[mytailorisrich]

The difference according to Rust doc itsef:

reserve()

> Reserves capacity for at least additional more elements to be inserted in the given Vec<T>. The collection may reserve more space to speculatively avoid frequent reallocations.

reserve_exact()

> Reserves the minimum capacity for at least additional more elements to be inserted in the given Vec<T>. Unlike reserve, this will not deliberately over-allocate to speculatively avoid frequent allocations... Prefer reserve if future insertions are expected.

So if you know you'll need n now but will probably need more later, use reserve(). If you know n is all you'll need, use reserve_exact().

Vec<T> is a contiguous growable array, so the optimization game is to minimise allocations, reallocations (to keep it contiguous), and unused allocated memory.

[kstrauser]

I could look this up, but I’m enjoying reading this conversation.

Do reserve and reserve_exact increment the capacity, or ensure there’s still at least that much capacity remaining?

If the former, if I reserve(1) 10 times in a row, does that mean it could be rounding up to a thousand elements (say, because of the page table size) each time?

[tialaramex]

At least that much remaining. For both Vec::reserve and Vec::reserve_exact the parameter is an unsigned integer representing how many more items you expect to need space for.

So reserve(1) 10 times in a row will just repeatedly ensure there's enough space for at least 1 more item, which after the first time there certainly is†

There's an excellent chance the capacity check in particular got inlined, so the optimized machine code will not "actually" call a function ten times, it'll call it once and then maybe emit a single capacity check inline, the subsequent checks even a crap 1980s optimiser will go "We do not need to check that again" and skip it.

† If the Vec is full and can't grow, which really can happen for Zero Size Types in particular, then this panics, and what happens next is a compile time choice, in debug likely it just tells you what went wrong and suggests how to make a backtrace. In the ordinary case that we instead got to keep running then it succeeded.

[kstrauser]

That makes sense, and it's how I'd hope it'd work. I can imagine all sorts of cases where I'm getting data from an external source, like an API request returning some mudball of data monstrosity, where the easiest path doesn't give all the information at once.

Nice. I truly do appreciate the developer ergonomics that went into Rust. Its APIs are pleasantly well thought out.