rust memory layout

trait object

a reference to a trait type (or Box) is called trait object

two ways convert concrete type to trait object

  1. assigning to variable
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    use std::io::Write;

    let mut buffer: Vec<u8> = vec![];
    let w: &mut dyn Write = &mut buffer;
  2. pass a concrete type as a argument to a function
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    fn main() {
       let mut buffer: Vec<u8> = vec![];
       writer(&mut buffer);
    }

    fn writer(w: &mut dyn Write) {
        // ...
    }
    in both ways, buffer is converted to a trait object implements Write

in memory, a trait object (in the example, w) is a fat point consists two pointers
trait_object_memory
the vtable is generated once at compile time and shared by all objects of the same type. the vtable contains pointers to the machine code of the functions that must be present for a type to be a “Writer”

references