How do I declare a "static" field in a struct in Rust?

Question

How do I declare a "static" field in a struct in Rust, preferably with a default value:

struct MyStruct {
    x: i32,               // instance
    y: i32,               // instance
    my_static: i32 = 123, // static, how?
}

fn main() {
    let a = get_value();
    if a == MyStruct::my_static {
        //...
    } else {
        //...
    }
}

Solution 1

Rust does not support static fields in structures, so you can't do that. The closest thing you can get is an associated method:

struct MyStruct {
    x: i32,
    y: i32,
}

impl MyStruct {
    #[inline]
    pub fn my_static() -> i32 {
        123
    }
}

fn main() {
    let a = get_value();
    if a == MyStruct::my_static() {
        //...
    } else {
        //...    
    }
}

Solution 2

You can declare an associated constant in an impl:

struct MyStruct {
    x: i32,
    y: i32,
}

impl MyStruct {
    const MY_STATIC: i32 = 123;
}

fn main() {
    println!("MyStruct::MY_STATIC = {}", MyStruct::MY_STATIC);
}

Solution 3

You can't declare a field static in a struct.

You can declare a static variable at module scope like this :

static FOO: int = 42;

And you can't have a static variable mutable without unsafe code : to follow borrowing rules it would have to be wrapped in a container making runtime borrowing checks and being Sync, like Mutex or RWLock, but these cannot be stored in static variable as they have non-trivial constructors.

Comments

[CinCout]

using the static keyword doesn't work?

[Incerteza]

do you have any idea why not?

[Vladimir Matveev]

@AlexanderSupertramp, probably because they are not really needed? Static fields can only used for scoping and encapsulation, but encapsulation unit in Rust is module, so just make a static in the module your struct is in, that's it.

[Shepmaster]

static and const are different things in Rust.

[aitvann]

@ Shepmaster, I know. I think the author of the question had in mind not static which is in Rust, but a member of the structure that is not stored in the object of the structure

[Kristianmitk]

doc.rust-lang.org/book/const-and-static.html for a working different things in Rust link

[jkmartindale]

If you are looking to make a static variable mutable with Mutex you can combine it with lazy-static! (which does additional wrapping of its own)

[user2284570]

@Levans, if the struct is declared static, will it apply to its members?

[user2284570]

@aitvann if the struct is declared static, will it apply to its members?

[MisterMiyagi]

This precisely matched my use-case of "constant Python class attribute".

[SharedRory]

This should be the accepted answer as it does what OP wants

[CATboardBETA]

I don't think it's "appropriate" for me to flag this saying that a mod should "change" this to accepted answer, given that isn't possible, but I agree. This should be made the accepted answer.