Rust中的胖指针怎么使用

trait SomeTrait { }fn main() {    println!("======== The size of different pointers in Rust: ========");    println!("&dyn Trait:-----{}", size_of::<&dyn SomeTrait>());    println!("&[&dyn Trait]:--{}", size_of::<&[&dyn SomeTrait]>());    println!("Box<Trait>:-----{}", size_of::<Box<SomeTrait>>());    println!("&i32:-----------{}", size_of::<&i32>());    println!("&[i32]:---------{}", size_of::<&[i32]>());    println!("Box<i32>:-------{}", size_of::<Box<i32>>());    println!("&Box<i32>:------{}", size_of::<&Box<i32>>());    println!("[&dyn Trait;4]:-{}", size_of::<[&dyn SomeTrait; 4]>());    println!("[i32;4]:--------{}", size_of::<[i32; 4]>());}AI代码助手复制代码

// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)trait Test {    fn add(&self) -> i32;    fn sub(&self) -> i32;    fn mul(&self) -> i32;}// This will represent our home brewn fat pointer to a trait object   #[repr(C)]struct FatPointer<'a> {    /// A reference is a pointer to an instantiated `Data` instance    data: &'a mut Data,    /// Since we need to pass in literal values like length and alignment it's    /// easiest for us to convert pointers to usize-integers instead of the other way around.    vtable: *const usize,}// This is the data in our trait object. It's just two numbers we want to operate on.struct Data {    a: i32,    b: i32,}// ====== function definitions ======fn add(s: &Data) -> i32 {    s.a + s.b}fn sub(s: &Data) -> i32 {    s.a - s.b}fn mul(s: &Data) -> i32 {    s.a * s.b}fn main() {    let mut data = Data {a: 3, b: 2};    // vtable is like special purpose array of pointer-length types with a fixed    // format where the three first values has a special meaning like the    // length of the array is encoded in the array itself as the second value.    let vtable = vec![        0,            // pointer to `Drop` (which we're not implementing here)        6,            // lenght of vtable        8,            // alignment        // we need to make sure we add these in the same order as defined in the Trait.        add as usize, // function pointer - try changing the order of `add`        sub as usize, // function pointer - and `sub` to see what happens        mul as usize, // function pointer    ];    let fat_pointer = FatPointer { data: &mut data, vtable: vtable.as_ptr()};    let test = unsafe { std::mem::transmute::<FatPointer, &dyn Test>(fat_pointer) };    // And voalá, it's now a trait object we can call methods on    println!("Add: 3 + 2 = {}", test.add());    println!("Sub: 3 - 2 = {}", test.sub());    println!("Mul: 3 * 2 = {}", test.mul());}AI代码助手复制代码