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Code Issues Releases Activity

Move Trait Objects to the Smart Pointers segment (#1756)

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Fixes #1516.
This commit is contained in:
Dustin J. Mitchell
2024-01-25 09:47:50 -05:00
committed by GitHub
parent 2313c0c3d0
commit 20f45521e5
3 changed files with 3 additions and 2 deletions
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---
minutes: 10
---
# Trait Objects
Trait objects allow for values of different types, for instance in a collection:
```rust,editable
struct Dog {
name: String,
age: i8,
}
struct Cat {
lives: i8,
}
trait Pet {
fn talk(&self) -> String;
}
impl Pet for Dog {
fn talk(&self) -> String {
format!("Woof, my name is {}!", self.name)
}
}
impl Pet for Cat {
fn talk(&self) -> String {
String::from("Miau!")
}
}
fn main() {
let pets: Vec<Box<dyn Pet>> = vec![
Box::new(Cat { lives: 9 }),
Box::new(Dog { name: String::from("Fido"), age: 5 }),
];
for pet in pets {
println!("Hello, who are you? {}", pet.talk());
}
}
```
Memory layout after allocating `pets`:
```bob
Stack Heap
.- - - - - - - - - - - - - -. .- - - - - - - - - - - - - - - - - - - - - - -.
: : : :
: pets : : +----+----+----+----+ :
: +-----------+-------+ : : +-----+-----+ .->| F | i | d | o | :
: | ptr | o---+---+-----+-->| o o | o o | | +----+----+----+----+ :
: | len | 2 | : : +-|-|-+-|-|-+ `---------. :
: | capacity | 2 | : : | | | | data | :
: +-----------+-------+ : : | | | | +-------+--|-------+ :
: : : | | | '-->| name | o, 4, 4 | :
: : : | | | | age | 5 | :
`- - - - - - - - - - - - - -' : | | | +-------+----------+ :
: | | | :
: | | | vtable :
: | | | +----------------------+ :
: | | '---->| "<Dog as Pet>::talk" | :
: | | +----------------------+ :
: | | :
: | | data :
: | | +-------+-------+ :
: | '-->| lives | 9 | :
: | +-------+-------+ :
: | :
: | vtable :
: | +----------------------+ :
: '---->| "<Cat as Pet>::talk" | :
: +----------------------+ :
: :
'- - - - - - - - - - - - - - - - - - - - - - -'
```
<details>
- 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:
```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 Pet>());
println!("{}", std::mem::size_of::<Box<dyn Pet>>());
```
[virtual method table]: https://en.wikipedia.org/wiki/Virtual_method_table
</details>