How to initialize a generic variable in Rust

Question

In a function generic on T , how can I properly create and initialize a variable of type T in safe (or unsafe) Rust? T can be anything. What is the idiomatic way of doing such thing?

fn f<T>() {
    let t: T = todo!("what to put here?");
}

One possible use case might be to use T as a temporary variable for swapping.

Answer 1

Putting a Default bound on T is the idiomatic way to construct generic types within a generic function.

There's nothing special about the Default trait though and you can declare a similar trait and use that within your generic functions.

Also, if a type implements Copy or Clone you can initialize as many copies and clones as you want from a single value.

Commented examples:

// use Default bound to call default() on generic type
fn func_default<T: Default>() -> T {
    T::default()
}

// note: there's nothing special about the Default trait
// you can implement your own trait identical to it
// and use it in the same way in generic functions
trait CustomTrait {
    fn create() -> Self;
}

impl CustomTrait for String {
    fn create() -> Self {
        String::from("I'm a custom initialized String")
    }
}

// use CustomTrait bound to call create() on generic type
fn custom_trait<T: CustomTrait>() -> T {
    T::create()
}

// can multiply copyable types
fn copyable<T: Copy>(t: T) -> (T, T) {
    (t, t)
}

// can also multiply cloneable types
fn cloneable<T: Clone>(t: T) -> (T, T) {
    (t.clone(), t)
}

playground