CODEWITHSUNDEEP
rust
Jonathan Wilbur
Jonathan Wilbur

Reputation: 1147

Can you disable constructor syntax for a type?

Say I define my own type in a Rust library, like so:

struct Date {
    year: u16,
    month: u8,
    day: u8
}

impl Date {
    fn new(y: u16, m: u8, d: u8) -> Date {
        // Do some validation here first
        Date { year: y, month: m, day: d }
    }
}

Is there a way to require users to use the Date::new constructor? In other words, can I somehow prohibit users from creating their own Date struct using the following constructor:

let d = Date { 2017, 7, 10 };

I ask because it seems to be a detrimental flaw if you cannot force developers to run their arguments through a battery of validation before setting the members of a struct. (Although, maybe there is some other pattern that I should use in Rust, like validating data when they are used rather than when they are created; feel free to comment on that.)

Upvotes: 41

Views: 4297

Answers (1)

Matthieu M.
Matthieu M.

Reputation: 299780

TL;DR: The "default constructor" is disabled by default already.

The struct syntax is only available to those who have access to all the fields of the struct.

As a result, it is only accessible in the same module, as per privacy rules, unless all fields are marked pub in which case it is accessible wherever the struct is.

Note that the same is true of functions, since new is not marked pub here, it's inaccessible to any module other than the current one.

Upvotes: 40

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