NonZeroUsize in std::num - Rust (original) (raw)

Type Alias NonZeroUsize

1.28.0 · Source

pub type NonZeroUsize = NonZero<usize>;

Expand description

A usize that is known not to equal zero.

This enables some memory layout optimization. For example, Option<NonZeroUsize> is the same size as usize:

assert_eq!(size_of::<Option<core::num::NonZeroUsize>>(), size_of::<usize>());

§Layout

NonZeroUsize is guaranteed to have the same layout and bit validity as usizewith the exception that 0 is not a valid instance.Option<NonZeroUsize> is guaranteed to be compatible with usize, including in FFI.

Thanks to the null pointer optimization,NonZeroUsize and Option<NonZeroUsize>are guaranteed to have the same size and alignment:

use std::num::NonZeroUsize;

assert_eq!(size_of::<NonZeroUsize>(), size_of::<Option<NonZeroUsize>>());
assert_eq!(align_of::<NonZeroUsize>(), align_of::<Option<NonZeroUsize>>());

struct NonZeroUsize(/* private fields */);