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| // Niche optimization | |
| // A clever way to make Option<T> (or similar enums) use the exact same memory | |
| // size as T itself, without wasting space for a "tag" | |
| // (like a boolean for Some/None). | |
| use std::mem::{size_of, MaybeUninit}; | |
| fn main() { | |
| // Seeing the size overhead of using Option types | |
| println!("size_of::<i32>(): {}", size_of::<i32>()); | |
| println!( | |
| "size_of::<Option<i32>>(): {}", | |
| size_of::<Option<i32>>() /* `i32` can be a zero value (0) | |
| therefore Niche optimization cannot work here. Instead | |
| `None` will be given another byte size. */ | |
| ); | |
| // The size of a pointer (& and &mut) is usize. | |
| println!("---\nsize_of::<&i32>(): {}", size_of::<&i32>()); | |
| println!( | |
| "size_of::<Option<&i32>>(): {}", | |
| size_of::<Option<&i32>>() /* Niche optimization. Why does it | |
| work here and not in `Option<i32>` because a pointer (& or &mut) | |
| cannot be null or zero. */ | |
| ); | |
| println!("size_of::<usize>(): {}", size_of::<usize>()); | |
| println!("---"); | |
| // No overhead with MaybeUnint types; as types are uninitialized | |
| println!("size_of::<i32>(): {}", size_of::<i32>()); | |
| println!( | |
| "size_of::<MaybeUninit<i32>>(): {}", size_of::<MaybeUninit<i32>>() | |
| ); | |
| } |
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