Rust size_of用法及代碼示例

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>>>());

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 (because the largest alignment amongst its
// fields 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>());