Why can impl trait not be used to return multiple / conditional types?

by

impl Trait is not equivalent to returning an interface or base class object. It’s a way of saying “I don’t want to write the name of the specific type I’m returning”. You’re still returning a value of a single, specific type; you just aren’t saying which type.

Each of those branches is returning different types, hence the problem. Implementing the same trait is not enough.

What you likely want in this specific case is a trait object like Box<dyn RngCore>.

extern crate rand; // 0.6.5

use rand::{rngs::OsRng, thread_rng, RngCore};

fn rng() -> Box<dyn RngCore> {
    match OsRng::new() {
        Ok(rng) => Box::new(rng),
        Err(_) => Box::new(thread_rng()),
    }
}

Note: if you are using a slightly older version of Rust, you may need to remove the dyn keyword. It’s optional in the previous (2015) edition of Rust.

More Related Contents:

  1. Traits and Rust
  2. How to get a reference to a concrete type from a trait object?
  3. How do I implement a trait I don’t own for a type I don’t own?
  4. What makes something a “trait object”?
  5. Is it possible to use `impl Trait` as a function’s return type in a trait definition?
  6. How do I return an instance of a trait from a method?
  7. I implemented a trait for another trait but cannot call methods from both traits
  8. Why would I implement methods on a trait instead of as part of the trait?
  9. How does the mechanism behind the creation of boxed traits work?
  10. Trait implementation for both a trait object and for direct implementors of the trait
  11. Why is the `Sized` bound necessary in this trait?
  12. How can I implement Rust’s Copy trait?
  13. Is there any trait that specifies numeric functionality?
  14. Understanding Traits and Object Safety
  15. How do I pass a closure through raw pointers as an argument to a C function?
  16. Can’t implement a trait I don’t own for all types that implement a trait I do own
  17. How do I specify that a struct field must implement a trait?
  18. Rust equivalent to Swift’s extension methods to a protocol?
  19. Cannot use moved BufReader after for loop with bufreader.lines()
  20. Do I have to implement a trait twice when implementing it for both reference and non-reference types?
  21. How to test for equality between trait objects?
  22. error [E0716]: temporary value dropped while borrowed (rust)
  23. Can I get a trait object of a multi-trait instance without using a generic type?
  24. How to iterate over and filter an array?
  25. How to fix lifetime error when function returns a serde Deserialize type?
  26. Is there another option to share an Arc in multiple closures besides cloning it before each closure?
  27. Is there a way to use existing structs as enum variants?
  28. How can I create enums with constant values in Rust?
  29. Use of undeclared type or module `std` when used in a separate module
  30. How do I cope with lazy iterators?

Leave a Comment