Golang data structures — slices/maps internals (capacity growth, preallocation, hash buckets), slices/maps packages, container/list/heap/ring, strings.Builder vs bytes.Buffer, generic collections, unsafe/weak pointers. Use when choosing or optimizing Go data structures or slice/map internals.
Golang data structures — slices/maps internals (capacity growth, preallocation, hash buckets), slices/maps packages, container/list/heap/ring, strings.Builder vs bytes.Buffer, generic collections, unsafe/weak pointers. Use when choosing or optimizing Go data structures or slice/map internals.
user-invocable
true
license
MIT
compatibility
Designed for Claude Code, Codex or similar harness, and for projects using Golang.
Persona: You are a Go engineer who understands data structure internals. You choose the right structure for the job — not the most familiar one — by reasoning about memory layout, allocation cost, and access patterns.
Go Data Structures
Built-in and standard library data structures: internals, correct usage, and selection guidance. For safety pitfalls (nil maps, append aliasing, defensive copies) see samber/cc-skills-golang@golang-safety skill. For channels and sync primitives see samber/cc-skills-golang@golang-concurrency skill. For string/byte/rune choice see samber/cc-skills-golang@golang-design-patterns skill.
Best Practices Summary
Preallocate slices and maps with make(T, 0, n) / make(map[K]V, n) when size is known or estimable — avoids repeated growth copies and rehashing
Arrays SHOULD be preferred over slices only for fixed, compile-time-known sizes (hash digests, IPv4 addresses, matrix dimensions)
NEVER rely on slice capacity growth timing — the growth algorithm changed between Go versions and may change again; your code should not depend on when a new backing array is allocated
Use container/heap for priority queues, container/list only when frequent middle insertions are needed, container/ring for fixed-size circular buffers
strings.Builder MUST be preferred for building strings; bytes.Buffer MUST be preferred for bidirectional I/O (implements both io.Reader and io.Writer)
Generic data structures SHOULD use the tightest constraint possible — comparable for keys, custom interfaces for ordering
unsafe.Pointer MUST only follow the 6 valid conversion patterns from the Go spec — NEVER store in a uintptr variable across statements
weak.Pointer[T] (Go 1.24+) SHOULD be used for caches and canonicalization maps to allow GC to reclaim entries
Slice Internals
A slice is a 3-word header: pointer, length, capacity. Multiple slices can share a backing array (→ see samber/cc-skills-golang@golang-safety for aliasing traps and the header diagram).
Each growth copies the entire backing array — O(n)
Preallocation
// Exact size known
users := make([]User, 0, len(ids))
// Approximate size known
results := make([]Result, 0, estimatedCount)
// Pre-grow before bulk append (Go 1.21+)
s = slices.Grow(s, additionalNeeded)
slices Package (Go 1.21+)
Key functions: Sort/SortFunc, BinarySearch, Contains, Compact, Grow. For Clone, Equal, DeleteFunc → see samber/cc-skills-golang@golang-safety skill.
Slice Internals Deep Dive — Full slices package reference, growth mechanics, len vs cap, header copying, backing array aliasing.
Map Internals
Maps are hash tables with 8-entry buckets and overflow chains. They are reference types — assigning a map copies the pointer, not the data.
Preallocation
m := make(map[string]*User, len(users)) // avoids rehashing during population
maps Package Quick Reference (Go 1.21+)
Function
Purpose
Collect (1.23+)
Build map from iterator
Insert (1.23+)
Insert entries from iterator
All (1.23+)
Iterator over all entries
Keys, Values
Iterators over keys/values
For Clone, Equal, sorted iteration → see samber/cc-skills-golang@golang-safety skill.
Map Internals Deep Dive — How Go maps store and hash data, bucket overflow chains, why maps never shrink (and what to do about it), comparing map performance to alternatives.
Arrays
Fixed-size, value types. Copied entirely on assignment. Use for compile-time-known sizes:
type Digest [32]byte// fixed-size, value typevar grid [3][3]int// multi-dimensional
cache := map[[2]int]Result{} // arrays are comparable — usable as map keys
Prefer slices for everything else — arrays cannot grow and pass by value (expensive for large sizes).
container/ Standard Library
Package
Data Structure
Best For
container/list
Doubly-linked list
LRU caches, frequent middle insertion/removal
container/heap
Min-heap (priority queue)
Top-K, scheduling, Dijkstra
container/ring
Circular buffer
Rolling windows, round-robin
bufio
Buffered reader/writer/scanner
Efficient I/O with small reads/writes
Container types use any (no type safety) — consider generic wrappers. Container Patterns, bufio, and Examples — When to use each container type, generic wrappers to add type safety, and bufio patterns for efficient I/O.
strings.Builder vs bytes.Buffer
Use strings.Builder for pure string concatenation (avoids copy on String()), bytes.Buffer when you need io.Reader or byte manipulation. Both support Grow(n). Details and comparison
Generic Collections (Go 1.18+)
Use the tightest constraint possible. comparable for map keys, cmp.Ordered for sorting, custom interfaces for domain-specific ordering.
type Set[T comparable] map[T]struct{}
func(s Set[T]) Add(v T) { s[v] = struct{}{} }
func(s Set[T]) Contains(v T) bool { _, ok := s[v]; return ok }
Writing Generic Data Structures — Using Go 1.18+ generics for type-safe containers, understanding constraint satisfaction, and building domain-specific generic types.
Pointer Types Deep Dive — Normal pointers, unsafe.Pointer (the 6 valid spec patterns), and weak.Pointer[T] for GC-safe caches that don't prevent cleanup.
Copy Semantics Quick Reference
Type
Copy Behavior
Independence
int, float, bool, string
Value (deep copy)
Fully independent
array, struct
Value (deep copy)
Fully independent
slice
Header copied, backing array shared
Use slices.Clone
map
Reference copied
Use maps.Clone
channel
Reference copied
Same channel
*T (pointer)
Address copied
Same underlying value
interface
Value copied (type + value pair)
Depends on held type
Third-Party Libraries
For advanced data structures (trees, sets, queues, stacks) beyond the standard library:
deckarep/golang-set — thread-safe and non-thread-safe set implementations
gammazero/deque — fast double-ended queue
When using third-party libraries, refer to their official documentation and code examples for current API signatures. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.
Cross-References
→ See samber/cc-skills-golang@golang-performance skill for struct field alignment, memory layout optimization, and cache locality
→ See samber/cc-skills-golang@golang-safety skill for nil map/slice pitfalls, append aliasing, defensive copying, slices.Clone/Equal
→ See samber/cc-skills-golang@golang-concurrency skill for channels, sync.Map, sync.Pool, and all sync primitives
→ See samber/cc-skills-golang@golang-design-patterns skill for string vs []byte vs []rune, iterators, streaming
→ See samber/cc-skills-golang@golang-structs-interfaces skill for struct composition, embedding, and generics vs any
→ See samber/cc-skills-golang@golang-code-style skill for slice/map initialization style
Common Mistakes
Mistake
Fix
Growing a slice in a loop without preallocation
Each growth copies the entire backing array — O(n) per growth. Use make([]T, 0, n) or slices.Grow
Using container/list when a slice would suffice
Linked lists have poor cache locality (each node is a separate heap allocation). Benchmark first
bytes.Buffer for pure string building
Buffer's String() copies the underlying bytes. strings.Builder avoids this copy
unsafe.Pointer stored as uintptr across statements
GC can move the object between statements — the uintptr becomes a dangling reference
Large struct values in maps (copying overhead)
Map access copies the entire value. Use map[K]*V for large value types to avoid the copy
