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< h2 id = "box-around-traits" > < a class = "header" href = "#box-around-traits" > Box around traits< / a > < / h2 >
< p > < code > Box< / code > is very useful for returning traits. You know that you can write traits in generic functions like in this example:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::fmt::Display;
struct DoesntImplementDisplay {}
fn displays_it< T: Display> (input: T) {
println!(" {}" , input);
}
fn main() {}
< / code > < / pre > < / pre >
< p > This only takes something with < code > Display< / code > , so it can't accept our struct < code > DoesntImplementDisplay< / code > . But it can take in a lot of others like < code > String< / code > .< / p >
< p > You also saw that we can use < code > impl Trait< / code > to return other traits, or closures. < code > Box< / code > can be used in a similar way. You can use a < code > Box< / code > because otherwise the compiler won't know the size of the value. This example shows that a trait can be used on something of any size:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > #![allow(dead_code)] // Tell the compiler to be quiet
use std::mem::size_of; // This gives the size of a type
trait JustATrait {} // We will implement this on everything
enum EnumOfNumbers {
I8(i8),
AnotherI8(i8),
OneMoreI8(i8),
}
impl JustATrait for EnumOfNumbers {}
struct StructOfNumbers {
an_i8: i8,
another_i8: i8,
one_more_i8: i8,
}
impl JustATrait for StructOfNumbers {}
enum EnumOfOtherTypes {
I8(i8),
AnotherI8(i8),
Collection(Vec< String> ),
}
impl JustATrait for EnumOfOtherTypes {}
struct StructOfOtherTypes {
an_i8: i8,
another_i8: i8,
a_collection: Vec< String> ,
}
impl JustATrait for StructOfOtherTypes {}
struct ArrayAndI8 {
array: [i8; 1000], // This one will be very large
an_i8: i8,
in_u8: u8,
}
impl JustATrait for ArrayAndI8 {}
fn main() {
println!(
" {}, {}, {}, {}, {}" ,
size_of::< EnumOfNumbers> (),
size_of::< StructOfNumbers> (),
size_of::< EnumOfOtherTypes> (),
size_of::< StructOfOtherTypes> (),
size_of::< ArrayAndI8> (),
);
}
< / code > < / pre > < / pre >
< p > When we print the size of these, we get < code > 2, 3, 32, 32, 1002< / code > . So if you were to do this, it would give an error:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" >
< span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // ⚠️
fn returns_just_a_trait() -> JustATrait {
let some_enum = EnumOfNumbers::I8(8);
some_enum
}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< p > It says:< / p >
< pre > < code class = "language-text" > error[E0746]: return type cannot have an unboxed trait object
--> src\main.rs:53:30
|
53 | fn returns_just_a_trait() -> JustATrait {
| ^^^^^^^^^^ doesn't have a size known at compile-time
< / code > < / pre >
< p > And this is true, because the size could be 2, 3, 32, 1002, or anything else. So we put it in a < code > Box< / code > instead. Here we also add the keyword < code > dyn< / code > . < code > dyn< / code > is a word that shows you that you are talking about a trait, not a struct or anything else.< / p >
< p > So you can change the function to this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" >
< span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // 🚧
fn returns_just_a_trait() -> Box< dyn JustATrait> {
let some_enum = EnumOfNumbers::I8(8);
Box::new(some_enum)
}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< p > And now it works, because on the stack is just a < code > Box< / code > and we know the size of < code > Box< / code > .< / p >
< p > You see this a lot in the form < code > Box< dyn Error> < / code > , because sometimes you can have more than one possible error.< / p >
< p > We can quickly create two error types to show this. To make an official error type, you have to implement < code > std::error::Error< / code > for it. That part is easy: just write < code > impl std::error::Error {}< / code > . But errors also need < code > Debug< / code > and < code > Display< / code > so they can give information on the problem. < code > Debug< / code > is easy with < code > #[derive(Debug)]< / code > but < code > Display< / code > needs the < code > .fmt()< / code > method. We did this once before.< / p >
< p > The code looks like this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > use std::error::Error;
use std::fmt;
#[derive(Debug)]
struct ErrorOne;
impl Error for ErrorOne {} // Now it is an error type with Debug. Time for Display:
impl fmt::Display for ErrorOne {
fn fmt(& self, f: & mut fmt::Formatter) -> fmt::Result {
write!(f, " You got the first error!" ) // All it does is write this message
}
}
#[derive(Debug)] // Do the same thing with ErrorTwo
struct ErrorTwo;
impl Error for ErrorTwo {}
impl fmt::Display for ErrorTwo {
fn fmt(& self, f: & mut fmt::Formatter) -> fmt::Result {
write!(f, " You got the second error!" )
}
}
// Make a function that just returns a String or an error
fn returns_errors(input: u8) -> Result< String, Box< dyn Error> > { // With Box< dyn Error> you can return anything that has the Error trait
match input {
0 => Err(Box::new(ErrorOne)), // Don't forget to put it in a box
1 => Err(Box::new(ErrorTwo)),
_ => Ok(" Looks fine to me" .to_string()), // This is the success type
}
}
fn main() {
let vec_of_u8s = vec![0_u8, 1, 80]; // Three numbers to try out
for number in vec_of_u8s {
match returns_errors(number) {
Ok(input) => println!(" {}" , input),
Err(message) => println!(" {}" , message),
}
}
}
< / code > < / pre > < / pre >
< p > This will print:< / p >
< pre > < code class = "language-text" > You got the first error!
You got the second error!
Looks fine to me
< / code > < / pre >
< p > If we didn't have a < code > Box< dyn Error> < / code > and wrote this, we would have a problem:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" >
< span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // ⚠️
fn returns_errors(input: u8) -> Result< String, Error> {
match input {
0 => Err(ErrorOne),
1 => Err(ErrorTwo),
_ => Ok(" Looks fine to me" .to_string()),
}
}
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< p > It will tell you:< / p >
< pre > < code class = "language-text" > 21 | fn returns_errors(input: u8) -> Result< String, Error> {
| ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
< / code > < / pre >
< p > This is not surprising, because we know that a trait can work on many things, and they each have different sizes.< / p >
< / main >
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