Use Types That Implement a Trait as Argument Type

[thekief]

Sorry for the probably slightly confusing title. I am looking for somewhat of the "best" option for the following problem.

I have some code looking something like this:

struct MD { ... }

struct XML { ... }

trait Format {
  fn do_fancy_stuff()
}

impl Format for MD {...}

impl Format for XML {...}

The idea would be that the type is specificed via an argument of a flag, e.g., -t xml. The type should then be retrievable by args.type and the do_fancy_stuff function could be called without any further boilerplate.

I tried to use generics for this and have the trait being the flag's type but that probably makes things even more complicated than necessary. It looked somewhat liked this:

struct Args<T: Format> {
  ...
    #[arg(
    short = 't',
    default_value_t = MD{},
    help = r#"Defines the output format:
    markdown (md)
    markdown tables (xml)
"#,
    )]
    output_type: T,
  ...

[epage]

Technically, there might be a way to support something like this but it would likely involve a lot of complexity and be fairly niche.

Is there a reason you couldn't make it output_type: Box<dyn Format> or have an enum Format instead of a trait?

[thekief]

Thank you for the blazing fast answer. No, the code does not have any of these restrictions.

I already thought about the both of these approaches. The Box one causes some issue further down the line I could not resolve:

69  |     output_type: Box<dyn Format>,
    |                  ^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `dyn internal::Format`

I will look into the enum Format variant again. I already had a solution with it but I was not really satisfied with it, as I ended up parsing the values into the enumeration and had to execute a match on it every time to call the proper function and that felt somewhat wrong.

[epage]

Note that Box<dyn Format> generally has worse overhead than match on an enum.