Can I use generic function arguments in Rust to create new variables?

Question

Is it possible to create an instance of a generic parameter type in Rust?

I'm coming from a C++ background where it's perfectly valid to use template types to create types in the actual function body.

I'm trying to create a variable with type T in this function but I'm not sure how.

I just want to be able to create an object of type T, load it, and then insert it into the HashMap:

fn load<T>(&mut self, id: String, path: String)
    where T: AssetTrait + 'static
{
    // let asset : T = T; this doesn't work?

    asset.load(path);

    // self.assets.insert(id, Box::new<T>(asset));
}

Here is all my code:

trait AssetTrait {
    fn load(&self, path: String) {
        // Do nothing
        // Implement this in child asset object
    }
}

struct AssetManager {
    assets: HashMap<String, Box<AssetTrait + 'static>>,
}

impl AssetManager {
    fn new() -> AssetManager {
        let asset_manager = AssetManager { assets: HashMap::new() };

        return asset_manager;
    }

    fn load<T>(&mut self, id: String, path: String)
        where T: AssetTrait + 'static
    {
        // let asset : T = T; this doesn't work?

        asset.load(path);

        // self.assets.insert(id, Box::new<T>(asset));
    }
}

malbarbo · Accepted Answer · 2016-05-20 12:00:51Z

5

In C++ when you declare a variable like T asset; you are assuming that T has a default constructor (compile time version of duck typing). It is an compiler error to instantiated T with a type that does not have a default constructor, but it's ok if no such instantiation happens.

In Rust you cannot "assume" that a type parameter supports an operation. You have to specify the supported operations with bounds on the type parameter.

That said, you have to options:

Define your own constructor like associated function in AssetTrait

For example, you can declare load as an associated function that do not take a self parameter and returns Self, and call T::load(path) to instantiate T:

use std::collections::HashMap;

trait AssetTrait {
    fn load(path: String) -> Self;
}

struct AssetManager {
    assets: HashMap<String, Box<AssetTrait + 'static>>,
}

impl AssetManager {
    fn new() -> AssetManager {
        let asset_manager = AssetManager { assets: HashMap::new() };
        return asset_manager;
    }

    fn load<T>(&mut self, id: String, path: String)
        where T: AssetTrait + 'static
    {
        let asset = T::load(path);
        self.assets.insert(id, Box::new(asset));
    }
}

Use a predefined trait that has a constructor like associated function

In Rust, the Default trait is used for this purpose:

use std::collections::HashMap;

trait AssetTrait {
    fn load(&mut self, path: String);
}

struct AssetManager {
    assets: HashMap<String, Box<AssetTrait + 'static>>,
}

impl AssetManager {
    fn new() -> AssetManager {
        let asset_manager = AssetManager { assets: HashMap::new() };
        return asset_manager;
    }

    fn load<T>(&mut self, id: String, path: String)
        where T: Default + AssetTrait + 'static
    {
        let mut asset = T::default();
        asset.load(path);
        self.assets.insert(id, Box::new(asset));
    }
}

edited May 20, 2016 at 12:00

answered May 19, 2016 at 22:41

malbarbo

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4 Comments

John Over a year ago

I got it to work but I had to change the "asset" variable in the second example to be mutable for the load function to work correctly.

malbarbo Over a year ago

That makes sense, load will modify self.

Steve Klabnik Over a year ago

Tiny note: every fn deccared in an impl block is an associated fn. we have a more specific name for those that take self, methods, but methods are associated functions.

malbarbo Over a year ago

@SteveKlabnik Thanks Steve. I updated the answer and this lapse was removed.

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