Merkmalsobjekte

We previously saw how trait objects can be used with references, e.g &dyn Pet. However, we can also use trait objects with smart pointers like Box to create an owned trait object: Box<dyn Pet>.

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Memory layout after allocating pets:

<Dog as Pet>::talk<Cat as Pet>::talkStackHeapFidoptrlives9len2capacity2data:name,4,4age5vtablevtablepets: Vec<Box<dyn Pet>>data: CatDogProgram text

Speaker Notes

This slide should take about 10 minutes.
  • Types that implement a given trait may be of different sizes. This makes it impossible to have things like Vec<dyn Pet> in the example above.
  • dyn Pet is a way to tell the compiler about a dynamically sized type that implements Pet.
  • In the example, pets is allocated on the stack and the vector data is on the heap. The two vector elements are fat pointers:
    • A fat pointer is a double-width pointer. It has two components: a pointer to the actual object and a pointer to the virtual method table (vtable) for the Pet implementation of that particular object.
    • The data for the Dog named Fido is the name and age fields. The Cat has a lives field.
  • Compare these outputs in the above example:
    println!("{} {}", std::mem::size_of::<Dog>(), std::mem::size_of::<Cat>()); println!("{} {}", std::mem::size_of::<&Dog>(), std::mem::size_of::<&Cat>()); println!("{}", std::mem::size_of::<&dyn Pet>()); println!("{}", std::mem::size_of::<Box<dyn Pet>>());