Can you disable constructor syntax for a type?

Question

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.)

Does this answer your question? How to restrict the construction of struct? — Herohtar
– Herohtar, Commented Sep 28, 2022 at 16:23

Matthieu M. · Accepted Answer · 2017-07-11 06:30:39Z

41

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.

edited Jul 11, 2017 at 6:30

answered Jul 10, 2017 at 16:29

Matthieu M.

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5 Comments

Jonathan Wilbur Over a year ago

Ah! Excellent. Thank you, sir!

Matthieu M. Over a year ago

@8bittree: No, it's an unless. Let me qualify it a bit more.

Matthieu M. Over a year ago

@8bittree: Great! Thanks for your feedback!

jmrah Over a year ago

So, assuming the Date struct was public, how would I make sure user's were only able to make a Date using Date::new, but still be able to access the properties of Date? Would I have to make a 'throw away' private field in Date, i.e. struct Date { private_field_just_so_i_cant_be_instantiated: bool, ... }? (go easy on me, I'm just learning Rust :)

Matthieu M. Over a year ago

@jrahhali: You have a choice between "getters" to expose the properties, or a private field which in Rust you'd start with an underscore _ to avoid warnings of being unused. I personally would recommend getters, as private fields are better encapsulated, allowing to change the internal representation without breaking the interface.

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