When code involves polymorphism, there needs to be a mechanism to determine which specific version is actually run. This is called 'dispatch.' There are two major forms of dispatch: static dispatch and dynamic dispatch. While Rust favors static dispatch, it also supports dynamic dispatch through a mechanism called 'trait objects.'
For the rest of this chapter, we'll need a trait and some implementations.
Let's make a simple one, Foo. It has one method that is expected to return a
String.
trait Foo { fn method(&self) -> String; }
We'll also implement this trait for u8 and String:
impl Foo for u8 { fn method(&self) -> String { format!("u8: {}", *self) } } impl Foo for String { fn method(&self) -> String { format!("string: {}", *self) } }
We can use this trait to perform static dispatch with trait bounds:
fn do_something<T: Foo>(x: T) { x.method(); } fn main() { let x = 5u8; let y = "Hello".to_string(); do_something(x); do_something(y); }
Rust uses 'monomorphization' to perform static dispatch here. This means that
Rust will create a special version of do_something() for both u8 and
String, and then replace the call sites with calls to these specialized
functions. In other words, Rust generates something like this:
fn do_something_u8(x: u8) { x.method(); } fn do_something_string(x: String) { x.method(); } fn main() { let x = 5u8; let y = "Hello".to_string(); do_something_u8(x); do_something_string(y); }
This has some upsides: static dispatching of any method calls, allowing for inlining and hence usually higher performance. It also has some downsides: causing code bloat due to many copies of the same function existing in the binary, one for each type.
Furthermore, compilers aren’t perfect and may “optimise” code to become slower.
For example, functions inlined too eagerly will bloat the instruction cache
(cache rules everything around us). This is part of the reason that #[inline]
and #[inline(always)] should be used carefully, and one reason why using a
dynamic dispatch is sometimes more efficient.
However, the common case is that it is more efficient to use static dispatch, and one can always have a thin statically-dispatched wrapper function that does a dynamic, but not vice versa, meaning static calls are more flexible. The standard library tries to be statically dispatched where possible for this reason.
Rust provides dynamic dispatch through a feature called 'trait objects.' Trait
objects, like &Foo or Box<Foo>, are normal values that store a value of
any type that implements the given trait, where the precise type can only be
known at runtime. The methods of the trait can be called on a trait object via
a special record of function pointers (created and managed by the compiler).
A function that takes a trait object is not specialised to each of the types
that implements Foo: only one copy is generated, often (but not always)
resulting in less code bloat. However, this comes at the cost of requiring
slower virtual function calls, and effectively inhibiting any chance of
inlining and related optimisations from occurring.
Trait objects are both simple and complicated: their core representation and layout is quite straight-forward, but there are some curly error messages and surprising behaviours to discover.
There's two similar ways to get a trait object value: casts and coercions. If
T is a type that implements a trait Foo (e.g. u8 for the Foo above),
then the two ways to get a Foo trait object out of a pointer to T look
like:
let ref_to_t: &T = ...; // `as` keyword for casting let cast = ref_to_t as &Foo; // using a `&T` in a place that has a known type of `&Foo` will implicitly coerce: let coerce: &Foo = ref_to_t; fn also_coerce(_unused: &Foo) {} also_coerce(ref_to_t);
These trait object coercions and casts also work for pointers like &mut T to
&mut Foo and Box<T> to Box<Foo>, but that's all at the moment. Coercions
and casts are identical.
This operation can be seen as "erasing" the compiler's knowledge about the specific type of the pointer, and hence trait objects are sometimes referred to "type erasure".
Let's start simple, with the runtime representation of a trait object. The
std::raw module contains structs with layouts that are the same as the
complicated build-in types, including trait objects:
pub struct TraitObject { pub data: *mut (), pub vtable: *mut (), }
That is, a trait object like &Foo consists of a "data" pointer and a "vtable"
pointer.
The data pointer addresses the data (of some unknown type T) that the trait
object is storing, and the vtable pointer points to the vtable ("virtual method
table") corresponding to the implementation of Foo for T.
A vtable is essentially a struct of function pointers, pointing to the concrete
piece of machine code for each method in the implementation. A method call like
trait_object.method() will retrieve the correct pointer out of the vtable and
then do a dynamic call of it. For example:
struct FooVtable { destructor: fn(*mut ()), size: usize, align: usize, method: fn(*const ()) -> String, } // u8: fn call_method_on_u8(x: *const ()) -> String { // the compiler guarantees that this function is only called // with `x` pointing to a u8 let byte: &u8 = unsafe { &*(x as *const u8) }; byte.method() } static Foo_for_u8_vtable: FooVtable = FooVtable { destructor: /* compiler magic */, size: 1, align: 1, // cast to a function pointer method: call_method_on_u8 as fn(*const ()) -> String, }; // String: fn call_method_on_String(x: *const ()) -> String { // the compiler guarantees that this function is only called // with `x` pointing to a String let string: &String = unsafe { &*(x as *const String) }; string.method() } static Foo_for_String_vtable: FooVtable = FooVtable { destructor: /* compiler magic */, // values for a 64-bit computer, halve them for 32-bit ones size: 24, align: 8, method: call_method_on_String as fn(*const ()) -> String, };
The destructor field in each vtable points to a function that will clean up
any resources of the vtable's type, for u8 it is trivial, but for String it
will free the memory. This is necessary for owning trait objects like
Box<Foo>, which need to clean-up both the Box allocation and as well as the
internal type when they go out of scope. The size and align fields store
the size of the erased type, and its alignment requirements; these are
essentially unused at the moment since the information is embedded in the
destructor, but will be used in future, as trait objects are progressively made
more flexible.
Suppose we've got some values that implement Foo, the explicit form of
construction and use of Foo trait objects might look a bit like (ignoring the
type mismatches: they're all just pointers anyway):
let a: String = "foo".to_string(); let x: u8 = 1; // let b: &Foo = &a; let b = TraitObject { // store the data data: &a, // store the methods vtable: &Foo_for_String_vtable }; // let y: &Foo = x; let y = TraitObject { // store the data data: &x, // store the methods vtable: &Foo_for_u8_vtable }; // b.method(); (b.vtable.method)(b.data); // y.method(); (y.vtable.method)(y.data);
If b or y were owning trait objects (Box<Foo>), there would be a
(b.vtable.destructor)(b.data) (respectively y) call when they went out of
scope.
The use of language like "fat pointer" implies that a trait object is always a pointer of some form, but why?
Rust does not put things behind a pointer by default, unlike many managed languages, so types can have different sizes. Knowing the size of the value at compile time is important for things like passing it as an argument to a function, moving it about on the stack and allocating (and deallocating) space on the heap to store it.
For Foo, we would need to have a value that could be at least either a
String (24 bytes) or a u8 (1 byte), as well as any other type for which
dependent crates may implement Foo (any number of bytes at all). There's no
way to guarantee that this last point can work if the values are stored without
a pointer, because those other types can be arbitrarily large.
Putting the value behind a pointer means the size of the value is not relevant when we are tossing a trait object around, only the size of the pointer itself.