Understanding Traits and Object Safety

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Why does this particular example not work? The chapter Trait Objects
states:

So what makes a method object-safe? Each method must require that Self: Sized

Isn’t that fulfilled?

This question really is: What is a trait object?

A trait object is an interface in the Object-Oriented paradigm:

  • it exposes a limited set of methods,
  • which are applied to an unknown concrete type.

The fact that the concrete type to which the operations is applied is unknown is specifically why one uses a trait object, as it allows manipulating a heterogeneous set of types in a uniform fashion down to the assembly level.

The fact the concrete type is unknown, however, means that the size of the memory area which contains the memory is also unknown; therefore a trait object can only be manipulated behind a reference or pointer such as &dyn TraitObject, &mut dyn TraitObject or Box<dyn TraitObject> for example.

At the memory level, each of them is represented identically:

  • a pointer to a virtual table, which is a structure holding one function pointer per “method” of the trait object at a fixed offset,
  • a pointer to the actual data of the object.

What is the difference between Trait: Sized and where Self: Sized? (Well, yes, one inherits the trait the other one is a parameter bound, but from Rust’s trait object perspective?)

There is no inheritance in Rust. In both cases those are bounds:

  • Trait: Sized states that the trait itself can only be implemented for a type that already implements Sized,
  • fn method(&self) where Self: Sized states that only types that implement Sized can implement this method.

Note: when implementing a trait, all methods must end up having a definition; the latter is therefore only really useful if a default implementation is provided for the method with the Self: Sized bound, as is shown here.

What is the preferred change I had to make object_safety_dynamic work?

You have to take the trait object by reference or pointer. Whether you use a reference or pointer depends on whether you want to transfer ownership or not.

More Related Contents:

  1. What makes something a “trait object”?
  2. Traits and Rust
  3. How to get a reference to a concrete type from a trait object?
  4. Why can impl trait not be used to return multiple / conditional types?
  5. How do I implement a trait I don’t own for a type I don’t own?
  6. Is it possible to use `impl Trait` as a function’s return type in a trait definition?
  7. How do I return an instance of a trait from a method?
  8. I implemented a trait for another trait but cannot call methods from both traits
  9. Sending trait objects between threads in Rust
  10. The trait cannot be made into an object
  11. Why would I implement methods on a trait instead of as part of the trait?
  12. How does the mechanism behind the creation of boxed traits work?
  13. Trait implementation for both a trait object and for direct implementors of the trait
  14. Why is the `Sized` bound necessary in this trait?
  15. How can I implement Rust’s Copy trait?
  16. Is there any trait that specifies numeric functionality?
  17. How do I pass a closure through raw pointers as an argument to a C function?
  18. Can’t implement a trait I don’t own for all types that implement a trait I do own
  19. How do I specify that a struct field must implement a trait?
  20. Rust equivalent to Swift’s extension methods to a protocol?
  21. Cannot use moved BufReader after for loop with bufreader.lines()
  22. How do I pass Rc to a function accepting Rc?
  23. Why does using Option::map to Box::new a trait object not work?
  24. What is the cited problem with using generic type parameters in trait objects?
  25. Do I have to implement a trait twice when implementing it for both reference and non-reference types?
  26. Conditionally iterate over one of several possible iterators
  27. When I can use either Cell or RefCell, which should I choose?
  28. Generics Error: expected type parameter, found struct
  29. How to create and write to memory mapped files?
  30. Is there any way to restrict a generic type to one of several types?

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