What is the internal structure of a slice? How does append work?

Q: What is the internal structure of a slice? How does append work?

// Slice header (conceptually) type slice struct { array unsafe.Pointer len int cap int } s := make([]int, 3, 5) // len=3, cap=5, pointer to [0,0,0,_,_] s = append(s, 10) // len=4, cap=5 — reuses same array s = append(s, 20) // len=5, cap=5 — still same array s = append(s, 30) // len=6, cap=10 — new array allocated (~2x growth), old array can be GC'd // Shared memory danger a := []int{1, 2, 3, 4, 5} b := a[:3] // shares same underlying array b[0] = 99 // also modifies a! fmt.Println(a[0]) // 99 // Safe copy to avoid sharing c := make([]int, 3) copy(c, a[:3]) Append always returns the new slice — the original slice header is not modified Sub-slices share the underlying array — mutations are visible across all sub-slices Use copy() to create an independent slice when you need isolation Pre-allocate with make([]T, 0, expectedSize) to avoid repeated reallocations in loops Growth factor is approximately 2x for small slices, decreasing for very large slices

Difficulty: 6/10

slice header, capacity growth, append semantics

A slice is a three-field struct — pointer to underlying array, length, and capacity. Append reuses the existing array if capacity allows, otherwise allocates a new larger array.

Slice internals and append behavior

Key points for interviews

1
Append always returns the new slice — the original slice header is not modified
2
Sub-slices share the underlying array — mutations are visible across all sub-slices
3
Use copy() to create an independent slice when you need isolation
4
Pre-allocate with make([]T, 0, expectedSize) to avoid repeated reallocations in loops
5
Growth factor is approximately 2x for small slices, decreasing for very large slices

Scenario Questions

0-2 years experience

2-5 years experience

1During a code review you notice a function that repeatedly appends to a slice inside a loop without pre‑allocating capacity. How would you diagnose the performance impact and what change would you recommend?

Follow-up Questions