            url: "www.google.com".to_string(),
            stream: None,
        }
    }

    pub fn initialize_stream(&mut self) {
        self.stream = Some(TcpStream::connect("127.0.0.1:34254").unwrap());
    }

    pub fn method_that_uses_stream(&self) {
        if let Some(ref stream) = self.stream {
            // can use the stream here
        } else {
            println!("the stream hasn't been initialized yet");
        }
    }
}

This is similar to what is done in Swift, in case you're familiar with that language.

Answer 2

All fields indeed have to be initialized when creating the struct instance (there is no null in Rust) so all the memory is allocated. There is often a dedicated method (like new) that sets default values for fields which are supposed to be modified at a later stage.

I'd use the Box when you don't know the size of the field (like Vec does).