How to store predefined instances of a struct in a way that they can be referenced by name?

Question

I would like to store many predefined instances of a struct as constants and group them in a way that I am able to reference them by name.

I could achieve it like this:

use std::collections::HashMap;

struct MyStruct {
    // ...
}

const INSTANCES: HashMap<&str, MyStruct> = [
    ("Instance1", MyStruct {
        // fields...
    }),
    ("Instance2", MyStruct {
        // fields...
    }),
    ("Instance3", MyStruct {
        // fields...
    }),
    // Add more instances as needed
].iter().cloned().collect();

fn main() {
    let instance = INSTANCES.get("Instance1");
    // ...
}

Or this:

struct MyStruct {
    // ...
}

enum MyStructInstance {
    Instance1,
    Instance2,
    Instance3,
    // Add more instances as needed
}

impl MyStructInstance {
    fn get_instance(&self) -> MyStruct {
        match self {
            MyStructInstance::Instance1 => MyStruct {
                // fields...
            },
            MyStructInstance::Instance2 => MyStruct {
                // fields...
            },
            MyStructInstance::Instance3 => MyStruct {
                // fields...
            },
            // ...
        }
    }
}

fn main() {
    let instance1 = MyStructInstance::Instance1.get_instance();
    // ...
}

But it does not seem well formed. What is the standard way for storing a named list of predefined instances?

I am looking for a better alternative, because I might have 40+ of them — Iter Ator, May 17 '23 at 20:34
Does this answer your question? [Idiomatic way to create a constant HashMap with values in Rust](https://stackoverflow.com/questions/76039999/idiomatic-way-to-create-a-constant-hashmap-with-values-in-rust) — drewtato, May 17 '23 at 21:07
I have a similar problem, my first thought was Vec, but i wish i could put all instances in an enum as values, but that's not possible, i guess. I am still looking for a solution. — Hamza Zubair, May 25 '23 at 04:18

xamgore · Answer 1 · 2023-05-19T16:06:58.387

I would like to store many predefined instances of a struct as constants and group them in a way that I am able to reference them by name.

pub mod group {
    pub struct MyStruct {
        // ...
    }

    pub const Instance1: MyStruct = MyStruct {};
    
    pub const Instance2: MyStruct = MyStruct {};
    
    pub const Instance3: MyStruct = MyStruct {};
}

fn main() {
    let instance1 = group::Instance1;
    let instance2 = group::Instance2;
    // ...
}

Playground. If you want to modify them, you can use static with unsafe updates. You may implement FromStr trait for MyStruct, if you need conversions from strings.

How to store predefined instances of a struct in a way that they can be referenced by name?

1 Answers1