In Rust, references can never be null, so in case where you actually need null, such as a linked list, you use the Option
type:
struct Element {
value: i32,
next: Option<Box<Element>>,
}
How much overhead is involved in this in terms of memory allocation and steps to dereference compared to a simple pointer? Is there some "magic" in the compiler/runtime to make Option
cost-free, or less costly than if one were to implement Option
by oneself in a non-core library using the same enum
construct, or by wrapping the pointer in a vector?
Yes, there is some compiler magic that optimises Option<ptr>
to a single pointer (most of the time).
use std::mem::size_of;
macro_rules! show_size {
(header) => (
println!("{:<22} {:>4} {}", "Type", "T", "Option<T>");
);
($t:ty) => (
println!("{:<22} {:4} {:4}", stringify!($t), size_of::<$t>(), size_of::<Option<$t>>())
)
}
fn main() {
show_size!(header);
show_size!(i32);
show_size!(&i32);
show_size!(Box<i32>);
show_size!(&[i32]);
show_size!(Vec<i32>);
show_size!(Result<(), Box<i32>>);
}
The following sizes are printed (on a 64-bit machine, so pointers are 8 bytes):
// As of Rust 1.22.1
Type T Option<T>
i32 4 8
&i32 8 8
Box<i32> 8 8
&[i32] 16 16
Vec<i32> 24 24
Result<(), Box<i32>> 8 16
Note that &i32
, Box
, &[i32]
, Vec<i32>
all use the non-nullable pointer optimization inside an Option
!