Collections

From Rust Community Wiki
Revision as of 23:50, 8 June 2020 by Aloso (talk | contribs) (Sentence case for headings)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

Rust's standard library contains common collection types, such as sequences, sets, and maps. These live in the std::collectionsThis links to official Rust documentation module. In no_std environments, they can be accessed under alloc::collectionsThis links to official Rust documentation.

All collections are owned and allocated on the heap.

Sequences[edit | edit source]

Sequences can contain an arbitrary number of items of the same type. The standard library contains three sequence types: Vec, VecDeque, and LinkedList.

Vec[edit | edit source]

VecThis links to official Rust documentation is the most ubiquitous collection type. It is backed by a heap-allocated array. When items are added or removed from the Vec, the array grows or shrinks dynamically. Even though pushing items to a Vec sometimes requires copying the array, the amortized time to push an item is constant. Vec is the most performant sequence type in the standard library for many purposes, and the only one that can be sliced as it is stored contiguously in memory.

Length and capacity[edit | edit source]

Vec manages to achieve a constant amortized push complexity by allocating space for more elements than it contains. Since most of the time the underlying storage is in fact larger than the required storage, the push operation is simply a matter of copying one item to the end and incrementing the length.

When writing performance-critical code, reallocating can be costly. It is therefore advised to manually control the capacity of your Vec using methods such as Vec::with_capacityThis links to official Rust documentation and Vec::reserveThis links to official Rust documentation.

VecDeque[edit | edit source]

VecDequeThis links to official Rust documentation is a circular buffer. It works similar to Vec, except that inserting or removing items at the start of the sequence is more efficient.

LinkedList[edit | edit source]

LinkedListThis links to official Rust documentation is a doubly-linked list. It allows adding and removing items at both ends in constant time, but doesn't allow indexing items in constant time. LinkedList is less memory efficient and less cache friendly than Vec and VecDeque, which very often leads to worse performance.

Maps[edit | edit source]

Maps are data structures that map keys to values. The standard library contains two map types: HashMap and BTreeMap.

HashMap[edit | edit source]

HashMapThis links to official Rust documentation is an efficient implementation of a hash map. It internally uses the Cargo vec.svghashbrown crate.

TODO

BTreeMap[edit | edit source]

BTreeMapThis links to official Rust documentation

TODO

Sets[edit | edit source]

Sets are unordered collections that can't contain the same item more than once. The standard library contains the HashSetThis links to official Rust documentation and BTreeSetThis links to official Rust documentation types, which internally use the same implementation as HashMap and BTreeMap (the items are stored in the keys, and the values are empty).

BinaryHeap[edit | edit source]

The BinaryHeapThis links to official Rust documentation is a priority queue implemented with a binary heap.