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There's two problems with this statement: - index checks aren't intrinsic for all cases where it is done, e.g. https://github.com/rust-lang/rust/blob/8f62c2920

by dbaupp 10y ago

There's two problems with this statement: - index checks aren't intrinsic for all cases where it is done, e.g. https://github.com/rust-lang/rust/blob/8f62c2920077eb5cb81323142fc5dbe6ae8813c0/src/libcore/slice.rs#L610-L618 https://github.com/rust-lang/rust/blob/8f62c2920077eb5cb8132... - being intrinsic is absolutely not required for the checks to be removed. Bounds checks are just a normal branch with a fairly straight-forward condition, and the always-true nature of those conditions is inferred in the same way for both the built-in indexing and non-built-in indexing. This is true in languages other than Rust. An iterator is designed to just not do any indexing at all (neither using the built-in [] operator or one of the functions that implements manual bounds checks), because it instead just manually (unsafely) walks a pointer along the array.

jdmichal 10y ago
> An iterator is designed to just not do any indexing at all (neither using the built-in [] operator or one of the functions that implements manual bounds checks), because it instead just manually (unsafely) walks a pointer along the array. It obviously still has to bound-check that it's not about to walk right out of the array. If you want to call this something other than a "bound-check", I think that's being overly pedantic.
- dbaupp 10y ago
  Yes, it checks that it's reached the end of the array as part of the loop, in the same place that `for (int i = 0; i < n; i++)` checks whether it's reached the end of the loop. I think this is different to the indexing bounds checks we've been discussing since the whole process is more controlled (compare and increment a pointer), rather than taking an arbitrary integer. But yes, strictly speaking you're right, an iterator does do check when it's reached the end of the array, it just doesn't do any indexing nor does it use the indexing checks built into the compiler (which is what you implied the iterators benefit from).

by dbaupp 10y ago

jdmichal 10y ago

> An iterator is designed to just not do any indexing at all (neither using the built-in [] operator or one of the functions that implements manual bounds checks), because it instead just manually (unsafely) walks a pointer along the array. It obviously still has to bound-check that it's not about to walk right out of the array. If you want to call this something other than a "bound-check", I think that's being overly pedantic.

dbaupp 10y ago
Yes, it checks that it's reached the end of the array as part of the loop, in the same place that `for (int i = 0; i < n; i++)` checks whether it's reached the end of the loop. I think this is different to the indexing bounds checks we've been discussing since the whole process is more controlled (compare and increment a pointer), rather than taking an arbitrary integer. But yes, strictly speaking you're right, an iterator does do check when it's reached the end of the array, it just doesn't do any indexing nor does it use the indexing checks built into the compiler (which is what you implied the iterators benefit from).