Go data structures

Go Data Structures

Pick the structure that fits the access pattern — not the most familiar one. Slices and maps are the workhorses; arrays, container types, and the slices/maps packages cover the rest. Understanding the header layout, growth costs, and copy semantics of each turns most performance questions into one-line decisions.

Core Rules

1. Slices and maps are reference types — assigning copies the header, not the data. Use slices.Clone / maps.Clone for a true copy.
2. Preallocate with make([]T, 0, n) and make(map[K]V, n) whenever the size is known or estimable.
3. Always assign the result of append — the backing array may move.
4. Use slices and maps packages (Go 1.21+) instead of hand-rolled helpers.
5. map[K]struct{} is the canonical set — zero-byte values, no boolean ambiguity.
6. strings.Builder for string building, bytes.Buffer when you need io.Reader/io.Writer.

Picking a Structure

What do you need?
├─ Ordered, fixed compile-time size      → [N]T  array
├─ Ordered, dynamic size                 → []T   slice
│  ├─ Known size               → make([]T, 0, n)
│  └─ JSON output must be []   → []T{} literal (not nil)
├─ Key/value lookup                      → map[K]V
│  ├─ Need a set            → map[K]struct{}
│  └─ Known size            → make(map[K]V, n)
├─ Priority queue / top-k                → container/heap
├─ Frequent middle insertion             → container/list
├─ Fixed-size rolling window             → container/ring
├─ Pure string building                  → strings.Builder
└─ Read+write of bytes                   → bytes.Buffer

Slice Internals

A slice is a 3-word header: pointer, length, capacity. Multiple slices can alias the same backing array — s[1:4] shares memory with s.

Capacity Growth

The exact algorithm has changed across versions; do not rely on it. As of recent Go:

• len < 256 → capacity roughly doubles.
• len ≥ 256 → grows by ~25%.
• Each growth allocates a new backing array and copies — O(n) per growth.

Preallocation

users := make([]User, 0, len(ids))         // exact size
results := make([]Result, 0, estimated)    // approximate
s = slices.Grow(s, additional)             // pre-grow before bulk append (Go 1.21+)

slices Package (Go 1.21+)

Function	Purpose
Sort, SortFunc, SortStableFunc	sorting
BinarySearch, BinarySearchFunc	sorted lookup
Contains, Index, IndexFunc	search
Compact, CompactFunc	dedupe adjacent equals
Clone, Equal	safe copy / comparison
Delete, DeleteFunc	removal preserving order
Grow	preallocate before append
Concat (1.22+)	concatenate slices

Prefer these over hand-rolled loops — they're tested, generic, and use the fastest available paths.

Read references/slices-and-maps.md for capacity growth, aliasing pitfalls, and 2-D slice patterns.

nil vs Empty Slice: The JSON Trap

Both have len == 0 and cap == 0, but they encode differently:

var nilSlice []string         // → JSON: null
emptySlice := []string{}      // → JSON: []

API contracts almost always want []. Initialise the slice explicitly in any struct that gets marshaled to JSON, and treat nil/empty as identical when reading (use len(s) == 0).

For internal computation where nil is never marshaled, the nil slice is conventional and slightly cheaper (no allocation until first append).

Maps

Maps are hash tables with 8-entry buckets and overflow chains. They are reference types — assigning a map copies a pointer.

Preallocation

m := make(map[string]*User, len(users)) // avoids rehashing during population

The size hint is approximate (it's about bucket count), but it still saves repeated rehashing in the common case.

Sets

type Set[T comparable] map[T]struct{}

func (s Set[T]) Add(v T)         { s[v] = struct{}{} }
func (s Set[T]) Has(v T) bool    { _, ok := s[v]; return ok }
func (s Set[T]) Remove(v T)      { delete(s, v) }

struct{} is zero bytes; the set is just the key set of the underlying map.

map[K]bool is also common but ambiguous: did false mean "explicitly excluded" or "not present"? struct{} removes the question.

maps Package (Go 1.21+)

Clone, Equal/EqualFunc, DeleteFunc; Keys, Values, Collect, Insert since 1.23 (iterators).

Read references/strings-bytes-builder.md for string-vs-bytes, Builder vs Buffer, and rune handling.

Arrays

Fixed-size, value type, copied on assignment. Useful for compile-time-known sizes:

type Digest [32]byte
type IP4 [4]byte
cache := map[[2]int]Result{} // arrays are comparable → usable as map keys

For anything dynamic, use a slice.

container/* and Third-Party

Package	Use case	Caveat
container/heap	priority queue, top-K	implement the interface yourself
container/list	LRU, frequent middle splice	poor cache locality
container/ring	rolling window, round-robin	fixed size
bufio	I/O with many small reads/writes	always check Flush errors

For typed sets/queues/trees beyond the stdlib, prefer well-tested libraries (emirpasic/gods, gammazero/deque) and benchmark before optimising.

Read references/containers-and-pointers.md for heap implementation, unsafe.Pointer's six valid patterns, and weak.Pointer[T].

Copy Semantics Cheat Sheet

Type	Copy behaviour
primitives, arrays, structs	value (deep for contained value fields)
slice	header copied, backing array shared — use slices.Clone
map, channel	reference copied — use maps.Clone for maps
*T, interface	address / (type, value) pair copied

Anti-Patterns

Anti-pattern	Why it hurts	Do this instead
s := append(s, x) ignoring return	Backing array may move; s becomes stale	Always reassign
var m map[K]V; m[k] = v	nil map panic	m := make(map[K]V) or map[K]V{}
var s []T then marshal to JSON as []	Encodes as null	s := []T{}
make([]T, 0, 10000) "just in case"	Wasted memory	Size by actual data
m := map[K]bool{} as a set	false is ambiguous	map[K]struct{}
bytes.Buffer for pure string building	Extra copy in String()	strings.Builder
Large struct values in a map	Each lookup copies the value	map[K]*V

Verification Checklist

• Every make([]T, ...) and make(map[K]V, ...) has a capacity hint when the size is known.
• Every append reassigns its result.
• Slices marshaled to JSON are initialised as []T{}, not var s []T.
• All "sets" use map[K]struct{} (or a generic Set[T] wrapper).
• No bytes.Buffer used purely for String() output.
• No *sync.Mutex copied via struct assignment (go vet copylocks).
• slices.Clone / maps.Clone used when handing data to callers that may mutate.

References

• references/slices-and-maps.md — internals, growth, aliasing, slices/maps packages
• references/strings-bytes-builder.md — strings.Builder, bytes.Buffer, rune handling
• references/containers-and-pointers.md — container/heap, generic wrappers, unsafe.Pointer, weak.Pointer