Return a generic from a function

1

I want to return a generic instance from a function, how should I do this?

I have the following 2 structs:

struct EX {
    val: u8,
}

struct EX2 {
    val: u8,
}

And the following function:

fn parse(input: &str) -> ...
{
    // HOW?
}

How cam I make the function parse return either EX or EX2?

2

One way would be to wrap both in a enum and return it.
Or, if both structs implement the same trait, you can return a Box<dyn CommonTrait>

fn parse(input: &str) -> Box<dyn MyTrait> {
    Box::new(Ex1{...})
}
3

Thx, I'm still new to rust. Does using dyn doesn't have any drawbacks on performance.

4

Does that matter if it is the way for accomplishing what you want?

By the way, if there's a common trait that both types implement, you can just require the type to be specified:

fn parse<T: MyTrait>(input: &str) -> T {
    …
}

Incidentally, this is exactly how str::parse() is implemented.

This approach uses static dispatch, while dyn Trait results in dynamic dispatch. This pretty much implies all the advantages and disadvantages of the two different methods.

1 Like
5

I don't know if it's the only solution.
Here's basically what I try to solve.

I have a string representing a XML and I want to return a struct that represents the input.
Off course, each input might represent a different struct.

Any thoughts?

6

When you need to dynamically decide the type that comes from deserializing loosely-typed data such as XML, the typical thing to do in Rust is creating an enum as @naim suggested above.

7

I'd add though that it might be more typical for this use case to define the related types themselves as a single enum.

enum Data {
  EX {
    val: u8,
  },
  EX2 {
    val: u8,
  }
}

This would be more appropriate if you will not be expecting to write separate functions to deal with the separate data types.

1 Like
8

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