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Trait Object的虚表探索

2022-11-15 15:59 2023-02-23 10:38 作者: xmh0511

````rust

use std::any::Any;

struct TestA{
    c:f64,
    c2:f64,
    c3:f64
}
impl TestA{
    fn memeber(&self){}
}
trait MyTrait{
    fn show(&self)->i32;
}
impl MyTrait for TestA{
    fn show(&self)->i32 {
        //self.v
        println!("invoke");
        0
    }
}

impl Drop for TestA{
    fn drop(&mut self) {
        println!("drop");
    }
}

#[repr(C)] // 保证数据和field一一对应
struct Vtable{
    drop:fn(* const ()),
    size:usize,
    alignment:usize,
    f:fn(* const())->i32
}

trait TraitForDyn{

}

impl<T:?Sized> TraitForDyn for &T{

}

fn test_trait<T:TraitForDyn>(v:T){}

fn main() {
    println!("{x}",x = std::mem::size_of::<TestA>());
    let i = TestA{c:0.0,c2:0.0,c3:0.0};
    unsafe {
      let pp = &i as * const TestA;
        println!("obj addresss: {:p}",pp);
    };
    let rf:& dyn MyTrait = &i;
   // let d = rf.show();
    unsafe {
      let ptr = &rf as * const &dyn MyTrait as *const * const();
      let data = *ptr;
      let vtable = *ptr.add(1);
      let vt = vtable as * const   Vtable;
      let vt = &*vt;
      let r = vt.size;
      let f2 = vt.f;
      let drp = vt.drop;
      let alignment = vt.alignment;
      f2(data);
      //drp(data);
      println!("obj size: {r}");
      println!("obj alignment {alignment}");
      println!("{vtable:?}");
    };

use std::fmt::Pointer;

unsafe {
        let ptr = rf as * const dyn MyTrait;
        let c = (ptr as * const ());
        println!("c addr: {c:p}");
        let size = std::mem::size_of::<* const dyn MyTrait>();
        println!("size: = {size}");
    };
}

````

https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/