Niche optimization

Shrink enum size with niche optimization.

Niche optimization

Some types have bit patterns that are never used: &T and Box<T> are never null, bool is only 0 or 1, NonZeroU32 can't be 0, and char lies outside certain ranges. The compiler uses these "niches" to store the enum discriminant, so Option needs no extra byte:

size_of::<Option<&u8>>()        // 8: None = null pointer
size_of::<Option<NonZeroU32>>()  // 4: None = 0
size_of::<Option<u32>>()         // 8: u32 has no unused pattern
size_of::<Option<bool>>()        // 1: None = 2

A niche in your own type

Holding ids as NonZeroU32 lets you express the "invalid id" case with Option for free:

use std::num::NonZeroU32;

struct KullaniciId(NonZeroU32);

let id = NonZeroU32::new(42).map(KullaniciId);   // Some
let bos = NonZeroU32::new(0);                     // None
println!("{}", size_of::<Option<KullaniciId>>()); // 4

Niches work when nested too: Option<Option<bool>> is still 1 byte.

Exercise Niche sizes

Print the size_of values of Option<&u8>, Option<NonZeroU32>, Option<u32> and Option<bool> on one line separated by spaces.

Show example solution

use std::mem::size_of;
use std::num::NonZeroU32;

fn main() {
    println!(
        "{} {} {} {}",
        size_of::<Option<&u8>>(),
        size_of::<Option<NonZeroU32>>(),
        size_of::<Option<u32>>(),
        size_of::<Option<bool>>()
    );
}

Test yourself

Why can <code>Option<Box<T>></code> be used in FFI in place of C's nullable pointer?
- Box exists in C
- It can't be
- It's always 16 bytes
- None is guaranteed to be represented as a null pointer
Show answer

None is guaranteed to be represented as a null pointer — This is one of a few documented niche guarantees: Option<&T>, Option<Box<T>>, Option<NonNull<T>>, Option<extern fn>.

Non-zero integer

use std::num::____;
let id = NonZeroU32::new(42);   // Some(42)

Show answer

use std::num::NonZeroU32;
let id = NonZeroU32::new(42);   // Some(42)

Type and size (64-bit)

Option<&u8>

8
Option<NonZeroU32>

4
Option<u32>

8 (extra tag)
Option<bool>

1

Next lesson: Summary - Memory layout

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