4 ms·

So from what I've played with Option<Infallible> in rust would be directly equivalent to std::nullopt_t and has similar semantics in vectors, ie it just increme

by jpc0 2y ago

So from what I've played with Option<Infallible> in rust would be directly equivalent to std::nullopt_t and has similar semantics in vectors, ie it just increments length. There seems to be slightly more compile time work done in Rust but using the use case you described the concept of a Vec<Option<Infallible>> can just as easily be represented as std::vector<std::nullopt_t> with very similar semantics although possibly slightly less optimisation, although I would think if it was actually used in the language extensively I would see library implementors makeing it entirely compile time just like in rust. I don't disagree that algebraic data types are nice and that Rust has an interesting implementation of them, however this specific example doesn't seem unrepresentable in C++ using std::option and std::variant although the actual semantics relating to usage is easier in Rust.

Measter 2y ago
Bear in mind that the optimization on `Option<Infallible>` isn't specific to that type; it applies to anything that results in one possible value. For example, `Foo` in this example is also zero-sized: pub enum Foo { Foo, Bar(A), Baz(B), Qux(C), } pub enum A {} pub enum B {} pub enum C {} The optimization applied to Vec also applies to any zero-sized type.
tialaramex 2y ago
How do you get to the idea that std::vector<std::nullopt_t> "just increments length" ? I spent some time trying this out in Godbolt†, and then by hand, and then reading the Microsoft STL, and exactly as I'd assumed before I read your comment it was a growable array of one byte objects, with the type std::nullopt_t, 400 objects? 400 bytes. 4 million objects? 4 million bytes. † Including longer than I'd like to admit forgetting that C++ silently copies large objects without telling you and so my diagnostic was actually making a new std::vector because I forgot to write an ampersand to make it a reference... It is still possible I missed something and so I invite you to tell me what that was. But overall my thought is that you've missed the whole point and are in the same place as when C++ programmers get confused and think Empty types (like Infallible) and Zero Size Types (like Option<Infallible>) are somehow the same, which is like confusing zero (The additive identity) with one (the multiplicative identity).
- jpc0 2y ago
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by jpc0 2y ago

Measter 2y ago

Bear in mind that the optimization on `Option<Infallible>` isn't specific to that type; it applies to anything that results in one possible value. For example, `Foo` in this example is also zero-sized: pub enum Foo { Foo, Bar(A), Baz(B), Qux(C), } pub enum A {} pub enum B {} pub enum C {} The optimization applied to Vec also applies to any zero-sized type.

tialaramex 2y ago

How do you get to the idea that std::vector<std::nullopt_t> "just increments length" ? I spent some time trying this out in Godbolt†, and then by hand, and then reading the Microsoft STL, and exactly as I'd assumed before I read your comment it was a growable array of one byte objects, with the type std::nullopt_t, 400 objects? 400 bytes. 4 million objects? 4 million bytes. † Including longer than I'd like to admit forgetting that C++ silently copies large objects without telling you and so my diagnostic was actually making a new std::vector because I forgot to write an ampersand to make it a reference... It is still possible I missed something and so I invite you to tell me what that was. But overall my thought is that you've missed the whole point and are in the same place as when C++ programmers get confused and think Empty types (like Infallible) and Zero Size Types (like Option<Infallible>) are somehow the same, which is like confusing zero (The additive identity) with one (the multiplicative identity).

jpc0 2y ago
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