Function std::mem::size_of

pub const fn size_of<T>() -> usize

Returns the size of a type in bytes.

More specifically, this is the offset in bytes between successive elements in an array with that item type including alignment padding. Thus, for any type T and length n, [T; n] has a size of n * size_of::<T>().

In general, the size of a type is not stable across compilations, but specific types such as primitives are.

The following table gives the size for primitives.

Type	size_of::<Type>()
()	0
u8	1
u16	2
u32	4
u64	8
i8	1
i16	2
i32	4
i64	8
f32	4
f64	8
char	4

Furthermore, usize and isize have the same size.

The types *const T, &T, Box<T>, Option<&T>, and Option<Box<T>> all have the same size. If T is Sized, all of those types have the same size as usize.

The mutability of a pointer does not change its size. As such, &T and &mut T have the same size. Likewise for *const T and *mut T.

Size of #[repr(C)] items

The C representation for items has a defined layout. With this layout, the size of items is also stable as long as all fields have a stable size.

Size of Structs

For structs, the size is determined by the following algorithm.

For each field in the struct ordered by declaration order:

1. Add the size of the field.
2. Round up the current size to the nearest multiple of the next field's alignment.

Finally, round the size of the struct to the nearest multiple of its alignment.

Unlike C, zero sized structs are not rounded up to one byte in size.

Size of Enums

Enums that carry no data other than the descriminant have the same size as C enums on the platform they are compiled for.

Size of Unions

The size of a union is the size of its largest field.

Unlike C, zero sized unions are not rounded up to one byte in size.

Examples

use std::mem;

// Some primitives
assert_eq!(4, mem::size_of::<i32>());
assert_eq!(8, mem::size_of::<f64>());
assert_eq!(0, mem::size_of::<()>());

// Some arrays
assert_eq!(8, mem::size_of::<[i32; 2]>());
assert_eq!(12, mem::size_of::<[i32; 3]>());
assert_eq!(0, mem::size_of::<[i32; 0]>());


// Pointer size equality
assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());

Using #[repr(C)].

use std::mem;

#[repr(C)]
struct FieldStruct {
    first: u8,
    second: u16,
    third: u8
}

// The size of the first field is 1, so add 1 to the size. Size is 1.
// The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
// The size of the second field is 2, so add 2 to the size. Size is 4.
// The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
// The size of the third field is 1, so add 1 to the size. Size is 5.
// Finally, the alignment of the struct is 2, so add 1 to the size for padding. Size is 6.
assert_eq!(6, mem::size_of::<FieldStruct>());

#[repr(C)]
struct TupleStruct(u8, u16, u8);

// Tuple structs follow the same rules.
assert_eq!(6, mem::size_of::<TupleStruct>());

// Note that reordering the fields can lower the size. We can remove both padding bytes
// by putting `third` before `second`.
#[repr(C)]
struct FieldStructOptimized {
    first: u8,
    third: u8,
    second: u16
}

assert_eq!(4, mem::size_of::<FieldStructOptimized>());

// Union size is the size of the largest field.
#[repr(C)]
union ExampleUnion {
    smaller: u8,
    larger: u16
}

assert_eq!(2, mem::size_of::<ExampleUnion>());