Comment by TazeTSchnitzel

15 hours ago

I'm not sure if this quite answers your question, but the difference in philosophy here is that C++ objects with pointers always have identity, whereas a Rust object only has identity if it has a non-zero size. It's an application of the zero-overhead principle, "you only pay for what you use".

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TazeTSchnitzel

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phire  8 hours ago

> whereas a Rust object only has identity if it has a non-zero size.

Rust also has the complication that function pointers are not guaranteed to have an unique identity; If multiple functions compile to the same code, the compiler is allowed de-duplicate them.

The documentation [0] also warns it's also possible for the compiler to create multiple versions of the same function. And while I've absolutely seen the compiler to create multiple optimised versions of functions in disassembled code (partial inlining based on the caller), I'm not sure it's possible to get pointers to more than one version.

[0] https://doc.rust-lang.org/std/ptr/fn.fn_addr_eq.html

ketzu  15 hours ago

"zero sized objects don't have identity" actually would answer my question and is a really interesting factoid, thanks!

  • tialaramex  14 hours ago

    This is why C++ doesn't have ZSTs, it wants all objects to have identities, the obvious way to distinguish them is by where they are in memory, but ZSTs don't have distinct addresses in memory.