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Comment by robinei | Hacker News Reader

Since you are talking about a heterogeneous [edit: homogeneous] array, you would store the concrete structs continously in Rust as well. Rust would also not waste space in the vector for storing a vtable-pointer, and would instead construct fat pointers dynamically when needed (since it knows the type, it knows which vtable to inject in the fat pointer).

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> you would store the concrete structs continously in Rust as well. Rust would also not waste space in the vector for storing a vtable-pointer, and would instead construct fat pointers dynamically when needed (since it knows the type, it knows which vtable to inject in the fat pointer).

Uh, what? How is the compiler just supposed to magically know which of the structures in the array are of what type, without any additional identifying information? I'm assuming that in this optimized case, there's a hidden type field in each struct, that it would use to index into a table of vtable pointers? If so, there you go, that's yet another level of indirection.

I meant to say homogenous array, which I assume you were talking about.

No, I was talking about iterating over an array of objects calling their virtual functions (or either of the additional cases listed above). Of course it's easy to "do the right thing" with homogeneous arrays, either in the compiler or by hand if need be. But if you're iterating over a homogeneous array, calling the same virtual function on every single one, and your compiler somehow manages to notice this before you do, you probably screwed up in your design somewhere, so that's not the kind of problem I'm talking about.

It usually is smarter for performance to do the "data oriented design" thing and break the heterogeneous arrays into separate homogeneous arrays, so that you can potentially avoid a few levels of indirection, hoist loop invariants out, and maybe even make use of SIMD. But the whole point of the conversation was to talk about a nontrivial abstraction that (supposedly) trades performance for clarity. So I gave a scenario that would exercise that overhead.

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