CODEWITHSUNDEEP
vectorrustmemory-management
Moold
Moold

Reputation: 107

How to best keep the first N elements in Vec and release unused capacity?

I want to keep only the first 2 elements in a Vec and release any unused capacity. Here is my current solution:

let mut data = vec![1, 2, 3, 4, 5, 6]; // produced by another function
data.truncate(2);
data.shrink_to_fit();

Is there a better way to do this?

Upvotes: 2

Views: 1103

Answers (2)

tmaj
tmaj

Reputation: 34987

Rust docs https://static.rust-lang.org/doc/master/std/vec/struct.Vec.html#guarantees

In general, Vec's allocation details are subtle enough that it is strongly recommended that you only free memory allocated by a Vec by creating a new Vec and dropping it.

I'm a Rust noob, but it seems to say that the solution would be:

let v = vec![v[0], v[1]];

(or vec![&v[0], &v[1]] if appropriate);

BTW. https://static.rust-lang.org/doc/master/std/vec/struct.Vec.html#guarantees also says:

push and insert will never (re)allocate if the reported capacity is sufficient. push and insert will (re)allocate if len()==capacity(). That is, the reported capacity is completely accurate, and can be relied on. It can even be used to manually free the memory allocated by a Vec if desired.

I don't understand how to use this information :)

Upvotes: 1

John Kugelman
John Kugelman

Reputation: 361605

Truncating and shrinking is the best way. Releasing unused capacity is a distinct operation; there's no way around it. Rust doesn't do it automatically since you might be removing and then adding more elements.

Upvotes: 2

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