Coming from C++ I assumed this was the only way but Rust has an interesting approach where the single objects do not pay any cost because virtual dispatch is handled by fat pointers. So you carry around the `vptr` in fat pointers (`&dyn MyTrait`) only when needed, not in every instance.
Do you know how is this exactly deduced?
A specific type/reference to a type will always use static dispatch.
fn foo(bar: &Baz) { bar.thing(); }
A dyn trait reference will always use dynamic dispatch and carry around the vtable pointer.
fn foo(bar: &dyn BazTrait) { bar.thing(); }
One of the papers I had bookmarked when toying with my own language design was someone that had worked out how to make interfaces as fast or faster than vtables by using perfect hashing and using the vtable as a hash table instead of a list.
You can also, when inlining a polymorphic call, put a conditional block in that bounces back to full dispatch if the call occasionally doesn’t match the common case. The problem with polymorphic inlining though is that it quickly resembles the exact sort of code we delete and replace with polymorphic dispatch:
if (typeof arg1 == “string”) {
} else if typeof arg1 === …) {
} else if {
} else if {
} else {
}Could you explain this a bit more? The word "list" makes me think you might be thinking that virtual method lookup iterates over each element of the vtable, doing comparisons until it finds a match -- but I'm certain that this is not how virtual method invocation works in C++. The vtable is constructed at compile time and is already the simplest possible "perfect hashtable": a short, dense array with each virtual method mapping to a function pointer at a statically known index.
So these guys essentially assigned a hashcode to every function of every interface and then you would do dispatch instead of obj.vtable[12] you would do modular math x = singature.hash % len(obj.vtable) and call that.
I believe this was sometime around 2005-2008 and they found that it was fast enough on hardware of that era to be usable.
One caveat with "hash vtables" is that you only really see a performance win when the interface has a lot of specializations.
For instance, if you treat some collections as read only, you can define comprehensions across them with a single implementation. But that means the mutators have to be contained in another type, which a subset will implement, and may have covariant inputs.
There's absolutely nothing wrong with this code. It's just that it's not as extensible
It's a 'closed world' representation where the code assumes it knows about every possibility. This make extension more difficult
The code itself is extraordinarily good and performant.
With the polymorphic approach, I just have to create the new subtype, and all the users can do the right thing (if they were written with polymorphism in mind, anyway - if they use virtual functions on the base class).
For me, the key insight was from the last paragraph of the article:
C++23 introduces "deducing this", which is a way to avoid the performance cost of dynamic dispatch without needing to use tricks like CRRT, by writing:
class Base {
public:
auto foo(this auto&& self) -> int { return 77 + self.bar(); }
};
class Derived : public Base {
public:
auto bar() -> int { return 88; }
};
With concepts, templates and compile time execution, there is no need for CRTP, and in addition it can cover for better error messages regarding what methods to dispatch to.
But still CRTP is widely used in low-latency environments :)