是什么使某物成为“特征对象”？

Question

最近的 Rust 更改使“特征对象”对我来说更加突出，但我对究竟是什么使某些东西成为特征对象只有模糊的把握。一项特别的更改是 upcoming change 允许 trait 对象将 trait 实现转发到内部类型。

给定一个特征Foo，我很确定Box<Foo> / Box<dyn Foo> 是一个特征对象。 &Foo / &dyn Foo 也是 trait 对象吗？像Rc 或Arc 这样的其他智能指针呢？我怎样才能使自己的类型算作 trait 对象？

reference 只提到一次 trait 对象，但没有像定义一样。

Answer 1

Trait 对象是动态调度的 Rust 实现。动态分派允许在运行时选择多态操作（特征方法）的一种特定实现。动态调度允许非常灵活的架构，因为我们可以在运行时交换函数实现。但是，动态调度的运行时成本很小。

保存特征对象的变量/参数是胖指针，由以下组件组成：

• 指向内存中对象的指针
• 指向该对象的 vtable 的指针，vtable 是一个带有指向实际方法实现的指针的表。

示例

struct Point {
    x: i64,
    y: i64,
    z: i64,
}

trait Print {
    fn print(&self);
}

// dyn Print is actually a type and we can implement methods on it
impl dyn Print + 'static {
    fn print_traitobject(&self) {
        println!("from trait object");
    }
}

impl Print for Point {
    fn print(&self) {
        println!("x: {}, y: {}, z: {}", self.x, self.y, self.z);
    }
}

// static dispatch (compile time): compiler must know specific versions
// at compile time generates a version for each type

// compiler will use monomorphization to create different versions of the function
// for each type. However, because they can be inlined, it generally has a faster runtime
// compared to dynamic dispatch
fn static_dispatch<T: Print>(point: &T) {
    point.print();
}

// dynamic dispatch (run time): compiler doesn't need to know specific versions
// at compile time because it will use a pointer to the data and the vtable.
// The vtable contains pointers to all the different different function implementations.
// Because it has to do lookups at runtime it is generally slower compared to static dispatch

// point_trait_obj is a trait object
fn dynamic_dispatch(point_trait_obj: &(dyn Print + 'static)) {
    point_trait_obj.print();
    point_trait_obj.print_traitobject();
}

fn main() {
    let point = Point { x: 1, y: 2, z: 3 };

    // On the next line the compiler knows that the generic type T is Point
    static_dispatch(&point);

    // This function takes any obj which implements Print trait
    // We could, at runtime, change the specfic type as long as it implements the Print trait
    dynamic_dispatch(&point);
}

Answer 2

当你有一个指向特性的指针时，你就有了特性对象。 Box、Arc、Rc 和引用 & 本质上都是指针。在定义“特征对象”方面，它们的工作方式相同。

“特征对象”是 Rust 对 dynamic dispatch 的看法。 这是一个示例，我希望能帮助说明什么是 trait 对象：

// define an example struct, make it printable
#[derive(Debug)]
struct Foo;

// an example trait
trait Bar {
    fn baz(&self);
}

// implement the trait for Foo
impl Bar for Foo {
    fn baz(&self) {
        println!("{:?}", self)
    }
}

// This is a generic function that takes any T that implements trait Bar.
// It must resolve to a specific concrete T at compile time.
// The compiler creates a different version of this function
// for each concrete type used to call it so &T here is NOT
// a trait object (as T will represent a known, sized type
// after compilation)
fn static_dispatch<T>(t: &T)
where
    T: Bar,
{
    t.baz(); // we can do this because t implements Bar
}

// This function takes a pointer to a something that implements trait Bar
// (it'll know what it is only at runtime). &dyn Bar is a trait object.
// There's only one version of this function at runtime, so this
// reduces the size of the compiled program if the function
// is called with several different types vs using static_dispatch.
// However performance is slightly lower, as the &dyn Bar that
// dynamic_dispatch receives is a pointer to the object +
// a vtable with all the Bar methods that the object implements.
// Calling baz() on t means having to look it up in this vtable.
fn dynamic_dispatch(t: &dyn Bar) {
    // ----------------^
    // this is the trait object! It would also work with Box<dyn Bar> or
    // Rc<dyn Bar> or Arc<dyn Bar>
    //
    t.baz(); // we can do this because t implements Bar
}

fn main() {
    let foo = Foo;
    static_dispatch(&foo);
    dynamic_dispatch(&foo);
}

供进一步参考，有一个很好的Trait Objects chapter of the Rust book

Answer 3

简短回答：您只能将对象安全的特征转换为特征对象。

对象安全特征：无法解析为具体实现类型的特征。在实践中，两个规则控制一个特征是否是对象安全的。

1. 返回类型不是 Self。
2. 没有泛型类型参数。

任何满足这两个规则的特征都可以用作特征对象。

对象安全的特征示例可用作特征对象：

trait Draw {
    fn draw(&self);
}

不能用作特征对象的特征示例：

trait Draw {
    fn draw(&self) -> Self;
}

详解：https://doc.rust-lang.org/book/second-edition/ch17-02-trait-objects.html