Generic function
- Rust course
- Intermediate
- Lesson 7/115
- 4 min
Write generic functions with <T>.
Generic functions
Generics let the same code work with different types. The type parameter is declared in angle brackets, and trait bounds say what can be done with it:
fn en_buyuk<T: PartialOrd + Copy>(v: &[T]) -> T {
let mut eb = v[0];
for &x in v {
if x > eb {
eb = x;
}
}
eb
}Without PartialOrd, > can't be used; without Copy, v[0] can't be copied out of the slice.
Monomorphization
The compiler generates the generic function separately for each concrete type it's used with. The calls en_buyuk(&[1, 2]) and en_buyuk(&['a', 'b']) produce two separate pieces of machine code:
// what the compiler produces (conceptually)
fn en_buyuk_i32(v: &[i32]) -> i32 { .. }
fn en_buyuk_char(v: &[char]) -> char { .. }That's why generics have no runtime cost; the price is slightly longer compile times and a larger binary.
Practice Find the largest
Write the function en_buyuk<T: PartialOrd + Copy> above. Print the results for [3, 9, 2], ['a', 'z', 'm'] and [1.5, 2.5, 0.5] on one line separated by spaces.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn en_buyuk<T: PartialOrd + Copy>(v: &[T]) -> T {
let mut eb = v[0];
for &x in v {
if x > eb {
eb = x;
}
}
eb
}
fn main() {
println!("{} {} {}", en_buyuk(&[3, 9, 2]), en_buyuk(&['a', 'z', 'm']), en_buyuk(&[1.5, 2.5, 0.5]));
}Expected output
9 z 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>fn f<T>(a: T, b: T) -> T { a + b }</code> compile?
- It compiles
- Addition isn't known to be defined for T; T: std::ops::Add<Output = T> is needed Correct answer
- A generic function can't return a value
- Two parameters can't share a type
Explanation: Every operation in generic code must be backed by a trait bound; the compiler assumes nothing else about T.
2 Type parameter
fn en_buyuk____(v: &[T]) -> T {
// ...
}fn en_buyuk<T: PartialOrd + Copy>(v: &[T]) -> T {
// ...
}Explanation: T needs the PartialOrd and Copy bounds for comparing and copying.
3 Call and T
| Term | Meaning |
|---|---|
en_buyuk(&[3, 9, 2]) |
i32 |
en_buyuk(&['a', 'z']) |
char |
en_buyuk(&[1.5, 2.5]) |
f64 |
en_buyuk(&[1u8, 7]) |
u8 |