How Go Slices Work Internally (Slice Header)

A Go slice is a lightweight descriptor containing a pointer to an underlying array, its length, and its capacity. It does not own the data; it only references a portion of an existing array. When you pass a slice to a function, you pass this descriptor by value, but the underlying array data is shared.

package main

import "fmt"

func main() {
    arr := [5]int{1, 2, 3, 4, 5}
    s := arr[1:4] // Pointer to arr[1], len=3, cap=4
    
    fmt.Printf("Slice: %v\n", s)      // [2 3 4]
    fmt.Printf("Len: %d, Cap: %d\n", len(s), cap(s))
    
    s[0] = 99 // Modifies underlying array
    fmt.Printf("Array: %v\n", arr)    // [1 99 3 4 5]
}

Think of a slice as a window looking at a row of seats in a theater. The window doesn't own the seats; it just shows you a specific range of them. If you change a seat through the window, the actual seat changes for everyone else looking at that row too. This makes slices efficient for sharing data without copying it.