Cost of abstraction
- Rust course
- Expert
- Lesson 13/46
- 4 min
Learn how to measure the cost of abstraction.
The cost of abstraction
Rust's design principle, inherited from C++, is the zero-cost abstraction: "What you don't use, you don't pay for; what you do use, you couldn't hand-code any better." Abstractions like iterators, closures, generics and Option compile into the same machine code as a hand-written loop:
// abstract
let a: u32 = (1..=10).filter(|n| n % 2 == 0).sum();
// by hand
let mut b = 0;
let mut i = 1;
while i <= 10 {
if i % 2 == 0 { b += i; }
i += 1;
}In a release build the two versions usually produce identical or nearly identical assembly.
Measuring
Verify claims by measuring. The criterion crate does statistical benchmarks; to see the assembly, use Compiler Explorer (godbolt.org) or cargo asm:
// benches/toplam.rs (criterion)
c.bench_function("yineleyici", |b| {
b.iter(|| (1..=black_box(1000u32)).filter(|n| n % 2 == 0).sum::<u32>())
});Zero cost doesn't hold for every abstraction: Box<dyn Trait>, Rc, RefCell and Mutex are deliberately chosen tools with a real cost.
Practice Two ways, same result
Compute the sum of the even numbers from 1 to 10 first with an iterator chain, then with a while loop. Print both results separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let a: u32 = (1..=10).filter(|n| n % 2 == 0).sum();
let mut b = 0;
let mut i = 1;
while i <= 10 {
if i % 2 == 0 {
b += i;
}
i += 1;
}
println!("{} {}", a, b);
}Expected output
30 30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which build does the zero-cost claim apply to?
- The optimized (release) build Correct answer
- The debug build
- Both equally
- Neither
Explanation: In a debug build there's no inlining; abstractions really stay as function calls and are slow.
2 Abstract sum
let a: u32 = (1..=10).____(|n| n % 2 == 0).sum();let a: u32 = (1..=10).filter(|n| n % 2 == 0).sum();Explanation: filter selects the even numbers; sum adds them up.
3 Tool and its cost
| Term | Meaning |
|---|---|
Yineleyici zinciri |
Zero cost |
Generic fonksiyon |
Zero (code grows) |
Box<dyn Trait> |
Indirect call |
RefCell |
Runtime check |