4

我想我可以或多或少地尝试从头开始构建一个特征对象而不使用这些impl块。详细说明：

trait SomeTrait {
    fn fn_1(&self);
    fn fn_2(&self, a: i64);
    fn fn_3(&self, a: i64, b: i64);
}

struct TraitObject {
    data: *mut (),
    vtable: *mut (),
}

fn dtor(this: *mut ()) {
    // ...
}

fn imp_1(this: *mut ()) {
    // ...
}

fn imp_2(this: *mut (), a: i64) {
    // ...
}

fn imp_3(this: *mut (), a: i64, b: i64) {
    // ...
}

fn main() {
    let data = &... as *mut (); // something to be the object
    let vtable = [dtor as *mut (),
                  8 as *mut (),
                  8 as *mut (),
                  imp_1 as *mut (),
                  imp_2 as *mut (),
                  imp_3 as *mut ()]; // ignore any errors in typecasting,
        //this is not what I am worried about getting right

    let to = TraitObject {
        data: data,
        vtable: vtable.as_ptr() as *mut (),
    };
    // again, ignore any typecast errors,

    let obj: &SomeTrait = unsafe { mem::transmute(to) };

    // ...

    obj.fn_1();
    obj.fn_2(123);
    obj.fn_3(123, 456);
}

据我了解，成员函数出现在特征定义中的顺序并不总是与函数指针出现在 VTable 中的顺序相同。有没有办法确定 VTable 中每个特征方法的偏移量？

4

1 回答 1

3

如果您不介意在运行时检测布局，则可以比较特定偏移处的函数地址，并将它们与已知的虚拟实现的地址进行比较以匹配它们。这假设您知道特征中有多少方法，因为您可能需要阅读所有方法。

use std::mem;

trait SomeTrait {
    fn fn_1(&self);
    fn fn_2(&self, a: i64);
    fn fn_3(&self, a: i64, b: i64);
}

struct Dummy;

impl SomeTrait for Dummy {
    fn fn_1(&self) { unimplemented!() }
    fn fn_2(&self, _a: i64) { unimplemented!() }
    fn fn_3(&self, _a: i64, _b: i64) { unimplemented!() }
}

struct TraitObject {
    data: *mut (),
    vtable: *mut (),
}

fn main() {
    unsafe {
        let fn_1 = Dummy::fn_1 as *const ();
        let fn_2 = Dummy::fn_2 as *const ();
        let fn_3 = Dummy::fn_3 as *const ();

        let dummy = &mut Dummy as &mut SomeTrait;
        let dummy: TraitObject = mem::transmute(dummy);
        let vtable = dummy.vtable as *const *const ();
        let vtable_0 = *vtable.offset(3);
        let vtable_1 = *vtable.offset(4);
        let vtable_2 = *vtable.offset(5);

        // Mapping vtable offsets to methods is left as an exercise to the reader. ;)
        println!("{:p} {:p} {:p}", fn_1, fn_2, fn_3);
        println!("{:p} {:p} {:p}", vtable_0, vtable_1, vtable_2);
    }
}
于 2016-09-27T00:33:18.073 回答