Comparators

(Supported by cpp, dart, and nanobind, queried with #[diplomat::cfg(supports=comparators)]. Intended to be supported by kotlin)

Some languages allow for overloading the </>/=/etc operators, similar to Rust’s PartialOrd.

To expose this over Diplomat, use the comparison attribute on a method that takes another Self parameter:

#![allow(unused)]
fn main() {
#[diplomat::bridge]
mod ffi {
    #[diplomat::opaque]
    #[derive(Ord, PartialOrd, Eq, PartialEq)]
    struct Foo(u8);

    impl Foo {
        #[diplomat::attr(auto, comparison)]
        pub fn compare(&self, other: &Foo) -> std::cmp::Ordering {
            self.cmp(other)
        }
    }
}
}

In Dart, this will provide an overloaded ==, and Foo will implement Comparable<Foo>.

We currently do not but Diplomat would like to also support the ability to just overload ==.

Partial Comparisons

(Supported by cpp, and nanobind, queried with #[diplomat::cfg(supports=partial_comparators)])

Some backends support partial comparisons, which return Option<std::cmp::Ordering>:

#![allow(unused)]
fn main() {
#[diplomat::bridge]
mod ffi {
    #[diplomat::opaque]
    #[derive(Ord, PartialOrd, Eq, PartialEq)]
    struct Foo(u8);

    impl Foo {
        #[diplomat::attr(auto, comparison)]
        pub fn compare(&self, other: &Foo) -> Option<std::cmp::Ordering> {
            self.cmp(other)
        }
    }
}
}

This is intended for comparison of types that implement PartialEq only, like f32; since NaN is a valid value for f32 and cannot be compared against other values (and so returns None).