[Question] When to pass copy vs ref?

[Indra-db]

Hi there,

I'm new to Rust and I'm implementing my own vec3 class for practice, but I'm wondering when to pass as copy vs const ref in functions owned by the class. so I figured to check popular math crates and stumbled upon here.

Most of the time I see you just copy the other vector and self but occasionally &self is passed and I don't understand when to use what 🤔

code I'm referring to; https://docs.rs/glam/0.24.1/src/glam/f32/vec3.rs.html#172-178

Could you explain it to me please?

[bitshifter]

There's not really a hard and fast rule and I don't remember the rules of the Rust ABI. Most of glam is inlined, so it shouldn't make too much difference which one is used. Usually I look at the generated assembly and maybe write a benchmark with both styles to see what seems best. Most likely in an optimised build they will be the same, there might be differences in debug builds which can be a consideration In glam I mostly only use &self with large structures like 4x4 matrices and even then, I don't recall if the asm was different, I made those decisions a while ago :)

Here's an example in godbolt, with one implementation of cross taking self and the other taking &self, the generated code is the same for both in debug and optimized builds. In the optimised build it looks like it's only generated one of them because the compiler has worked out they're exactly the same.

[Indra-db]

Thank you!