Trait bound
- Rust course
- Intermediate
- Lesson 19/115
- 4 min
Constrain a generic parameter with <T: Trait>.
Trait bounds
A trait bound says which behaviours a generic type must have. Writing T: Display lets T values be printed with {} inside the function:
use std::fmt::Display;
fn cerceve<T: Display>(x: T) -> String {
format!("[{}]", x)
}
println!("{} {}", cerceve(5), cerceve("a")); // [5] [a]The bound both permits the function body and restricts the caller: calling with a type that doesn't implement Display is a compile error.
Without a bound
Without a bound the compiler assumes nothing about T, and the error message says what's missing:
fn cerceve<T>(x: T) -> String {
format!("[{}]", x)
}
// error[E0277]: `T` doesn't implement `std::fmt::Display`
// help: consider restricting type parameter `T`
// fn cerceve<T: std::fmt::Display>(x: T)Following the compiler's suggestion is often the fastest way to find the right bound.
Practice Frame it
Write the function fn cerceve<T: Display>(x: T) -> String above. Print the results for 5, "a" and 2.5 on one line separated by spaces.
Starter code
use std::fmt::Display;
fn main() {
// buraya yaz
}Sample solution
use std::fmt::Display;
fn cerceve<T: Display>(x: T) -> String {
format!("[{}]", x)
}
fn main() {
println!("{} {} {}", cerceve(5), cerceve("a"), cerceve(2.5));
}Expected output
[5] [a] [2.5]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why doesn't <code>cerceve(vec![1, 2])</code> compile?
- It compiles
- Vec<i32> doesn't implement Display Correct answer
- Vec can't be generic
- Two elements is too many
Explanation: Vectors print with {:?} (Debug); {} needs Display, which Vec doesn't implement.
2 Printable
fn cerceve<T: ____>(x: T) -> String {
format!("[{}]", x)
}fn cerceve<T: Display>(x: T) -> String {
format!("[{}]", x)
}Explanation: The {} format requires the Display trait.
3 Bound and what it permits
| Term | Meaning |
|---|---|
T: Display |
Printing with {} |
T: Debug |
Printing with {:?} |
T: PartialOrd |
< and > comparison |
T: Clone |
.clone() |