pub fn trait_object()Expand description
§trait 对象本质
示例 1:
struct CloningLab {
subjects: Vec<Box<Mammal>>,
// subjects: Vec<Box<Mammal + Clone>>,
}
trait Mammal {
fn walk(&self);
fn run(&self);
}
#[derive(Clone)]
struct Cat {
meow_factor: u8,
purr_factor: u8
}
impl Mammal for Cat {
fn walk(&self) {
println!("Cat::walk");
}
fn run(&self) {
println!("Cat::run")
}
}
示例2:
#![allow(unused)]
#![feature(raw)]
use std::{mem, raw};
// an example trait
trait Foo {
fn bar(&self) -> i32;
}
impl Foo for i32 {
fn bar(&self) -> i32 {
*self + 1
}
}
fn main() {
let value: i32 = 123;
// let the compiler make a trait object
let object: &dyn Foo = &value;
// look at the raw representation
let raw_object: raw::TraitObject = unsafe { mem::transmute(object) };
// the data pointer is the address of `value`
assert_eq!(raw_object.data as *const i32, &value as *const _);
let other_value: i32 = 456;
// construct a new object, pointing to a different `i32`, being
// careful to use the `i32` vtable from `object`
let synthesized: &dyn Foo = unsafe {
mem::transmute(raw::TraitObject {
data: &other_value as *const _ as *mut (),
vtable: raw_object.vtable,
})
};
// it should work just as if we had constructed a trait object out of
// `other_value` directly
assert_eq!(synthesized.bar(), 457);
}
正常 trait Object 布局图：
cat's vtable
Cat Layout Trait Object
+------------+ +--------------+ +---------------+
| | | | | |
| | | | +-----> | drop pointer |
| meow_factor| | | | | size |
| +<--------------+data pointer | | | align |
| purr_factor| | | | | |
| | | vtable pointer-----+ | run fn pointer|
| | | | |walk fn pointer|
| | | | | |
+------------+ +--------------+ | |
| |
| |
| |
+---------------+
假设：trait Mammal + Clone 布局图：
注意：非法
Mammal
cat's vtable
Cat Layout Trait Object
+------------+ +--------------+ +---------------+
| | | | | |
| | | | +-----> | drop pointer |
| meow_factor| | | | | size |
| +<--------------+data pointer | | | align |
| purr_factor| | | | | |
| | | vtable pointer-----+ | run fn pointer|
| | | | |walk fn pointer|
| | | +-----+ | |
+------------+ +--------------+ | | |
| | |
| | |
| | |
| +---------------+
|
|
| Clone Vtable
|
+-----> +--------------+
| |
| drop pointe |
| |
| size |
| align |
| |
| clone |
| fn pointer |
| |
+--------------+
MammalClone
cat's vtable
Cat Layout Trait Object
+------------+ +--------------+ +-----------------+
| | | | | |
| | | | +-----> | drop pointer |
| meow_factor| | | | | size |
| +<--------------+data pointer | | | align |
| purr_factor| | | | | |
| | | vtable pointer-----+ | run fn pointer |
| | | | |walk fn pointer |
| | | | | |
+------------+ +--------------+ |clone fn pointer |
| |
| |
| |
+-----------------+