Static distribution and dynamic distribution in trust

rust The paradigm in belongs to static polymorphism , It is compile time polymorphism , No run-time performance loss . At compile time , Whether it's paradigm enumeration 、 Canonical functions and canonical structures , Will be singled out （Monomorphization）.
Take the following generic function as an example ：

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

#[derive(Debug)]
struct Foo;

trait Bar {
    
    fn baz(&self);
}

impl Bar for Foo {
    
    fn baz(&self) {
    
        println!("{:?}", self);
    }
}

//  Static distribution 
fn static_dispatch<T>(t: &T) where T: Bar {
    
    t.baz();
}

//  Dynamic distribution 
fn dynamic_dispatch(t: &dyn Bar) {
    
    t.baz();
}

static_dispatch The generic function is compiled , Will find Bar trait All implementations of , And generate an implementation for each of them , For example, in the above example , Only Foo A structure realizes Bar trait, In fact, at compile time static_dispatch Will generate

fn static_dispatch(t: &Foo){
    
    t.baz();
}

This method , If there are others Bar trait The implementation will also generate its corresponding concrete methods . That is, when compiling, each Bar trait All implementations of generate a specific method , There is no overhead when running .
Put the above method into https://play.rust-lang.org/ Choose ASM：

You can see that the generated compiled code is indeed singlet ：

and dynamic_dispatch The method input parameter of is t:&dyn Bar, It is a kind of dynamic distribution . Parameters t Type of dimension &Bar yes trait object .trait It is also an abstract rather than a concrete type , But its type size cannot be determined at compile time , therefore trait Objects must use fat pointers . You can use the reference operator & or Box<T> To make a trait object .trait Object is equivalent to the following structure ：

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

TraitObject Include two pointers ：data Pointers and vtable The pointer . With impl MyTrait for T For example ,data Pointer to trait Object saves type data T,vtable Pointer to include T Realized MyTrait Of vtable（virtual table, The name comes from C++, So it can be called virtual table ）. The essence of virtual table is a structure , Contains destructors , size , Information such as alignment and method .
At compile time , The compiler only knows TraitObject Contains information about pointers , And the size of the pointer is also determined （ Because it needs to be allocated on the stack , The size must be certain ）, But I don't know which method to call . During program operation , When there is trait_object.method() Method is called ,TraitObject Will find the correct function pointer from the virtual table according to the virtual table pointer , Then make dynamic calls （Java The runtime polymorphism of is similar ）. It's also going to be trait The reason why objects become dynamic distribution .

Reference material ：
1.《Rust The tao of programming 》（ Written by Zhang Hantong P61 P71）