Add speaker notes for trait objects (#194)

src/traits.md

@@ -35,3 +35,20 @@ fn main() {
     }
 }
 ```
+
+<details>
+
+* Traits may specify pre-implemented (default) methods and methods that users are required to implement themselves. Methods with default implementations can rely on required methods.
+* Types that implement a given trait may be of different sizes. This makes it impossible to have things like `Vec<Greet>` in the example above.
+* `dyn Greet` is a way to tell the compiler about a dynamically sized type that implements `Greet`.
+* In the example, `pets` holds Fat Pointers to objects that implement `Greet`. The Fat Pointer consist of two components, a pointer to the actual object and a pointer to the virtual method table for the `Greet` implementation of that particular object.
+
+Compare these outputs in the above example:
+```rust,ignore
+    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 Greet>());
+    println!("{}", std::mem::size_of::<Box<dyn Greet>>());
+```
+
+</details>