Is there a faster/shorter way to initialize variables in a Rust struct?

Question

In the following example, I would much prefer to assign a value to each field in the struct in the declaration of the fields. Alternatively, it effectively takes one additional

Answer 1

You can provide default values for your struct by implementing the Default trait. The default function would look like your current new function:

impl Default for cParams {
    fn default() -> cParams {
        cParams {
            iInsertMax: -1,
            iUpdateMax: -1,
            iDeleteMax: -1,
            iInstanceMax: -1,
            tFirstInstance: false,
            tCreateTables: false,
            tContinue: false,
        }
    }
}

You can then instantiate the struct by giving only the non-default values:

let p = cParams { iInsertMax: 10, ..Default::default() };

With some minor changes to your data structure, you can take advantage of an automatically derived default implementation. If you use #[derive(Default)] on a data structure, the compiler will automatically create a default function for you that fills each field with its default value. The default boolean value is false, the default integral value is 0.

An integer's default value being 0 is a problem here since you want the integer fields to be -1 by default. You could define a new type that implements a default value of -1 and use that instead of i64 in your struct. (I haven't tested that, but it should work).

However, I'd suggest to slightly change your data structure and use Option instead of i64. I don't know the context of your code, but it looks like you're using the special value of -1 to represent the special meaning "infinite", or "there's no max". In Rust, we use an Option to represent an optionally present value. There's no need for a -1 hack. An option can be either None or Some(x) where x would be your i64 here. It might even be an unsigned integer if -1 was the only negative value. The default Option value is None, so with the proposed changes, your code could look like this:

#[derive(Default)]
struct cParams {
    iInsertMax: Option,
    iUpdateMax: Option,
    iDeleteMax: Option,
    iInstanceMax: Option,
    tFirstInstance: bool,
    tCreateTables: bool,
    tContinue: bool,
}

let p = cParams { iInsertMax: Some(10), ..Default::default() };