[throwaway173738]

Suppose I think I may need 128 entries at some point, but the vector is allocated with room for 16 entries by default. I may not want to allocate and then immediately allocate again. But if I get to a 17th entry then I’m already causing allocation. So I might as well allocate 128 at that time so there are no more allocations at all.

[demurgos]

I believe that you are describing `Vec::with_capacity` which allows to change the initial reserved memory on construction.

`reserve` and `reserve_exact` are used when mutating an existing vec. What you provide is not the total wanted capacity but the additional wanted capacity.

`reserve` allows to avoid intermediate allocation.

Let's say that you have a vec with 50 items already and plan to run a loop to add 100 more (so 150 in total). The initial internal capacity is most likely 64, if you just do regular `push` calls without anything else, there will be two reallocations: one from 64 to 128 and one from 128 to 256.

If you call `reserve(100)`, you'll be able to skip the intermediate 64 to 128 reallocation: it will do a single reallocation from 64 to 256 and it will be able to handle the 100 pushes without any reallocation.

If you call `reserve_exact(100)`, you'll get a single reallocation for from 64 to 150 capacity, and also guarantee no reallocation during the processing loop.

The difference is that `reserve_exact` is better if these 100 items were the last ones you intended to push as you get a full vec of capacity 150 and containing 150 items. However, if you intend to push more items later, maybe 100 more, then you'd need to reallocate and break the amortized cost guarantees. With `reserve`, you don't break the amortized cost if there are follow-up inserts; at the price of not being at 100% usage all the time. In the `reserve` case, the capacity of 256 would be enough and let you go from 150 to 250 items without any reallocation.

In short, a rule of thumb could be:

- If creating a vec and you know the total count, prefer `Vec::with_capacity`

- If appending a final chunk of items and then no longer adding items, prefer `Vec::reserve_exact`

- If appending a chunk of items which may not be final, prefer `Vec::reserve`

[rsyring]

This should be in the docs or a blog post somewhere. Very clear explanation.

[demurgos]

That's a nice idea, thank you. I have personal blog, I'll try to clean it up a bit and provide performance measurements so it's worth posting.

Regarding the official documentation, I've returned to read them. I agree that the docs would benefit from more discussion about when to use each method. In particular, the code examples are currently exactly the same which is not great. Still, the most critical piece of information is there [0]

> Prefer `reserve` if future insertions are expected.

If anyone wants to reuse my explanation above, feel free to do it; no need to credit.

[0]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.res...

[unshavedyak]

It is, though not worded as nicely as the GP comment.

docs: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.res...

[arka2147483647]

All the functions mentioned above, even the cpp one, will reserve atleast the number of elements given to resize() or resize_exact(), but may reserve more than that.

After some pondering, and reading the rust documentation, I came to the conclusion that te difference is this: reserve() will grow the underlaying memory area to the next increment, or more than one increment, while reserve_exact() will only grow the underlaying memory area to the next increment, but no more than that.

Eg, if grow strategy is powers of two, and we are at pow(2), then reserve() may skip from pow(2) to pow(4), but reserve_exact() would be constrained to pow(3) as the next increment.

Or so i read the documentation. Hopefully someone can confirm?

[swiftcoder]

> even the cpp one, will reserve atleast the number of elements given

The C++ one, however, will not reserve more than you ask for (in the case that you reserve greater than the current capacity). It's an exact reservation in the rust sense.

> reserve() will grow the underlaying memory area to the next increment, or more than one increment, while reserve_exact() will only grow the underlaying memory area to the next increment, but no more than that

No, not quite. Reserve will request as many increments as it needs, and reserve_exact will request the exact total capacity it needs.

Where the docs get confusing, is that the allocator also has a say here. In either case, if you ask for 21 items, and the allocator decides it prefers to give you a full page of memory that can contain, say, 32 items... then the Vec will use all the capacity returned by the allocator.

[kbolino]

As far as I can tell, in the current implementation, reserve_exact is indeed exact. The only situation in which the capacity after calling reserve_exact will not equal length + additional is when it was already greater than that. Even if the allocator gives more than the requested amount of memory, the excess is ignored for the purposes of Vec's capacity: https://github.com/rust-lang/rust/blob/4b57d8154a6a74d2514cd...

Of course, this can change in the future; in particular, the entire allocator API is still unstable and likely won't stabilize any time soon.

[tialaramex]

Maybe more interestingly, line 659, slightly above that, explains that we know we got [u8] but today the ordinary Rust allocator promises capacity is correct, so we just ignore the length of that slice.

We could, as that comment suggests, check the slice and see if there's enough room for more than our chosen capacity. We could also debug_assert that it's not less room, 'cos the Allocator promised it would be big enough. I dunno if that's worthwhile.

[arka2147483647]

https://en.cppreference.com/w/cpp/container/vector/reserve.h...

says

> Increase the capacity of the vector (the total number of elements that the vector can hold without requiring reallocation) to a value that's greater or equal to new_cap.

I belive that the behaviour of reserve() is implementation defined.

[tialaramex]

Because there's only a single function here, it has to either be Vec::reserve or Vec::reserve_exact

If you don't offer Vec::reserve_exact then people who needed that run out of RAM and will dub your stdlib garbage. If you don't offer Vec::reserve as we've seen C++ programmers will say "Skill issue" whenever a noob gets awful performance as a result. So, it's an easy choice.

[mandarax8]

That said MSVC,GCC and clang all implement it to allocate an exact value.

[vlovich123]

> In either case, if you ask for 21 items, and the allocator decides it prefers to give you a full page of memory that can contain, say, 32 items... then the Vec will use all the capacity returned by the allocator.

It would be nice if this were true but AFAIK the memory allocator interface is busted - Rust inherits the malloc-style from C/C++ which doesn’t permit the allocator to tell the application “you asked for 128 bytes but I gave you an allocation for 256”. The alloc method just returns a naked u8 pointer.

[tialaramex]

The global allocator GlobalAlloc::alloc method does indeed return a naked pointer

But the (not yet stable) Allocator::allocate returns Result<NonNull<[u8]>, AllocError> --- that is, either a slice of bytes OR a failure.

Vec actually relies on Allocator not GlobalAlloc (it's part of the standard library so it's allowed to use unstable features)

So that interface is allowed to say you asked for 128 bytes but here's 256. Or, more likely, you asked for 940 bytes, but here's 1024. So if you were trying to make a Vec<TwentyByteThing> and Vec::with_capacity(47) it would be practical to adjust this so that when the allocator has 1024 bytes available but not 940 we get back a Vec with capacity 51 not 47.

[vlovich123]

You misread the documentation. Reserve-exact is precisely that - the growth strategy is ignored and you are ensured that at least that many more elements can be inserted without a reallocation. Eg reserve_exact(100) on an empty Vec allocates space for 100 elements.

By contrast reserve will allocate space for the extra elements following the growth strategy. If you reserve(100) on an empty Vec the allocation will be able to actually insert 128 (assuming the growth strategy is pow(n))

[tialaramex]

Actually that's not quite correct.

Vec::reserve(100) on an empty Vec will give you capacity 100, not 128 even though our amortization is indeed doubling.

The rules go roughly like this, suppose length is L, present capacity is C, reserve(N):

1. L + N < C ? Enough capacity already, we're done, return

2. L + N <= C * 2 ? Ordinary doubling, grow to capacity C * 2

3. Otherwise, try to grow to L + N

This means we can grow any amount more quickly than the amortized growth strategy or at the same speed - but never less quickly. We can go 100, 250, 600, 1300 and we can go 100, 200, 400, 800, 1600 - but we can''t do 100, 150, 200, 250, 300, 350, 400, 450, 500...